12.2: Phosphoryl Group Transfers and ATP

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Learning Goals (ChatGPT o3-mini)

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

ATP Structure, Energetics, and the Phosphoryl Transfer Reaction

• Describe the structural basis for ATP's role as a universal free energy carrier, and explain why the term "high-energy bond" is mechanistically misleading.
• Explain the thermodynamic factors — competing resonance, charge density, and differential hydration — that make ATP hydrolysis highly exergonic and distinguish phosphoanhydride bonds from phosphoester bonds in terms of reactivity.
• Compare the ΔG° of hydrolysis across carboxylic acid anhydrides, mixed anhydrides, phosphoric acid anhydrides, and thioesters, and rationalize the relative values using structural arguments.
• Write coupled reaction schemes showing how ATP hydrolysis drives thermodynamically unfavorable reactions such as glucose phosphorylation or peptide bond formation, applying Hess's Law to calculate net ΔG°.

Active Transport: Energy Sources and Mechanisms

• Distinguish active transport from facilitated diffusion and explain why active transport requires an energy input, framing the process in terms of ΔG and electrochemical gradients.
• Identify the four major energy sources that biological systems use to drive active transport — oxidation, ATP hydrolysis, light absorption, and collapse of an ion gradient — and provide a biochemical example of each.
• Define symport, antiport, electrogenic transport, and electroneutral transport, and explain how the collapse of one ion gradient can be coupled to the active transport of a second species against its gradient.

ATP and Phosphoryl Transfer reactions

Biological oxidation reactions serve two functions, as described in the previous chapter. Oxidation of organic molecules can produce new molecules with different properties. For example, increases in solubility are observed on the hydroxylation of aromatic substrates by cytochrome P450 (which we will explore in another chapter section). Likewise, amino acids can be oxidized to produce neurotransmitters. Many biological oxidation reactions occur, however, to produce energy to drive thermodynamically unfavored biological processes such as protein and nucleic acid synthesis or motility. Chemical potential energy is not just released in biological oxidation reactions. Instead, it is transduced into a more useful form of chemical energy in the molecule ATP (adenosine triphosphate). We will discuss the properties that make ATP so useful biologically and how exergonic biological oxidation reactions are coupled to ATP synthesis.

ATP is universally used as a "carrier" of free energy, meaning it has a higher free energy than its hydrolysis products.  Using alternative language, ATP has a high-energy transfer potential.  The structure of ATP and a simplified reaction mechanism for the cleavage of the terminal (γ) phosphoanhydride bond by water (hydrolysis) or alcohol (alcoholysis) is shown in Figure \(\PageIndex{1}\) below.

Figure \(\PageIndex{1}\): Mechanism for the hydrolysis and alcoholysis of the terminal phosphoanhydride bond in ATP

The reaction is also a nucleophilic substitution. From a different perspective, the water or alcohol is phosphorylated by ATP. Hence, the reaction can also be called a phosphoryl transfer. Chapter 6.5 shows that two different Enzyme Commission numbers might apply to reactions involving ATP. These are

• EC2 - transferases: transfer/exchange of a group from one molecule to another. More specifically, they are in category EC2.7 - transferring phosphorus-containing groups.
• EC3- hydrolases: hydrolysis reactions. More specifically, they are in category EC3.6 - acting on acid anhydride.s

Most of the examples in this book would best be classified as phosphoryl transfer reactions.

The terminal phosphate is shown in an alternative resonance form with the P atom sp³ hybridized with a tetrahedral geometry and a +1 formal charge. The attack on the electrophilic P atom by the incoming nucleophile leads to the formation of a trigonal planar transition state, with the dashed lines representing bond formation and breaking in an S_N2-like reaction. The final products are ADP, inorganic phosphate (Pi), or a phosphoester.

The hydrolysis reaction can be represented by a chemical equation or by a more typically written "biochemical" equation. Here is the chemical equation as recommended by the IUBMB/IUPAC:

\[\ce{ATP^{4-}(aq) + 2H2O (l)<=> ATP^{3-}(aq) + HPO4^{2-} (aq) + H3O^{+} (aq)} \nonumber\]

This equation is written to have both charge and mass balance. For example, the sum of charges on the left-hand side (-4) is the same as on the right-hand side (-4).

Of course, this is even a simplified equation since the reaction depends on the pH and the presence of divalent cations such as Mg²⁺. Other species that could be included just for ATP^4- include HATP³⁻, H₂ATP²⁻, MgHATP⁻, and Mg₂ATP, for example. The actual equilibrium constant would depend on pH, Mg²⁺ concentration^, and the solution's total ionic strength. If these were all fixed, the reaction could be written as a simplified biochemical equation as shown below:

\[\ce{ATP + H2O <=> ADP + Pi} \nonumber\]

We will most often use simplified biochemical equations when discussing metabolism.

