CONCEPT MAP: AMINO ACIDS AND PROTEINS
20.2 Energy and Biochemical Reactions
In order to progress, chemical reactions either release or absorb energy. For a reaction to be favorable and proceed on its own is dependent on either the release or absorp- tion of energy as heat (the change in enthalpy, ⌬H), together with the increase or decrease in disorder (⌬S, the entropy change) caused by the reaction. The net effect of these changes is given by the free-energy change of a reaction: ⌬G = ⌬H - T⌬ S.
Reactions in living organisms are no different from reactions in a chemistry labora- tory. Both follow the same laws, and both have the same energy requirements. Sponta- neous reactions—that is, those that are favorable in the forward direction—release free energy, and the energy released is available to do work. Such reactions, described as
Sun
Solar energy
Plants — photosynthesis
Transport across membrane
Mechanical work Chemical synthesis
Chemical energy
Animals—
release of chemical energy in catabolism
Work
Maintain body temperature
Lost to surroundings +
Heat
A + B + energy AB
▲ Figure 20.1
The flow of energy through the biosphere.
Energy from the sun is ultimately stored in chemical bonds, used for cellular or mechanical work, used to maintain body temperature, or lost as heat.
Review entropy, enthalpy, and free-energy change in Sections 7.2–7.4.
S E C T I O N 2 0 . 2 Energy and Biochemical Reactions 625
exergonic, are the source of our biochemical energy. Remember the difference between the terms exergonic and exothermic. “Exergonic” applies to the release of free energy, represented by a negative ⌬G. “Exothermic” applies only to the release of heat, repre- sented by a negative value for the heat of reaction, ⌬H.
Biochemists use transformed values of ⌬G calculated for standard state conditions, which are typically indicated by the symbol ⌬G ⬚. The free-energy values used in the biochemistry chapters of this text are those transformed values, but for simplicity we will continue to use ⌬G to symbolize the free-energy change of biochemical reactions in our discussions of energy.
As shown by the energy diagram in Figure 20.2a, the products of a favorable, exer- gonic reaction are farther downhill on the energy scale than the reactants. That is, the products are more stable than the reactants, and as a result the free-energy change 1⌬G2 has a negative value. Oxidation reactions, for example, are usually downhill reactions that release energy. Oxidation of glucose, the principal source of energy for animals, produces 686 kcal (2870 kJ) of free energy per mole of glucose:
C6H12O6 + 6 O2 h 6 CO2 + 6 H2O ⌬G = -686 kcal>mol 1-2870 kJ>mol2 The greater the amount of free energy released, the farther a reaction proceeds toward product formation before reaching equilibrium.
Reaction progress (a)
Free energy
Reactants
A favorable reaction
Products Negative
ΔG
Reaction progress (b)
Free energy
An unfavorable reaction
Reactants
Products Positive
ΔG Endergonic
reaction Exergonic
reaction
◀ Figure 20.2
Energy diagrams for favorable and unfavorable reactions.
(a) In a favorable reaction, the products have less energy than the reactants. (b) In an unfavo- rable reaction, the products have more energy than the reactants.
Reactions in which the products are higher in energy than the reactants can also take place, but such unfavorable reactions cannot occur without the input of energy from an external source. In other words, energy has to be added to the reactants for an energetically uphill change to occur (Figure 20.2b). You might think of these as reac- tions that have to be pushed up the hill. Such reactions are described as endergonic.
The larger the positive free-energy change, the greater the amount of energy that must be added to convert the reactants to products. Remember the difference here also:
“Endergonic” applies to reactions that require an input of free energy and have a posi- tive value of ⌬G. “Endothermic” refers to reactions that absorb heat from their sur- roundings and have a positive value for the heat of reaction, ⌬H.
Like the heat of reaction, the free-energy change switches sign for the reverse of a reaction, but the value does not change. Photosynthesis, the process whereby plants con- vert CO2 and H2O to glucose plus O2, is the reverse of the oxidation of glucose. Its ⌬G is therefore positive and of the same numerical value as that for the oxidation of glucose (see Chemistry in Action: Plants and Photosynthesis on p. 649). The sun provides the necessary external energy for photosynthesis (686 kcal/mol [2870kJ/mol] of glucose formed):
ΔG = −686 kcal/mol (−2870 kJ/mol) (exergonic, energy released) ΔG = +686 kcal/mol (+2870 kJ/mol) (endergonic, energy required) 6 CO2 6 H2O
Photosynthesis
Oxidation
+ C6H12O6 + 6 O2
Endergonic A nonspontaneous reaction or process that absorbs free energy and has a positive ⌬G.
Exergonic A spontaneous reaction or process that releases free energy and has a negative ⌬G.
Living systems make constant use of this principle in the series of chemical reac- tions we know as the biochemical pathways. Energy is stored in the products of an overall endergonic reaction pathway. This stored energy is released as needed in an overall exergonic reaction pathway that regenerates the original reactants. It is not nec- essary that every reaction in the pathways between the reactants and products be the same, so long as the pathways connect the same reactants and products.
PROBLEM 20.1
The following reactions occur in the citric acid cycle, an energy-producing sequence of reactions that we will discuss later in this chapter. Which of the reactions listed is (are) exergonic? Which is (are) endergonic? Which will release the most energy?
Write the complete equation for the reverse of reaction (c). (Recall that organic acids are usually referred to in biochemistry with the -ate ending because they exist as anions in body fluids.)
