# 2020_Winter_Bis2A_Facciotti_Lecture_12

### Learning Goals Associated with 2020_Winter_Bis2A_Facciotti_Lecture_12

• Define a redox reaction and identify common biological redox reactions.
• Given a redox  reaction, identify the reducing agent, oxidizing agent, molecule that becomes oxidized, and the reduced species. Identify which species the electron(s) “starts” in, and to which species it “goes.”
• Write a composite chemical equation when given two redox half-reactions.
• Calculate the ΔE0 for a given redox reaction using the equation ΔE0’ = E0(oxidant) - E0(reductant)
• Predict whether a directional transfer of electrons between two chemical species is endergonic or exergonic by applying the concept of redox potential to provided data.
• Qualitatively relate the difference in redox potentials with a corresponding delta of Gibbs enthalpy.
• Define each variable and its role in the equation: ΔG0 = -nFΔE0
• Convert between ΔG0 and ΔE0 for a given redox reaction using the equation ΔG0’ = -nFΔE0.
• Tell an energy story for a redox reaction that utilizes the electron carrier NAD+/NADH as the second substrate in the simple, generic  reaction scheme: AH + NAD+ -> A + NADH.
• Identify NAD+ from its molecular structure and identify the functional group involved in its function as an oxidizing or reducing agent.
• Create a thermodynamic argument for how ATP hydrolysis can be coupled to drive endergonic reactions.
• Explain the important contribution of water in determining the negative ΔG0 of the hydrolysis of a phosphoanhydride bond in ATP.

## Metabolism in BIS2A

Cellular metabolism represents roughly 1/3 of the BIS2A curriculum. While this may seem like a lot, we cover very little of what a classic course in metabolism (like BIS103) covers, and a minuscule fraction of the metabolism that occurs on the planet. What we cover, however, is the very important foundational knowledge. You will learn about common chemical reactions that are associated with the transformation of life's molecular building blocks and about different core modes of energy transfer that you will encounter often in biology. The energy story and the design challenge rubrics introduced earlier will become increasingly important in these next few modules and beyond.

### What have we learned? How will it relate to metabolism?

1. We have focused on the identification and chemical properties of common biological functional groups. As we dive into metabolism, this will help you be familiar with and sometimes even predict the chemical nature/reactivity of compounds you have never seen before.
2. We have practiced recognizing and classifying molecules into four major functional groups. This will help you as we discuss how to build and break down these molecules.
3. We have learned some basic thermodynamics. This gives us a common set of concepts with which to discuss whether a biochemical reaction or process is likely to occur, and if so in which direction and how fast. This will be critical as we consider some key reactions that take place in metabolism.
4. We have learned and practiced the energy story rubric. This will allow us to study new biochemical reactions and to discuss them with a common, consistent language and approach which also reinforces the lessons we learned about thermodynamics.

### An overview of this section

• We will introduce an important concept called reduction potential and you will be given the opportunity to use a redox tower. There is also a discussion on redox chemistry in your discussion manual. Use both resources.
• We will introduce two major players in metabolism, ATP and NADH. We expect you to recognize their structures if shown on an exam.
• We will cover the metabolic pathway glycolysis. Keep in mind that we want you to look at any reaction and tell us an energy story of that reaction. You should not try to memorize these pathways (though it will help to remember some big picture things - we will stress these). Often we will give you the pathway as a figure on the exams. Glycolysis produces 2 ATP via a process called substrate level phosphorylation, 2 NADH and 2 pyruvate compounds.
• We will use the reactions of the TCA cycle to create multiple examples of energy stories. The TCA cycle will also produce more ATP, NADH and oxidize glucose into CO2.
• We will look at an alternative pathway to that of the TCA cycle, fermentation. Here, for the first time, we will see NADH used as a reactant in a metabolic reaction.
• We will follow NADH to the end of its journey, as it donates its electrons to the electron transport chain (ETC). In this module, you will need to use a redox tower. The ETC produces a proton gradient. No ATP is directly generated in this process. However, the proton gradient is then used by the cell to run an enzyme called ATP synthase, which catalyzes the reaction ADP + Pi --> ATP. This method of ATP production (called oxidative phosphorylation) results in more ATP being produced than through substrate level phosphorylation.
• And finally, we will go through the process of photosynthesis.

