Worked Example 20.1 Determining Reaction Energy Are the following reactions exergonic or endergonic? Analysis Exergonic reactions release free energy, and ΔG is negative. Endergonic reactions gain free energy, and so ΔG is positive. Solution Reaction (a), the conversion of glucose 6-phosphate to fructose 6-phosphate has a positive ΔG; therefore it is endergonic. Reaction (b), the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate has a negative ΔG; therefore it is exergonic.

Worked Example 20.2 Determining Reaction Energy for Reverse Reactions Write the reverse reaction for each reaction in Worked Example 20.1. For each reverse reaction, determine ΔG 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 ΔG for the forward reaction is positive, ΔG for the reverse reaction has the same numeric value but is negative. If ΔG for the forward reaction is negative, ΔG for the reverse reaction has the same number value but is positive. Negative ΔG values indicate exergonic reactions, and positive ΔG values indicate endergonic reactions. Solution

Worked Example 20.3 Identifying Metabolic Pathways That Convert Basic Molecules to Energy (a) Identify in Figure 20.5 the stages in the catabolic pathway in which lipids ultimately yield ATP. (b) Identify in Figure 20.5 the place at which the products of lipid catabolism can join the common metabolism pathway. Analysis Look at Figure 20.5 and find the pathway for lipids. Follow the arrows to trace the flow of energy. Note that Stage 3 is the point at which the products of lipid, carbohydrate, and protein catabolism all feed into a central, common metabolic pathway, the citric acid cycle. The lipid molecules that feed into Stage 3 do so via acetyl-CoA (Stage 2). Note also that most products of Stage 3 catabolism feed into Stage 4 catabolism to produce ATP. Solution The lipids in food are broken down in Stage 1 (digestion) to fatty acids and glycerol. Stage 2 (acetyl-CoA production) results in fatty acid oxidation to acetyl-CoA. In Stage 3 (citric acid cycle) acetyl-CoA enters the citric acid cycle (the common metabolism pathway), which produces ATP, reduced coenzymes, and CO2. In Stage 4 (ATP production) the energy stored in the reduced coenzymes (from the citric acid cycle) is converted to ATP energy.

Worked Example 20.4 Determining if a Coupled Reaction Is Favorable The hydrolysis of succinyl-CoA is coupled with the production of GTP (guanosine triphosphate—closely related to ATP). The equations for the reactions are given below. Combine the equations appropriately and determine if the coupled reaction is favorable. Analysis Add the two equations together to produce the equation for the coupled reaction. Also add the ΔG values together, paying close attention to the signs. If the ΔG is positive, the reaction is not favorable and will not occur; if the ΔG is negative, the reaction is favorable and will occur. Solution Since ΔG is negative, the coupled reaction will occur as written.

Worked Example 20.5 Identifying Reactants and Products in the Citric Acid Cycle What substance(s) are the substrate(s) for the citric acid cycle? What are the products of the citric acid cycle? Analysis Study Figure 20.9. Note that acetyl-CoA feeds into the cycle, but does not come out anywhere. Can you see that all of the other reaction substrates are integral to the cycle and are always present, being continuously synthesized and degraded? Note also that the coenzymes NAD+ and FAD are reduced and the reduced versions are considered energy-carrying products of the cycle. Also, CO2 is produced at two different steps in the cycle. Finally, GDP is converted to GTP in Step 5 of the cycle. Solution Acetyl-CoA is the substrate for the cycle. Along with GDP and CoA, the oxidized coenzymes NAD+ and FAD might also be considered substrates, despite their status as coenzymes, because these substances cycle between the reduced and oxidized states. The products of the cycle are CO2 and the energy-rich reduced coenzymes NADH/H+ and FADH2 as well as GTP.

Worked Example 20.6 Determining Phosphorylation Type Analysis Both substrate-level phosphorylation and oxidative phosphorylation involve the transfer of a phosphate group and its energy to another molecule. Both may result in the production of ATP. The key difference is that substrate-level phosphorylation involves the transfer of a phosphate group from one molecule to another, whereas oxidative phosphorylation adds a phosphate ion directly to ADP with the aid of ATP synthase. Solution Reaction (a), involving the formation of GTP coupled with the conversion of succinyl-CoA to succinate, is an example of substrate-level phosphorylation. Reaction (b), involving the direct addition of phosphate to ADP by ATP synthase, is oxidative phosphorylation.