Just as there are standard state conditions for chemical reactions (1 bar pressure for a gas, 1 M for a solute in solution), there are biochemical standard states for biochemical reactions. They are pressure = 1 bar, pH = 7 (i.e., H₃O⁺=10^-7 M), Mg²⁺ = 1 mM, and ionic strength of either 0 or 0.25 M.

ATP contains two phosphoanhydride bonds (connecting the three phosphates) and one phosphoester bond (connecting a phosphate to the ribose ring). The pK_as for the reactions HATP^3- → ATP^4- + H⁺ and HADP^2- → ADP^3- + H⁺ are about 7.0, so the overall charges of ATP and ADP at physiological pH are -3.5 and -2.5, respectively. Each phosphorus atom is highly electrophilic and can react with nucleophiles, such as the OH group of water or an alcohol.

As we discussed earlier, anhydrides are thermodynamically more reactive than esters, which are more reactive than amides. The large negative ΔG^o (-7.5 kcal/mol, -31kJ/mol) for the hydrolysis of one of the phosphoanhydride bonds can be attributed to the relative destabilization of the reactants (ATP and water) and relative stabilization of the products (ADP = Pi). Specifically

• The reactants cannot be stabilized to the same extent as the products by resonance, due to competing resonance stabilization of the bridging anhydride O's.
• The charge density on the reactants is greater than that of the products
• Theoretical studies show that the products are more hydrated than the reactants.

The ΔG^o for ATP hydrolysis depends on divalent ion concentration and pH, which affect the stabilization and the charge states of the reactants and products.

Carboxylic acid anhydrides are even more unstable to hydrolysis than ATP (-20 kcal/mol, -84 kJ/mol), followed by mixed anhydrides (-12 kcal/mol, -50 kJ/mol) and phosphoric acid anhydrides (-7.5 kcal/mol or -31 kK/mol). The terminal anhydride bond is often called a "high-energy bond."  This is absolutely wrong and has created deep misconceptions about molecules like anhydrides.   What is true is that the anhydride reactants are high-energy, but only relative to the energy of their cleavage products, so the reaction proceeds with a large negative ΔG^o.

There is no such thing as a high-energy bond. All covalent bonds lower the energy of a system of two separated atoms. Figure \(\PageIndex{2}\) shows high-energy molecules compared to their hydrolysis products.  Think of a graph of free energy.  Carboxylic acid anhydrides and water (reactants) have higher energy than their products, two acetic acids.

Figure \(\PageIndex{2}\): Molecules that are high energy compared to their hydrolysis products.

Some older books state that the terminal anhydride bond is "high energy."  There is no such thing as a "high energy" bond.  When a bond forms between two molecules, the energy of the system decreases.  Energy input is required to break any bond.

Each molecule above, except the thioester, has a similar motif outlined with the red dotted rectangle. The thioester is also considered high-energy compared to its hydrolysis product, since the reactant is effectively destabilized relative to a carboxylic acid ester. This arises because the sulfur atom is larger in the thioester than the oxygen atom in the carboxylic acid ester. Hence, the C-S bond length (1.82 Å) is larger than that of C-O (1.43 Å), so the C-S bond is weaker. In addition, the lone pairs on the S, which is more polarizable than O, are less likely to be shared with the C as part of the resonance stabilization of the ester. Both effects raise the energy of the thioester relative to that of the carboxylic acid ester. The hydrolysis products of both esters have similar energies. Hence, the ΔG^o for the hydrolysis of the thioester is more negative and about the same for the hydrolysis of ATP.

How can ATP be used to drive thermodynamically unfavored reactions? First consider how the hydrolysis of a carboxylic acid anhydride, which has a ΔG^o = -12.5 kcal/mol (-52 kJ/mol) can drive the synthesis of a carboxylic acid amide, with a ΔG^o = + 2-3 kcal/mol (+ 4-12 kJ/mol). The reaction is:

anhydride + amine --> amide + carboxylic acid

This can be broken into two reactions, the hydrolysis of the anhydride and the synthesis of the amide, as shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Individual and net reactions for the formation of an amide from an anhydride and an amine

Now consider the reaction of glucose + Pi to form glucose-6-P. A phosphoester is formed in this reaction, so that the reaction would proceed with a positive ΔG^o = 3.3. If ATP was used to transfer the terminal (gamma) phosphate to glucose to form Glc-6-P, the reaction proceeds with a ΔG^o = -4.2 kcal/mol (-17.6 kJ/mol). This can be calculated since ΔG and ΔG^o are state functions and path-independent. Adding the reactions and the ΔG^os gives:

• glucose + Pi → glucose-6-P, ΔG⁰ = 3.3
• ATP + H₂O → ADP + Pi , ΔG⁰ = -7.5
• NET: glucose + ATP -→ Glucose-6-P + ADP, ΔG^o = -4.2

In most biological reactions involving ATP, the terminal phosphate group of ATP is transferred to a substrate via an enzyme called a kinase. Hence, hexokinase transfers the gamma phosphate from ATP to a hexose sugar. A protein kinase is an enzyme that transfers the gamma phosphate to a protein substrate.