## Reduction-Oxidation Reactions

In this class, most of the oxidation/reduction reactions that we discuss occur in metabolic pathways (connected sets of metabolic reactions) where compounds consumed by the cell are broken down into smaller parts and then reassembled into larger macromolecules.

Transferring electrons between two compounds results in one of these compounds losing an electron and one compound gaining an electron. For example, look at the figure below. If we use the energy story rubric to look at the overall reaction, we can compare the before and after characteristics of the reactants and products. What happens to the matter (stuff) before and after the reaction? Compound A starts as neutral and becomes positively charged. Compound B starts as neutral and becomes negatively charged. Because electrons are negatively charged, we can follow the movement of electrons from compound A to B by looking at the change in charge. A loses an electron (becoming positively charged), and we say that A has become oxidized. Oxidation is associated with the loss of electron(s). B gains the electron (becoming negatively charged), and we say that B has become reduced. Reduction is associated with the gain of electrons. We also know, since something happened that energy must have been transferred and/or reorganized in this process and we'll consider this shortly.

Figure 1. A generic red/ox reaction. The full reaction is A +B goes to A+ + B-. The two half reactions are shown in the blue box. A is oxidized by the reaction and B is reduced by the reaction.

Put another way, when an electron(s) is lost, or a molecule is oxidized, the electron(s) must then passed to another molecule. We say that the molecule gaining the electron becomes reduced. ***Oxidation and reduction reactions are always paired Together these paired reactions are known as an oxidation-reduction reaction (also called a red/ox reaction).****

In Bis2A we expect you to become familiar with this terminology. Try to learn it and learn to use it as soon as possible - we will use the terms frequently and will not have the time to define them each time.

Remember the Definitions:

### The Half Reaction

To formalize our common understanding of red/ox reactions, we introduce the concept of the half reaction. A full red/ox reaction requires two half reactions. We can think each half reaction as a description of what happens to one of the two molecules involved in the full red/ox reaction. We illustrate this below. In this example, compound AH is being oxidized by compound B+; electrons are moving from AH to B+ to generate A+ and BH. Each reaction can be thought of as two half reactions: Where AH is being oxidized and a second reaction where B+ is being reduced to BH. These two reactions are considered coupled, a term that shows that these two reactions occur together, at the same time.

Figure 2. Generic red/ox reaction where compound AH is being oxidized by compound B+. Each half reaction represents a single species or compound to either lose or gain electrons (and a subsequent proton as shown in the figure above). In half reaction #1 AH loses a proton and 2 electrons: in the second half reaction, B+ gains 2 electrons and a proton. In this example HA is oxidized to A+ while B+ is reduced to BH.

### Reduction Potential

By convention we analyze and describe red/ox reactions with respect to reduction potentials, a term that quantitatively describes the “ability” of a compound to gain electrons. This value of the reduction potential is determined experimentally but for the purpose of this course we assume that the reader will accept that the reported values are reasonably correct. We can anthropomorphize the reduction potential by saying that it is related to the strength with which a compound can “attract” or “pull” or “capture” electrons. Not surprisingly this is is related to but not identical to electronegativity.

What is this intrinsic property to attract electrons?

Different compounds, based on their structure and atomic composition have intrinsic and distinct attractions for electrons. This quality is termed reduction potential or E0and is a relative quantity (relative to some “standard” reaction). If a test compound has a stronger "attraction" to electrons than the standard (if the two competed, the test compound would "take" electrons from the standard compound), we say that the test compound has a positive reduction potential whose magnitude is proportional to how much more it "wants" electrons than the standard compound. The relative strength of the compound compared to the standard is measured and reported in units of Volts (V)(sometimes written as electron volts or eV) or milliVolts (mV). The reference compound in most red/ox towers is H2.

