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32.11: A Warmer World: Temperature Effects On Chemical Reactions

  • Page ID
    102054
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    Search Fundamentals of Biochemistry

    Inspiration for the chapter comes from Biochemical Adaptation by Hochachka and Somero.

    Organisms adapt to their environment, with one of the main drivers being temperature. This has occurred over geological time (think of arctic camels 3.4 million years ago!) and space with temperature gradients in terrestrial and aquatic environments. This is evident in the different species that thrive at different mountain heights and ocean depths.  Species that can move have advantages in selecting an environment best suited to their thermal needs. Historically, homo sapiens have engaged in seasonal migration, and aquatic species in vertical migrations.

    Temperature effects are universal throughout life, and physiology and biochemistry adaptations are ubiquitous. Metabolically-active life can exist from around -15oC to about 121oC (thermal saline springs). Unless greenhouse gas emissions are significantly decreased from present levels, parts of the world will become increasingly uninhabitable due to high temperatures and sea level rise. Estimates for the number of climate refugees range up to 1 billion people by 2050.   

    Two similar questions arise.  Can organisms adapt to increasing temperatures as the climate changes, and are organisms living close to their maximal survivable temperatures?

    Before we study the effects of temperature on chemical/biochemical reactions, let's review the basics of thermoregulation.  The following classification of organisms by types of thermoregulation is from BioLibre text.

    Types of Thermoregulation (Ectothermy vs. Endothermy) 

    Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, and are colloquially referred to as “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. Ectotherms are colloquially referred to as “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.

    Ectotherm 

    An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments in which temperatures are constant, such as the tropics or ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.  The cCommon frog is an ecotherm and regulates its body based on the temperature of the external environment

    Endotherms 

    In contrast to ectotherms, endotherms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is usually generated from the animal’s normal metabolism, but under conditions of excessive cold or low activity, an endotherm generate additional heat by shivering. Many endotherms have a larger number of mitochondria per cell than ectotherms. These mitochondria enables them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food more often. For example, a mouse (endotherm) must consume food every day to sustain high its metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.

    Homeothermy vs. Poikilothermy 

    Two other descriptors are also used.   A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Poikilotherm’s internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs that have such stable ambient temperatures that their internal temperature remains constant.  Figure \(\PageIndex{1}\) below shows the energy output vs temperature for a homeotherm (mouse) and poikilotherm (lizard).

    imageFigure \(\PageIndex{1}\): Homeotherm vs. Poikilotherm: Sustained energy output of an endothermic animal (mammal) and an ectothermic animal (reptile) as a function of core temperature. In this scenario, the mammal is also a homeotherm because it maintains its internal body temperature in a very narrow range. The reptile is also a poikilotherm because it can withstand a large range of temperatures.

    Another term is also employed, heterothermy, in which the temperature of a homeotherm can vary in different regions of the body (spatially) and also at different times (daily or seasonally as in hibernation).  The core body of a homeotherm is usually warmer than the extremities that allow cooling when needed.  In hibernation (or sustained torpor), both the body temperature and metabolic rates are decreased.  

    Means of Heat Transfer 

    Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and from dry skin the same manner. When a mammal sweats, evaporation removes heat from a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

    Key Points 

    • In response to varying body temperatures, processes such as enzyme production can be modified to acclimate to changes in temperature.
    • Endotherms regulate their own internal body temperature, regardless of fluctuating external temperatures, while ectotherms rely on the external environment to regulate their internal body temperature.
    • Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.
    • Heat can be exchanged between the environment and animals via radiation, evaporation, convection, or conduction processes.

    Key Terms 

    • ectotherm: An animal that relies on the external environment to regulate its internal body temperature.
    • endotherm: An animal that regulates its own internal body temperature through metabolic processes.
    • homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
    • poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

    These terms are diagramed in Figure \(\PageIndex{2}\) below.

