1. Worked Example 22.1 Relating Enzyme Names with Reaction Steps of Glycolysis
How do the names of the enzymes involved in the first two steps of glycolysis relate to the reactions involved?
Analysis
Look at the names of the enzymes and the reactions. Also recall the enzyme classification scheme from Chapter 19 (see p. 590).
Solution
In the first reaction, a phosphoryl group is added to glucose. The enzyme name is hexokinase; kinase because kinases transfer phosphoryl groups, and hexo- for a hexose sugar as the substrate. In the second reaction glucose 6-phosphate is rearranged to fructose 6-phosphate by phosphoglucose isomerase. This enzyme belongs to the enzyme class of isomerases, enzymes that rearrange molecules to an isomer of the original molecule. The phosphoglucose part of the name tells us that a phosphorylated glucose molecule will be rearranged; inspection of the reaction shows that this is true.

2. Worked Example 22.2 Identifying Catabolic Stages
Complete oxidation of glucose produces six molecules of carbon dioxide. Describe the stage of catabolism at which each one is formed.
Analysis
Look at each stage of catabolism for the complete oxidation of glucose to carbon dioxide. Notice how many molecules of carbon dioxide are produced and by which step. Pathways to consider (in order) are glycolysis, conversion of pyruvate to acetyl-CoA, and the citric acid cycle. (There is no need to consider oxidative phosporylation because glucose is completely oxidized at the end of the citric acid cycle.)
Solution
No molecules of carbon dioxide are produced during glycolysis. Conversion of one molecule of pyruvate to one molecule of acetyl-CoA yields one molecule of carbon dioxide. In the citric acid cycle, two molecules of carbon dioxide are released for each molecule of acetyl-CoA oxidized. One is released in Step 3 when isocitrate is converted to α-ketoglutarate and the other when α-ketoglutarate is converted to succinyl-CoA in Step 4. Since each glucose molecule produces two pyruvate molecules, the total is three molecules twice, or six molecules of carbon dioxide.

3. Worked Example 22.3 Analyzing a Reaction
The following overall reaction is a good example of the coupling of endergonic and exergonic reactions:
The coupled reactions are
What is the common intermediate in these coupled reactions? What is ΔG for the coupled reactions? Based on these ΔG values, is the change favorable or unfavorable?
Analysis
To find the common intermediate, look for a moiety that appears on the left-hand side of one of the equations and on the right-hand side of the other. In this case OP2O6 4– (pyrophosphate or diphosphate) appears in each equation, but on opposite sides. It must be the common intermediate. To determine the ΔG for the coupled reactions, simply add the ΔGs. If the sign of the sum is negative, the reaction is exergonic and favored; if the sign of the sum is positive, the reaction is endergonic and unfavored.
Solution
Inspection of the coupled reactions shows OP2O6 4– to be the common intermediate because it appears on opposite sides of the arrow in the two reactions. The sum of the ΔGs is –6.9 kcal/mol (–28.9 kJ/mol). The reaction is exergonic and is favored.