ATP is also used to drive peptide bond (amide) synthesis during protein synthesis. From an energetic perspective, anhydride cleavage can provide the energy for amide bond formation. Peptide bond synthesis in cells is accompanied by cleavage of both phosphoanhydride bonds in ATP in complicated reactions catalyzed by ribosomes in the cells. (This topic is considered in depth in Unit III). Figure \(\PageIndex{4}\) is a grossly simplified mechanism of how peptide bond formation can be coupled to ATP cleavage.

Figure \(\PageIndex{4}\): Individual and net reactions for forming a dipeptide from separate amino acids coupled with the cleavage of ATP. 

In the first reaction, the carboxylic acid end of amino acid 1 is activated to form a mixed carboxylic ester. The leaving group, Pi, is hydrolyzed in reaction 2 to help drive the reaction. An amide bond is formed in reaction 3, with the expulsion of an excellent leaving group, AMP. Phosphorylation reactions using ATP are nucleophilic substitution reactions that proceed through a pentavalent transition state. These reactions are also called phosphoryl transfer reactions.

One last note. ATP exists in cells as just one member of a pool of adenine nucleotides, which consists of not only ATP but also ADP and AMP (along with Pi). These constituents are readily interconvertible. We break down ATP at a rate equal to our body weight each day. Likewise, we make about the same amount from the turnover products. When energy is needed, carbohydrates and lipids are oxidized, producing ATP that can be used immediately for motility, biosynthesis, and other processes. It is important to realize that although ATP is converted to ADP in a thermodynamically spontaneous process, it is kinetically slow without an enzyme. Hence, ATP is stable in solution. However, its biological half-life is short, as it is rapidly cleared as described above. This recapitulates a theme we have seen before. Many reactions (such as oxidation with dioxygen, protein denaturation in nonpolar solvents, and ATP hydrolysis) are thermodynamically favored but kinetically slow. This kinetic slowness is a necessary, but of course insufficient, condition for life.

Introduction to Active Transport

We previously discussed how chemical potential energy in the form of reduced organic molecules can be transduced into ATP's chemical potential energy. This ATP can drive reductive biosynthesis and movement (from individual cells to whole organisms). ATP has two other significant uses in the cell.

Active Transport: Molecules must often move across membranes against a concentration gradient - from low to high chemical potential - in a process characterized by a positive ΔG. As protons could be "pumped" across the inner mitochondrial membrane against a concentration gradient, powered by the ΔG associated with electron transport (passing electrons from NADH to dioxygen), other species can cross membranes against a concentration gradient - a process called active transport - if coupled to ATP hydrolysis or the collapse of another gradient. Remember that active transport differs from facilitated diffusion, which we studied earlier, and occurs down a concentration gradient across the membrane. Many such species must be transported into the cell or intracellular organelles against a concentration gradient, as illustrated in Figure \(\PageIndex{5}\):

Figure \(\PageIndex{5}\): Examples of active transport reactions (source unfortunately lost)

Signal Transduction: All cells must know how to respond to their environment. They must be able to divide, grow, secrete, synthesize, degrade, differentiate, cease growth, and even die when the appropriate signal is given. This signal invariably is a molecule that binds to a receptor, typically on the cell surface. (Exceptions include light transduction in retinal cells when the signal is a photon and lipophilic hormones that pass through the membrane.) Shape changes follow ligand binding in transmembrane protein receptors, effectively transmitting the signal into the cytoplasm. We will discuss two main types of signal transduction pathways:

• nerve conduction, in which a presynaptic neuron releases a neurotransmitter, causing a postsynaptic neuron to "fire";
• signaling at the cell surface, which leads to the activation of kinases within the cytoplasm;

We will discuss signal transduction in the final two chapters.

A membrane receptor is required for active transport to occur, which recognizes the ligand to be transported. However, the energy source driving transport against a concentration gradient is of major interest to us. The biological world has adapted to use almost any available energy source.