    The_naked_truth_a_comprehensive_clarification_andFig2_.svg

    Figure \(\PageIndex{2}\): Thermoregulatory Term.  Buffenstein et al., Biol. Rev. (2021), doi: 10.1111/brv.12791.  Creative Commons Attribution License

    We have discussed in previous chapter sections how temperature can affect macromolecules such as proteins (Chapter 4), nucleic acids (Chapter 9.1) as well as supramolecular assemblies such as membranes (Chapter 10.3).  Temperature effects on small molecules and ions (such as salts in the Hofmeister series and glycerol, Chapter 4.9) in the environment that regulate the function/activity of these larger molecules and assemblies are also important. Hence we'll review and discuss the effects of temperature on these key molecular species in the next chapter section.  First, we'll delve deeper into the general impact of temperature on chemical and biochemical reactions.

    Temperature Effects on the Rates of Chemical Reactions

    To understand temperature effects on metabolic processes, let's first review temperature effects on ordinary chemical and biochemical reactions.  You may remember the general rule that the rate of a chemical reaction approximately doubles when the temperature is increased 10o C (10 K).  How does that arise?  This is generally true in a specific temperature range, as we will see below.

    The rates of reactions, either endothermic or exothermic, depend on the activation energy (Ea).  The activation energy is required to move from a reactant to the transition state, which then can go on to form product.

    The activation energy can be obtained from the Arrhenius equation (that you learned in introductory chemistry), which shows how the rate of an individual chemical reaction depends on temperature.

    \begin{equation}
    k=A e^{-E_a / R T}
    \end{equation}

    where k is the rate constant, Ea is the activation energy, Ea/RT is the average kinetic energy, and A is a constant (the "preexponential" factor). 

    By taking the natural log (ln) of each side and rearranging the equation, you get a "linearized" equation that is easier for most.

    \begin{equation}
    \ln k=\ln A-\frac{E_a}{R T}
    \end{equation}

    A plot of ln k vs 1/T has a slope = Ea/R, from which the activation energy can be calculated.  

    An alternative form can be derived:

    \begin{equation}
    \ln \frac{k_2}{k_1}=\frac{E_a}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)
    \end{equation}

    A derivation  

    Here it is!

    Derivation

    From 

    \begin{equation}
    \ln k_1=\ln (A)-E_a / R T_1
    \end{equation}

    solve for lnA

    \begin{equation}
    \ln (A)=\ln \left(k_1\right)+E_a / R T_1
    \end{equation}

    Substitute into the equation for ln(k2) gives 

    \begin{equation}
    \ln \left(k_2\right)=\ln \left(k_1\right)+E_a / R T_1-E_a / R T_2
    \end{equation}

    Rearrange to get

    \begin{equation}
    \ln \left(k_2\right)-\ln \left(k_1\right)=E_a / R T_1-E_a / R T_2
    \end{equation}

    Simplify to get the final equation!

    \begin{equation}
    \ln \left(\frac{k_2}{k_1}\right)=\frac{E_a}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)
    \end{equation}

    Solving for Ea gives

    \begin{equation}
    E_a=\frac{R \ln \frac{k_2}{k_1}}{\frac{1}{T_1}-\frac{1}{T_2}}
    \end{equation}

    Let's use this equation to calculate an Ea that will give a doubling of the reaction rate (k2/k1 = 2) going from T1 = 295 K (21.90 C, 71.3F) to T2 = 305 K (21.90 C, 89.3F), a 10oC temperature rise

    \begin{equation}
    \begin{aligned}
    E_a & =\frac{(8.314)(\ln 2)}{\frac{1}{295}-\frac{1}{305}} \\
    & =\frac{\left(8.314 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}\right)(0.693)}{0.00339 \mathrm{~K}^{-1}-0.00328 \mathrm{~K}^{-1}} \\
    & =\frac{5.76 \mathrm{Jmol}^{-1} \mathrm{~K}^{-1}}{\left(0.00011 \mathrm{~K}^{-1}\right)} \\
    & =52,400 \mathrm{Jmol}^{-1}=52.4 \mathrm{~kJ} \mathrm{~mol}^{-1}
    \end{aligned}
    \end{equation}

    Hence if a reaction has an activation energy Ea of about 54 kJ/mol, increasing the temperature from 295 to 305oC (i.e, by 10oC) doubles the reaction rate.