Possible NB Discussion Point

Rephrase for yourself: How do you describe or think about the difference between the concept of electronegativity and red/ox potential?

### The Red/ox Tower

All kinds of compounds can take part in red/ox reactions. Scientists have developed a graphical tool to tabulate red/ox half reactions based on their E0' values and to help us predict the direction of electron flow between potential electron donors and acceptors. Whether a particular compound can act as an electron donor (reductant) or electron acceptor (oxidant) depends critically on what other compound it is interacting with. The electron tower usually ranks a variety of common compounds (their half reactions) from most negative E0', compounds that readily get rid of electrons, to the most positive E0', compounds most likely to accept electrons. In addition, each half reaction is written by convention with the oxidized form on the left/followed by the reduced form on the right of the slash.
For example, we write the half reaction for the reduction of NAD+ to NADH:
NAD+/NADH. The tower below also lists the number of electrons that are transferred in each reaction. For example, the reduction of NAD+ to NADH involves two electrons, written in the table as 2e-.

A biochemical electron tower is shown below.

oxidized form

reduced form

n (electrons)

Eo´ (volts)

PS1* (ox)

PS1* (red)

-

-1.20

Acetate + CO2

pyruvate

2

-0.7

ferredoxin (ox) version 1

ferredoxin (red) version 1

1

-0.7

succinate + CO2 + 2H+

a-ketoglutarate + H2O

2

-0.67

PSII* (ox)

PSII* (red)

-

-0.67

P840* (ox)

PS840* (red)

-

-0.67

acetate

acetaldehyde

2

-0.6

glycerate-3-P

glyceraldehyde-3-P + H2O

2

-0.55

O2

O2-

1

-0.45

ferredoxin (ox) version 2

ferredoxin (red) version 2

1

-0.43

CO2

glucose

24

-0.43

CO2

formate

2

-0.42

2H+

H2

2

-0.42 (at [H+] = 10-7; pH=7)

Note: at [H+] = 1; pH=0  the Eo' for hydrogen is ZERO.  You will see this in chemistry class.

α-ketoglutarate + CO2 + 2H+

isocitrate

2

-0.38

acetoacetate

b-hydroxybutyrate

2

-0.35

Cystine

cysteine

2

-0.34

Pyruvate + CO2

malate

2

-0.33

2

-0.32

2

-0.32

Complex I FMN (enzyme bound)

FMNH2

2

-0.3

Lipoic acid, (ox)

Lipoic acid, (red)

2

-0.29

1,3 bisphosphoglycerate + 2H+

glyceraldehyde-3-P + Pi

2

-0.29

Glutathione, (ox)

Glutathione, (red)

2

-0.23

2

-0.22

Acetaldehyde + 2H+

ethanol

2

-0.2

Pyruvate + 2H+

lactate

2

-0.19

Oxalacetate + 2H+

malate

2

-0.17

α-ketoglutarate + NH4+

glutamate

2

-0.14

2

0.003-0.09

Methylene blue, (ox)

Methylene blue, (red)

2

0.01

Fumarate + 2H+

succinate

2

0.03

CoQ (Ubiquinone - UQ + H+)

UQH.

1

0.031

UQ + 2H+

UQH2

2

0.06

Dehydroascorbic acid

ascorbic acid

2

0.06

Plastoquinone; (ox)

Plastoquinone; (red)

-

0.08

Ubiquinone; (ox)

Ubiquinone; (red)

2

0.1

Complex III Cytochrome b2; Fe3+

Cytochrome b2; Fe2+

1

0.12

Fe3+ (pH = 7)

Fe2+ (pH = 7)

1

0.20

Complex III Cytochrome c1; Fe3+

Cytochrome c1; Fe2+

1

0.22

Cytochrome c; Fe3+

Cytochrome c; Fe2+

1

0.25

Complex IV Cytochrome a; Fe3+

Cytochrome a; Fe2+

1

0.29

1/2 O2 + H2O

H2O2

2

0.3

P840GS (ox)

PS840GS (red)

-

0.33

Complex IV Cytochrome a3; Fe3+

Cytochrome a3; Fe2+

1

0.35

Ferricyanide

ferrocyanide

2

0.36

Cytochrome f; Fe3+

Cytochrome f; Fe2+

1

0.37

PSIGS (ox)

PSIGS (red)

.