Energy released by oxidation: We have already encountered the active transport of protons driven by oxidative processes. In electron transport in respiring mitochondria, NADH is oxidized as it passes electrons to a series of mobile electron carriers (ubiquinone, cytochrome C, and eventually dioxygen) using Complexes 1, 3, and 4 in the inner membrane of the mitochondria. Somehow, the energy lost in this thermodynamically favored process is coupled to conformational changes in the complex, which leads to the ejection of protons from the matrix into the inner membrane space. One can imagine a series of conformation-sensitive pKa shifts in various side chains within the complexes, which, in concert, lead to the vectorial discharge of protons.

ATP hydrolysis: One would expect that this ubiquitous carrier of free energy would be used to drive active transport. This is one of ATP's predominant roles in the biological world. 70% of all ATP turnover in the brain is used for the creation and maintenance of a Na and K ion gradient across nerve cell membranes using the membrane protein Na+/K+ ATPase.

Light: Photosynthetic bacteria have a membrane protein called bacteriorhodopsin, which contains retinal, a conjugated polyene derived from beta-carotene. It is analogous to the visual pigment protein rhodopsin in retinal cells. Absorption of light by the retinal induces a conformation change in the retinal and protein, which leads to the vectorial discharge of protons.

Collapse of an ion gradient: The favorable collapse of an ion gradient can drive the transport of a different species against a concentration gradient. We have already observed that the collapse of a proton gradient across the inner mitochondrial membrane (through FoF1ATP synthase) can drive the thermodynamically unfavored synthesis of ATP. The collapse of a proton gradient provides a proton-motive force that can drive the active transport of sugars. Likewise, a sodium-motive force can drive the active transport of metal ions. Since the energy to make the initial ion gradients usually comes from ATP hydrolysis, ATP indirectly powers the transport of the other species against a gradient.

Figure \(\PageIndex{6}\): Types of coupled active transport

Often, the transport of one species is coupled to that of another. If the species are charged, a net charge imbalance across the membrane may occur. Several terms are used to describe various types of transport, as we saw previously in Chapter 12, and which are illustrated in Figure \(\PageIndex{6}\).

• symport - two species are cotransported in the same direction by the same transport protein
• antiport - two species are cotransported in opposite directions by the same transport protein
• electrogenic - a net electrical imbalance is generated across the membrane by symport or antiport of charged species
• electroneutral - no net electrical imbalance is generated across the membrane by symport or antiport of charged species

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

This chapter establishes two foundational concepts in bioenergetics: how ATP functions as the cell's principal free energy currency, and how that energy — along with other sources — powers the movement of molecules across membranes against concentration gradients.

ATP occupies a central position in cellular metabolism because it is thermodynamically poised between high-energy donor molecules and lower-energy products. The hydrolysis of ATP's terminal phosphoanhydride bond releases approximately −7.5 kcal/mol (−31 kJ/mol) under standard biochemical conditions. This favorable ΔG° arises not from any intrinsic property of the bond itself — there is no such thing as a "high-energy bond" — but rather from the relative destabilization of the reactants compared to the products. Competing resonance among the bridging oxygens, high charge density, and greater hydration of the products all contribute. Thioesters, carboxylic acid anhydrides, and mixed anhydrides exhibit analogous destabilization of reactants relative to products and therefore also carry large, negative ΔG° values for hydrolysis.

Because ΔG is a state function, ATP hydrolysis can be thermodynamically coupled to otherwise unfavorable reactions. Hexokinase, for example, couples glucose phosphorylation (ΔG° = +3.3 kcal/mol) to ATP hydrolysis, yielding a net ΔG° of −4.2 kcal/mol. Similarly, peptide bond synthesis — unfavorable as an isolated reaction — is driven by cleavage of both phosphoanhydride bonds of ATP during ribosomal protein synthesis. In each case, the enzyme-catalyzed phosphoryl transfer proceeds through an S_N2-like pentavalent transition state at phosphorus. The cell maintains a dynamic pool of adenine nucleotides (ATP, ADP, AMP, and Pi) that is continuously regenerated through oxidative catabolism; ATP itself is kinetically stable in solution but turns over rapidly in vivo.

The second major topic is active transport — the movement of molecules across membranes against their electrochemical gradients, a process with a positive ΔG that must therefore be coupled to an exergonic energy source. Four such sources are used in biology: the free energy of oxidation reactions (as in proton pumping during mitochondrial electron transport), ATP hydrolysis (as in the Na⁺/K⁺-ATPase, which accounts for 70% of brain ATP turnover), light absorption (as in bacteriorhodopsin-mediated proton pumping), and the collapse of a pre-existing ion gradient (as when a proton-motive or sodium-motive force drives secondary active transport of sugars or metal ions). Transport proteins mediate these processes and are classified by directionality — symport moves two species in the same direction, while antiport moves them in opposite directions. Transport is further described as electrogenic when a net charge imbalance is generated across the membrane, or electroneutral when charge balance is maintained.