    Assuming that the activation energy is constant, the rate constants increase with temperature since a larger fraction of the molecules have the energy (> Ea) necessary to react.  This is illustrated in Figure \(\PageIndex{3}\) below.

    Maxwell-Boltzmann-Distribution-simple-axis-labels.svg

    Figure \(\PageIndex{3}\): Plot of a Maxwell-Boltzmann distribution of speeds for different temperatures T=100K, T=1200K, T=5000K.  Points along the curve show (1) most likely speed, (2) average speed, and (3) thermal speed (velocity that a particle in a system would have if its kinetic energy were equal to the average energy of all the particles of the system).  https://commons.wikimedia.org/wiki/F...xis-labels.svg.   Creative Commons Attribution-Share Alike 4.0 International license.

    Let's look a the brown vertical line around 950 m/s.  If we take that as the activation energy, very few molecules in the blue distribution have the required kinetic energy > Eact. Ar progressively higher temperatures, great fractions (as measured by the area under the curve to the right of the dotted line at 950 m/s) have the required energy, hence the rates increase with temperature.

    When the temperature change is 10oC, the ratio of the rate constants (or rates), k2/k1 is often called Q10, the temperature coefficient (unitless).   Q10  is not a constant, since it depends on the two temperatures that differ by 100 C (10 K).  Hence the Q10  value for the 100 range from 273-283K is different than the Q10 value from 373-383K) .  Q10 for many reaction is around 2 (doubling of the reaction rate) - 3 (tripling the reaction rate) at physiological temperature .  Q10 =2 for a given Ea only at one set of temperatures that differ by 10oC.  The variation in Q10 values is illustrated in Table \(\PageIndex{1}\) below for a reaction in which Ea = 44.5 kJ/mol. Q10 decreases from 2 as the temperatures T1 and T2=T1+10oC increase.  

    T1 in K (oC)  T2 (K) (oC) k2/k1 (Q10)
    273 (- 0.15 oC)  283 (9.85 oC)oC 2
    373 (99.9 oC) 383 (110 oC) 1.45
    473 (200 oC) 483 (210 oC) 1.26

    Table \(\PageIndex{1}\):  Q10 = k2/k1 values at different temperatures T1 and T2 that differ by 10oC.

    We will see how this is important in biological settings in a bit.  If Q10 = 1, the reaction is independent of temperature, and a Q10 <1 shows a reaction that is not functioning.  An example might be an enzyme-catalyzed reaction in which the threshold is reached at a higher temperature T2 = T1+10, at which the enzymes lose an active conformation and starts to unfold.

    The same equation and the Q10 parameters apply to enzyme-catalyzed reactions.  The activation energies (Ea) for four enzymes involved in the degradation of lignocellulose in the surface soil and subsoil are shown in Table \(\PageIndex{2}\) below.  The enzymes include two hydrolases, β-glucosidase (BG) and cellobiohydrolase (CB), which cleave cellulose, and two oxidases, peroxidase (PER) and phenol oxidase (POX), which help degrade lignin. The overall average Ea for these enzymes is about 44.7 kJ/mol, similar to the example used in Table 1 above.

    Soil Type Ea (kJ/mol)
    BG CB PER POX
    Arctic surface 35.4 39.4 12.7 81.8
    Subarctic surface 36.5 38.6 21.1 45.7
    subsoil 52.2 41.5 22.4 39.4
    Temperate 1 surface 40.9 38 64.9 102
    subsoil 49.4 21.2 28 94.8
    Temperate 2 surface 31 43.4 25.4 49.5
    subsoil 40.9 39.9 19.8 47.5
    Temperate 3 surface 51.5 53.6 28.8 73.2
    subsoil 58.8 46.7 54.2 29
    Tropical 1 surface 47.8 50.5 26.5 47.7
    subsoil 56.6 47 47.1 27.1
    Tropical 2 surface 39.3 42.5 58.3 82.5
    subsoil 42.8 43.3 22.8 45.5
    Avg 44.9 42.0 33.2 58.9