0.37

Nitrate

nitrite

1

0.42

Fe3+ (pH = 2)

Fe2+ (pH = 2)

1

0.77

1/2 O2 + 2H+

H2O

2

0.816

PSIIGS (ox)

PSIIGS (red)

-

1.10

* Excited State, after absorbing a photon of light

GS Ground State, state prior to absorbing a photon of light

PS1: Oxygenic photosystem I

P840: Bacterial reaction center containing bacteriochlorophyll (anoxygenic)

PSII: Oxygenic photosystem II

Table 1. Common Red/ox tower used in Bis2A. By convention the tower half reactions are written with the oxidized form of the compound on the left and the reduced form on the right. Compounds that make good electron donors have highly negative reduction potentials. Compounds such as Glucose and Hydrogen gas are excellent electron donors. By contrast compounds that make excellent electron acceptors, such as Oxygen and Nitrite are excellent electron acceptors.

#### Video on electron tower

For a short video on how to use the electron tower in red/ox problems click here or below. This video was made by Dr. Easlon for Bis2A students. (This is quite informative.)

### What is the relationship between ΔE0' and ΔG?

The question now becomes: how do we know if any given red/ox reaction is energetically spontaneous or not (exergonic or endergonic) and regardless of direction, what the free energy difference is? The answer lies in the difference in the reduction potentials of the two compounds. The difference in the reduction potential for the reaction or E0' for the reaction, is the difference between the E0' for the oxidant (the compound getting the electrons and causing the oxidation of the other compound) and the reductant (the compound losing the electrons). In our generic example below, AH is the reductant and B+ is the oxidant. Electrons are moving from AH to B+. Using the E0' of -0.32 for the reductant and +0.82 for the oxidant the total change in E0' or ΔE0' is 1.14 eV.

Figure 3. Generic red/ox reaction with half reactions written with reduction potential (E0') of the two half reactions indicated.

The change in ΔE0' correlates to changes in Gibbs free energy, ΔG. In general a large positive ΔE0' is proportional to a large negative ΔG. The reactions are exergonic and spontaneous. For a reaction to be exergonic the reaction needs to have a negative change in free energy or -ΔG, this will correspond to a positive ΔE0'. In other words, when electrons flow "downhill" in a red/ox reaction from a compound with a lower (more negative) reduction potential to a second compound with a larger (more positive) reduction potential, they release free energy. The greater the voltage, E0', between the two components, the greater the energy available when electron flow occurs. It is, in fact, possible to quantify the amount of free energy available. The relationship is given by the Nernst equation:

Figure 4. The Nernst equation relates free energy of a red/ox reaction to the difference in reduction potential between the reduced products of the reaction and oxidized reactant.

Where:

• n is the number of moles of electrons transferred
• F is the Faraday constant of 96.485 kJ/V. Sometimes it is given in units of kcal/V which is 23.062 kcal/V, which is the amount of energy (in kJ or kcal) released when one mole of electrons passes through a potential drop of 1 volt

Note:

What you should notice is that ΔG and ΔE have an inverse relationship: When ΔG is positive, ΔE is negative and when ΔG is negative, ΔE is positive. For additional review see the red/ox discussion in the Bis2A Discussion Manual.