(a) Acetyl coenzyme A + Oxaloacetate + H2O h Citrate + Coenzyme A
⌬G = -9 kcal>mol 1-37.7 kJ>mol2
(b) Citrate h Isocitrate ⌬G = +3 kcal>mol 1+12.6 kJ>mol2
(c) Fumarate + H2O h l@Malate ⌬G = -0.9 kcal>mol 1-3.77 kJ>mol2 Worked Example 20.1Determining Reaction Energy
Are the following reactions exergonic or endergonic?
(a) Glucose 6@phosphateSFructose 6-phosphate
⌬G = +0.5 kcal>mol 1+2.09 kJ>mol2
(b) Fructose 6@phosphate + ATPSFructose 1,6@bisphosphate + ADP
⌬G = -3.4 kcal>mol 1-14.2 kJ>mol2
ANALYSIS Exergonic reactions release free energy, and ⌬G is negative. Endergonic reactions gain free energy, and so ⌬G is positive.
SOLUTION
Reaction (a), the conversion of glucose 6-phosphate to fructose 6-phosphate has a positive ⌬G; therefore it is endergonic. Reaction (b), the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate has a negative ⌬G; therefore it is exergonic.
Worked Example 20.2Determining Reaction Energy for Reverse Reactions
Write the reverse reaction for each reaction in Worked Example 20.1. For each reverse reaction, determine ⌬G and characterize the reaction as either exergonic or endergonic.
ANALYSIS First, remember that reactions are written left to right, with the reaction arrow pointing to the right. Second, remember that the compounds that are products in the original reaction are reactants in the reverse reaction and the compounds that are reactants in the original reaction are products in the reverse reaction. (We are assuming the reaction is directly reversible; this is not always true inside cells.) Third, remember that if ⌬G for the forward reaction is positive, ⌬G for the reverse reaction has the same numeric value but is negative. If ⌬G for the forward reaction is negative, ⌬G for the reverse reaction has the same number value but is positive. Negative ⌬G values indicate exergonic reactions, and positive ⌬G values indicate endergonic reactions.
SOLUTION
(a) Fructose 6@phosphateSGlucose 6@phosphate
⌬G = -0.5 kcal>mol 1-2.09 kJ>mol2 This reaction is exergonic.
(b) Fructose 1,6@bisphosphate + ADPSFructose 6@phosphate + ATP
⌬G = +3.4 kcal>mol 1+14.2 kJ>mol2 This reaction is endergonic.
S E C T I O N 2 0 . 2 Energy and Biochemical Reactions 627
KEY CONCEPT PROBLEM 20.2
In a cell, glucose can be oxidized via metabolic pathways. Alternatively, you could burn glucose in the laboratory. Which of these methods consumes or produces more energy? (Hint: all of the energy comes from converting the energy stored in the reduced bonds in glucose into the most oxidized form, carbon dioxide.)
KEY CONCEPT PROBLEM 20.3 The overall equation in this section,
C6H12O6+ 6 O2, 6 CO2+ 6 H2O photosynthesis
oxidation
shows the cycle between photosynthesis and oxidation. Pathways operating in opposite directions cannot be exergonic in both directions.
(a) Which of the two pathways in this cycle is exergonic and which is endergonic?
(b) Where does the energy for the endergonic pathway come from?
Life without Sunlight
Before we had the equipment to descend deep into the ocean, no one imagined that life existed there. What could provide the food and energy? Textbooks firmly stated that all life depends on sunlight.
Not true! In 1977, hydrothermal vents—openings spewing water heated to 400 °C deep within the earth—were found on the ocean floor. The hydrothermal vents were dubbed “black smokers” because the water was black with mineral sulfides precipitating from the hot, acidic water as it exited the vents.
At 2200 m below the ocean surface, there is no chance for the penetration of energy from sunlight. Therefore, the discovery of thriving clusters of tube worms, giant clams, mussels, and other creatures surrounding the black smokers was a great surprise.
Distinctive types of bacteria form the basis for the web of life in these locations. What replaces sunlight as their source of energy? The hot water is rich in dissolved inorganic substances that are reducing agents and therefore electron donors. Life- supporting energy is set free by their oxidation. Hydrogen sul- fide, for example, is abundant in the hot seawater, which has passed through sulfur-bearing mineral deposits on its way to the surface. This is the same gas produced during anaerobic decomposition of organic matter in a swamp; it is also the gas that gives the awful odor to rotten eggs. As the hydrogen sulfide is converted to sulfate ions in sulfate-reducing bacteria, the elec- trons set free in the oxidation move through an electron-trans- port chain that makes ATP formation possible for these bacteria.
Carbon dioxide dissolved in the seawater is the raw mate- rial used by the bacteria to make their own essential carbon- containing biomolecules. Experiments have shown that the tube worms, giant clams, and other creatures surrounding the black
smokers do not eat the bacteria. Rather, the bacteria colonize their digestive organs, where bacterial waste products and cell remnants are the carbon source for biosynthesis by their hosts.
An opportunity to observe the colonization of a hot deep- ocean environment came in 1991 when scientists discovered a volcano erupting underneath the ocean. Initially, all life in the vicinity was wiped out, yet soon afterward, the area was thriv- ing with bacteria. This discovery and others have raised some intriguing questions. The same black smoker bacteria have been found in the vicinity of the Mount St. Helens volcanic eruption, and hydrothermal vents with their communities of living things have been found in the fresh waters of the deepest lake on earth, Lake Baikal in Russia. Could it be that a thriving popula- tion of bacteria has been living in the hot interior of the earth ever since it formed? Were these anaerobic bacteria earth’s first inhabitants, and could they exist beneath the surface of other planets? Research will eventually answer these questions.
See Chemistry in Actions Problems 20.79 and 20.80 at the end of the chapter.
CHEMISTRY IN ACTION
▲ Sea life near a hydrothermal vent at the ocean’s floor.