    Table \(\PageIndex{2}\): Activation Energies (Ea, kJ mol−1) for extracellular soil enzymes involved in the degradation of lignocellulose. Adapted from Steinweg JM et al. (2013) PLOS ONE 8(3): e59943. https://doi.org/10.1371/journal.pone.0059943.  Creative Commons CC0 public domain

    Q10 temperature coefficients are also used to describe biological processes like respiration, speed of neural signal propagation, metabolic rates, etc. Many biological processes are affected by temperature, especially for ectotherms that adjust temperatures to outside environments, including daily and seasonal temperature shifts.  Mammals and birds alter metabolic rates with temperature.  This is true for hibernating animals. 

    The Q10 temperature coefficient can be considered to be the factor by which the reaction rates (k or R) increase (factor of 2, 3, 1.5, etc) for each 10-degree K or C temperature increase.  It is given by the following equation:

    \begin{equation}
    Q_{10}=\left(\frac{k_2}{k_1}\right)^{10^{\circ} \mathrm{C} /\left(T_2-T_1\right)}
    \end{equation}

    It is also called the van't Hoff's temperature coefficient.  To help understand Q10, let's consider some examples.

    • If T2-T1=10o, then Q10 is simply k2/k1 for the specified temperature pairs separated by a 100 C range (T1 and T2=T1+10).  Remember that Q10 is not a constant but depends on the temperature pairs and that it goes down with increasing temperature.
    • If the temperature range is > 100 C, the the measured ratio k2/k1 is a factor > 1 x Q10
    • If the temperature range is < 100 C, the the measured ratio k2/k1 is a fraction of Q10

    This equation can be converted to 

    \begin{equation}
    k_2=k_1 Q_{10}^{\left(T_2-T_1\right) / 10^{\circ} \mathrm{C}}
    \end{equation}

    where the rate constant k2 is related to a "base" rate k1 at a base temperature of T1.  An interactive graph of the above equation is shown in Figure \(\PageIndex{4}\) below.

    Figure \(\PageIndex{4}\): Interactive graph of k2 (rate 2) vs. delta T at different base rates k1

    Change the base rate constant, k1, at a base temperature of T1, and Q10 coefficient to see how they change k2.   

    Note that if Q10 =1,  k2 at T1+10 = k1 at T1, the rate is independent of the temperature.

    For most biological systems, the Q10 value is ~ 2 to 3 under physiological relevant conditions.  The ratios of the rates (R2/R1) for different Q10 values are shown in Figure \(\PageIndex{5}\) below.

    ​​​​​​Q10TemperatureCoefficientPlot.svg

    Figure \(\PageIndex{5}\):  Idealized graphs showing the dependence on temperature of the rates of chemical reactions and various biological processes for several different Q10 temperature coefficients. The dots on the graph show how the rate change with a temperature difference of 10oC.   Wikipedia. https://en.wikipedia.org/wiki/Q10_(t...e_coefficient).  CC BY-SA 4.0

    Again this hypothetical graph is meant to show the general meaning of Q10 values.

    The "Q" model has been used to fit complex reaction systems, not just individual reactions.  Figure \(\PageIndex{6}\) below shows the daily mean soil respiration rate as a function of soil temperature. In these graphs, the x-axis is Temperature, not ΔT.

    Temperature Response of Soil RespirationQ10Fig2.svg

    Figure \(\PageIndex{6}\): Relationships between daily mean soil respiration (Rs) and soil temperature (Ts).  Jia X et al.,  PLoS ONE 8(2): e57858. https://doi.org/10.1371/journal.pone.0057858.  Creative Commons Attribution License

    The soil temperature, Ts, was measured at a 10-cm depth. Open circles are from January to June; closed circles are from July to December. The solid lines use a Q10 model, in which the observed Rs vs Ts data are fit with an equation that optimizes the Q10 parameter  The dashed lines are fitted by a logistic model, which we used in Chapter 5.7 for fitting ELISAs data. Rs is significantly different between the first and second half of the year.  