## Introduction to Mobile Energy Carriers

Section Summary

Energy moves and transfers within the cell in a variety of ways. One critical mechanism that nature has developed is the use of recyclable molecular energy carriers. While there are several major recyclable energy carriers, they all share some common functional features:

### Properties of Key Cellular Molecular Energy Carriers

• We think of the energy carriers as existing in "pools" of available carriers. One could, by analogy, consider these mobile energy carriers analogous to the delivery vehicles of parcel carriers—the company has a certain "pool" of available vehicles at any one time to pickup and make deliveries.
• Each individual carrier in the pool can exist in one of multiple distinct states: it is either carrying a "load" of energy, a fractional load, or is "empty". The molecule can interconvert between "loaded" and empty and thus can be recycled. Again by analogy, the delivery vehicles can be either carrying packages or be empty and switch between these states.
• The balance or ratio in the pool between "loaded" and "unloaded" carriers is important for cellular function, is regulated by the cell, and can often tell us something about the state of a cell. Likewise, a parcel carrier service keeps close tabs on how full or empty their delivery vehicles are—if they are too full, there may be insufficient "empty" trucks to pick up new packages; if they are too empty, business must not be going well or they shut it down. There is an appropriate balance for different situations.

In this course, we will examine two major types of molecular recyclable energy carriers: (1) the adenine nucleotides: nicotinamide adenine dinucleotide (NAD+), a close relative, nicotinamide adenine dinucleotide phosphate (NADP+), and flavin adenine dinucleotide (FAD2+) and (2) nucleotide mono-, di-, and triphosphates, with particular attention paid to adenosine triphosphate (ATP). These two types of molecules participate in a variety of energy transfer reactions. We primarily associate adenine nucleotides with red/ox chemistry and nucleotide triphosphates with energy transfers linked to the hydrolysis or condensation of inorganic phosphates.

## Red/ox chemistry and electron carriers

The oxidation of, or removal of an electron from, a molecule (whether accompanied with the removal of an accompanying proton or not) results in a change of free energy for that molecule—matter, internal energy, and entropy have all changed. Likewise, the reduction of a molecule also changes its free energy. The magnitude of change in free energy and its direction (positive or negative) for a red/ox reaction dictates the spontaneity of the reaction and how much energy it transfers. In biological systems, where a great deal of energy transfer happens via red/ox reactions, it is important to understand how these reactions are mediated and to consider ideas or hypotheses for why these reactions are mediated most times by a small family of electron carriers.

Note: BURNING BEAR

Relate the burning of (the full oxidation of the sugar in) a gummy bear with the last paragraph above. What does that demonstration have to do with our upcoming discussion on red/ox carriers? There is some mention above already—can you find it?

Note: DESIGN CHALLENGE

The problem alluded to in the previous discussion question is a great place to bring in the design challenge rubric. If you recall, the first step of the rubric asks that you define a problem or question. Here, let's imagine that there is a problem to define for which the mobile electron carriers below helped Nature solve.

***Remember, evolution DOES NOT forward-engineer solutions to problems, but in retrospect, we can use our imagination and logic to infer that what we see preserved by natural selection provided a selective advantage, because the natural innovation "solved" a problem that limited success.***

Design challenge for red/ox carriers

• What was a problem(s) that the evolution of mobile electron carriers helped solve?
• The next step of the design challenge asks you to identify criteria for successful solutions. What are criteria for success in the problem you've identified?
• Step 3 in the design challenge asks you to identify solutions. Well, here Nature has identified some for us—we consider three in the reading below. It looks like Nature is happy to have multiple solutions to the problem.
• The penultimate step of the design challenge rubric asks you to test the proposed solutions against the criteria for success. This should make you think/discuss why there are multiple different electron carriers. Are there different criteria for success? Are they each solving slightly different problems? What do you think? Be on the lookout as we go through metabolism for clues.

In living systems, a small class of compounds function as electron shuttles: they bind and carry electrons between compounds in different metabolic pathways. The principal electron carriers we will consider derive from the B vitamin group and nucleotides. These compounds can both become reduced (that is, they accept electrons) or oxidized (they lose electrons) depending on the reduction potential of a potential electron donor or acceptor that they might transfer electrons to and from. Nicotinamide adenine dinucleotide (NAD+) (we show the structure below) derives from vitamin B3, niacin. NAD+ is the oxidized form of the molecule; NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron).

We are expecting you to memorize the two forms of NAD+/NADH, know which form is oxidized and which is reduced, and be able to recognize either form on the spot in a chemical reaction.

NAD+ can accept electrons from an organic molecule according to the general equation:

Here is some vocabulary review: when electrons are added to a compound, we say the compound has been reduced. A compound that reduces (donates electrons to) another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD+ becomes reduced to NADH. When electrons leave a compound, it becomes oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD+ is an oxidizing agent, and RH is oxidized to R. Put another way, the reducing agent gets oxidized and the oxidizing agent gets reduced.

You need to get this down! We will (a) test specifically on your ability to do so (as "easy" questions), and (b) we will use the terms with the expectation that you know what they mean and can relate them to biochemical reactions correctly (in class and on tests).

You will also encounter a second variation of NAD+, NADP+. It is structurally very similar to NAD+, but it contains an extra phosphate group and plays an important role in anabolic reactions, such as photosynthesis. Another nucleotide-based electron carrier that you will also encounter in this course and beyond, flavin adenine dinucleotide (FAD+), derives from vitamin B2, also called riboflavin. Its reduced form is FADH2. Learn to recognize these molecules as electron carriers.

Figure 1. The oxidized form of the electron carrier (NAD+) is shown on the left, and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD+.

The cell uses NAD+ to "pull" electrons off of compounds and to "carry" them to other locations within the cell; thus we call it an electron carrier.  Many metabolic processes we will discuss in this class involve NAD(P)+/H compounds. For example, in its oxidized form, NAD+ is used as a reactant in glycolysis and the TCA cycle, whereas in its reduced form (NADH), it is a reactant in fermentation reactions and electron transport chains (ETC). We will discuss each of these processes in later modules.

### Energy story for a red/ox reaction

***As a rule of thumb, when we see NAD+/H as a reactant or product, we know we are looking at a red/ox reaction.***

When NADH is a product and NAD+ is a reactant, we know that NAD+ has become reduced (forming NADH); therefore, the other reactant must have been the electron donor and become oxidized. The reverse is also true. If NADH has become NAD+, then the other reactant must have gained the electron from NADH and become reduced.

Figure 2. This reaction shows the conversion of pyruvate to lactic acid coupled with the conversion of NADH to NAD+. Source: https://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme/sequential_reactions

In the figure above, we see pyruvate becoming lactic acid, coupled with the conversion of NADH into NAD+. LDH catalyses this reaction. Using our "rule of thumb" above, we categorize this reaction as a red/ox reaction. NADH is the reduced form of the electron carrier, and NADH is converted into NAD+. This half of the reaction results in the oxidation of the electron carrier. Pyruvate is converted into lactic acid in this reaction. Both sugars are negatively charged, so it would be difficult to see which compound is more reduced using the charges of the compounds. However, we know that pyruvate has become reduced to form lactic acid, because this conversion is coupled to the oxidation of NADH into NAD+. But how can we tell that lactic acid is more reduced than pyruvate? The answer is to look at the carbon-hydrogen bonds in both compounds. As electrons transfer, they are often accompanied by a hydrogen atom. Pyruvate has a total of three C-H bonds, while lactic acid has four C-H bonds. When we compare these two compounds in the before and after states, we see that lactic acid has one more C-H bond; therefore, lactic acid is more reduced than pyruvate. This holds true for multiple compounds. For example, in the figure below, you should be able to rank the compounds from most to least reduced using the C-H bonds as your guide.

Figure 3. Above are a series of compounds than can be ranked or reorganized from most to least reduced. Compare the number of C-H bonds in each compound. Carbon dioxide has no C-H bonds and is the most oxidized form of carbon we will discuss in this class. Answer: the most reduced is methane (compound 3), then methanol (4), formaldehyde (1), carboxylic acid (2), and finally carbon dioxide (5).

Figure 4. This reaction shows the conversion of G3P, NAD+, and Pi into NADH and 1,3-BPG. This reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase.

#### Energy story for the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase:

Lets make an energy story for the reaction above.