    The soil respiration rate, Rs, at 10 cm depth was strongly affected by temperature, with an annual Q10 value of 2.76.   Daily estimates of Q10 averaged 2.04 and decreased with increasing Ts. A study of seagrass showed the Q10 values are affected by plant tissue age and that Q10 varied significantly with the initial temperature and temperature ranges.  

    The use of Q10 values from the Arrhenius equation is based on the assumption that the chemical/biochemical processes are exponential functions of temperature.  For complex processes like the decay of organic matter, it would be better to model the whole system by looking at the individual enzymes involved.  One problem with using Q10 values for very complex systems is the choice of the base temperature value for rate comparisons.  The anaerobic decomposition of organic matter is generally a linear function of temperature between 5°C and 30°C, which shows that a Q10 modeling system is not ideal.  A more complex systems biology approach using programs like Vcell and COPASI would be better and less likely to cause errors in predicted CH4 emissions from the decomposition process, for example.

    Getting Back to Proteins

    In Chapter 6.1 we explored the mechanisms used by enzymes to catalyze chemical reactions. These included general acid/base catalysis, metal ion (electrostatic) catalysis, covalent (nucleophilic) catalysis, and transition state stabilization. Some physical processes included intramolecular catalysis and strain/distortion.  The rate-limiting step in enzyme-catalyzed reactions can include actual bond breaking in the substrate, dissociation of product, and conformational change required to facilitate binding, catalysis, and dissociation.  A rate-limiting conformational change may occur not in the active site pocket but in nearby loops that modulate the accessibility of reactant to and dissociation of product from the active site.  These all may be influenced by temperature, with localized conformational flexibility especially important.  

    An interesting example of localized conformational changes affecting enzyme activity is RNase A.  His 48, 18 Å from the enzyme active site, is involved in the rate-limiting enzymatic step involving product release.

    Figure \(\PageIndex{7}\) shows an interactive iCn3D model of bovine pancreatic Ribonuclease A in complex with 3'-phosphothymidine (3'-5')-pyrophosphate adenosine 3'-phosphate (1U1B)

    Bovine pancreatic Ribonuclease A in complex with 3phosphothymidine 35-pyrophosphate adenosine 3phosphate (1U1B).png

    NIH_NCBI_iCn3D_Banner.svg Figure \(\PageIndex{7}\): Bovine pancreatic Ribonuclease A in complex with 3'-phosphothymidine (3'-5')-pyrophosphate adenosine 3'-phosphate (1U1B). (Copyright; author via source).  Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...SnrGYXSVXcLCk6

    The substrate is shown in spacefill.  The active site side chains and the distal His 48 are shown as sticks and labeled.  Two flexible loops, Loop 1 (magenta) near His 48, and Loop 4 (cyan) near the active site are highlighted.  On ligand binding, the loops move a few angstroms to make the active site more closed, inhibiting product release. Product release is associated with mobile regions including Loops 1 (20 Å from the active site) and 2.  Loop 4, near the active site, is involved in the specificity for purines 5' to the substrate cleavage site.  His 48 is conserved in pancreatic RNase A.  If mutated to alanine, the kcat decreases greater than 10X, indicating a change in the rate-determining conformational motion. The enzyme is still very active compared to the uncatalyzed reaction.  His 48 appears to regulate coupled motions in the protein that are rate-limiting.

    Figure \(\PageIndex{8}\) below shows the subtle shift in the conformation of apo-RNase A (magenta, no ligand, 1FS3) to the substrate-bound form (cyan, ligand in sticks), 1U1B). Note the small motion in His 48 shown in sticks at the bottom of the image.

    RNaseA_1FS3to1U1BCrop.gif

    Figure \(\PageIndex{8}\):  Conformational changes apo-RNase A (magenta, no ligand, 1FS3) on conversion to the substrate-bound form (cyan, ligand in sticks, 1U1B).

    We will explore temperature effects on protein structure and function more in the next chapter section.


    This page titled 32.11: A Warmer World: Temperature Effects On Chemical Reactions is shared under a not declared license and was authored, remixed, and/or curated by Henry Jakubowski.

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