First, let us characterize the reactants and products. The reactants are glyceraldehyde-3-phosphate (a carbon compound), Pi (inorganic phosphate), and NAD+. These three reactants enter a chemical reaction to produce two products, NADH and 1,3-bisphosphoglycerate. If you look closely, you can see that the 1,3-BPG contains two phosphates. This is important since a chemical reaction should lose no mass between its beginning and its end. There are two phosphates in the reactants, so there must be two phosphates in the products (conservation of mass!). You can double check the book keeping of mass for all other atoms. It should also tabulate correctly. An enzyme called glyceraldehyde-3-phosphate dehydrogenasethat catalyzes this reaction. The standard free energy change of this reaction is ~6.3 kJ/mol, so under standard conditions, we can say that the free energy of the products is higher than that of the reactants and that this reaction is not spontaneous under standard conditions.

This is a red/ox reaction. We know that because we have produced a reduced electron carrier (NADH) as a product and NAD+ is a reactant. Where did the electron come from to make NADH? The electron must have come from the other reactant (the carbon compound).

## ATP

An important chemical compound is adenosine triphospate (ATP). The main cellular role of ATP is as a “short-term” energy transfer device for the cell. The hydrolysis reactions that liberate one or more of ATP's phosphates are exergonic and many, many cellular proteins have evolved to interact with ATP in ways that help facilitate the transfer of energy from hydrolysis to myriad other cellular functions. In this way, ATP is often called the “energy currency” of the cell: it has reasonably fixed values of energy to transfer to or from itself and can exchange that energy between many potential donors and acceptors. We will see many examples of ATP "at work" in the cell, so be looking for them. As you see them, try to think of them as functional examples of Nature's uses for ATP that you could expect to see in another reaction or context.

### ATP structure and function

At the heart of ATP is the nucleotide called adenosine monophosphate (AMP). Like the other nucleotides, AMP is composed of a nitrogenous base (an adenine molecule) bonded to a ribose molecule and a single phosphate group. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP); the addition of a third phosphate group forms adenosine triphosphate (ATP).

Figure 1. ATP (adenosine triphosphate) has three phosphate groups that can be removed by hydrolysis to form ADP (adenosine diphosphate) or AMP (adenosine monophosphate).

The phosphorylation (or condensation of phosphate groups onto AMP) is an endergonic process. By contrast, the hydrolysis of one or two phosphate groups from ATP, a process called dephosphorylation, is exergonic. Why? Let's recall that the terms endergonic and exergonic refer to the sign on the difference in free energy of a reaction between the products and reactants, ΔG. Here we are explicitly assigning a direction to the reaction, either toward phosphorylation or dephosphorylation of the nucleotide. In the phosphorylation reaction, the reactants are the nucleotide and an inorganic phosphate while the products are a phosphorylated nucleotide and WATER. In the dephosphorylation/hydrolysis reaction, the reactants are the phosphorylated nucleotide and WATER while the products are inorganic phosphate and the nucleotide minus one phosphate.

Since Gibbs free energy is a state function, it doesn't matter how the reaction happens; you just consider the beginning and ending states. As an example, let's examine the hydrolysis of ATP. The reactants ATP and water are characterized by their atomic makeup and the kinds of bonds between the constituent atoms. We can associate some free energy with each of the bonds and their possible configurations—likewise for the products. If we examine the reaction from the standpoint of the products and reactants and ask "how can we recombine atoms and bonds in the reactants to get the products?," we find that a phosphoanhydride bond between an oxygen and a phosphorus must be broken in the ATP, a bond between an oxygen and hydrogen must be broken in the water, a bond must be made between the OH (that came from the splitting of water) and the phosphorus (from the freed PO3-2), and a bond must be formed between the H (derived from the splitting of water) and the terminal oxygen on the phosphorylated nucleotide. It is the sum of energies associated with all of those bond rearrangements (including those directly associated with water) that makes this reaction exergonic. We could make a similar analysis with the reverse reaction.

Is there something special about the specific bonds involved in these molecules? Much is made in various texts about the types of bonds between the phosphates of ATP. Certainly, the properties of the bonds in ATP help define the molecule's free energy and reactivity. However, while it is appropriate to apply concepts like charge density and availability of resonance structures to this discussion, trotting these terms out as an "explanation" without a thorough understanding of how these factors influence the free energy of the reactants is a special kind of hand-waving that we shouldn't engage in. Most BIS2A students have not had any college chemistry and those who have are not likely to have discussed those terms in any meaningful way. So, explaining the process using the ideas above only gives a false sense of understanding, assigns some mystical quality to ATP and its "special" bonds that don't exist, and distracts from the real point: the hydrolysis reaction is exergonic because of the properties of ATP and ALSO because of the chemical properties of water and those of the reaction products. For this class, it is sufficient to know that dedicated physical chemists are still studying the process of ATP hydrolysis in solution and in the context of proteins and that they are still trying to account for the key enthalpic and entropic components of the component free energies. We'll just need to accept a certain degree of mechanistic chemical ignorance and be content with a description of gross thermodynamic properties. The latter is perfectly sufficient to have deep discussions about the relevant biology.

### "High-Energy" bonds

What about the term "high-energy bonds" that we so often hear associated with ATP? If there is nothing "special" about the bonds in ATP, why do we always hear the term "high-energy bonds" associated with the molecule? The answer is deceptively simple. In biology the term "high-energy bond" is used to describe an exergonic reaction involving the hydrolysis of the bond in question that results in a "large," negative change in free energy. Remember that this change in free energy has not only to do with the bond in question but rather the sum of all bond rearrangements in the reaction. What constitutes a large change? It is a rather arbitrary assignment usually associated with an amount of energy associated with the types of anabolic reactions we typically observe in biology. If there is something special about the bonds in ATP, it is not uniquely tied to the free energy of hydrolysis, as there are plenty of other bonds whose hydrolysis results in greater negative differences in free energy.

Figure 2. The free energy of hydrolysis of different types of bonds can be compared to that of the hydrolysis of ATP. Source: http://bio.libretexts.org/Core/Biochemistry/Oxidation_and_Phosphorylation/ATP_and_Oxidative_Phosphorylation/Properties_of_ATP

Table 1. Table of common cellular phosphorylated molecules and their respective free energies of hydrolysis.

#### External link discussing the energetics of coupling ATP hydrolysis to other reactions

http://bio.libretexts.org/Core/Biochemistry/Oxidation_and_Phosphorylation/ATP_and_Oxidative_Phosphorylation/Properties_of_ATP

Possible NB Discussion Point

### The cycling of ATP pools

Estimates for the number of ATP molecules in a typical human cell range from ~3x107 (~5x10-17 moles ATP/cell) in a white blood cell to 5x109 (~9x10-15 moles ATP/cell) in an active cancer cell. While these numbers might seem large, and already amazing, consider that it is estimated that this pool of ATP turns over (becomes ADP and then back to ATP) 1.5 x per minute. Extending this analysis yields the estimate that this daily turnover amounts to roughly the equivalent of one body weight of ATP getting turned over per day. That is, if no turnover/recycling of ATP happened, it would take one body weight worth of ATP for the human body to function, hence our previous characterization of ATP as a "short-term" energy transfer device for the cell.

While the pool of ATP/ADP may be recycled, some of the energy that is transferred in the many conversions between ATP, ADP, and other biomolecules is also transferred to the environment. In order to maintain cellular energy pools, energy must transfer in from the environment as well. Where does this energy come from? The answer depends a lot on where energy is available and what mechanisms Nature has evolved to transfer energy from the environment to molecular carriers like ATP. In nearly all cases, however, the mechanism of transfer has evolved to include some form of redox chemistry.

In this and the sections that follow we are concerned with learning some critical examples of energy transfer from the environment, key types of chemistry and biological reactions involved in this process, and key biological reactions and cellular components associated with energy flow between different parts of the living system. We focus first on reactions involved in the (re)generation of ATP in the cell (not those involved in the creation of the nucleotide per se but rather those associated with the transfer of phosphates onto AMP and ADP).