1.  Out of thin air. Define nitrogen fixation. What organisms are responsible for nitrogen fixation? ✓ 3

2.  Fixing a problem. Write the equation for biological nitrogen fixation, and explain the role of ATP. ✓ 3 ✓ 4

3.  Like Starsky and Hutch. Match each term with its description. ✓ 3

4.  Teamwork. Identify the two components of the nitrogenase complex, and describe their specific tasks. ✓ 3

5.  The fix is in. “The mechanistic complexity of nitrogenase is necessary because nitrogen fixation is a thermodynamically unfavorable process.” True or false? Explain. ✓ 3

6.  Siphoning resources. Nitrogen-fixing bacteria on the roots of some plants can consume as much as 20% of the ATP produced by the plant—consumption that does not seem very beneficial to the plant. Explain why plants tolerate this loss of valuable resources and what the bacteria are doing with the ATP. ✓ 3

7.  Vital, in the truest sense. Why are certain amino acids defined as essential for human beings? ✓ 4

8.  From few, many. What are the seven precursors of the 20 amino acids? ✓ 4

9.  Common component. What cofactor is required by all aminotransferases? ✓ 4

10.  Common resource. If an animal is fed ¹⁵N-labeled aspartate, many amino acids bearing the ¹⁵N label subsequently appear. What reactions take part in the transfer of the label? ✓ 4

11.  One carbon at a time. What is the role of tetrahydrofolate in biochemical systems?

12.  The same but different. Differentiate between S-adenosylmethionine and tetrahydrofolate.

13.  Telltale tag. In the reaction catalyzed by glutamine synthetase, an oxygen atom is transferred from the side chain of glutamate to orthophosphate, as shown by the results of ¹⁸O-labeling studies. Account for this finding.

14.  Direct synthesis. Which of the 20 amino acids can be synthesized directly from a common metabolic intermediate by a transamination reaction? ✓ 4

15.  Lines of communication. For the following example of a branched pathway, propose a feedback-inhibition scheme that would result in the production of equal amounts of Y and Z. ✓ 4

16.  Cumulative feedback inhibition. Consider the branched pathway in problem 15. The first common step (A → B) is partly inhibited by both of the final products, each acting independently of the other. Suppose that a high level of Y alone decreased the rate of the A → B step from 100 to 60 s⁻¹ and that a high level of Z alone decreased the rate from 100 to 40 s⁻¹. What would the rate be in the presence of high levels of both Y and Z?

17.  One carbon, and only one carbon. We have identified three biomolecules that carry activated one-carbon units of some sort. Name these three carriers.

18.  I’ve seen that face before. Vitamin B₁₂ is required by methionine synthase to regenerate methionine from homocysteine. What other enzyme that we have encountered in our studies requires vitamin B₁₂?

19.  Further ramifications. A person on a diet lacking in methionine would not be able to synthesize adequate amounts of proteins. However, insufficient protein synthesis would not be the only biochemical problem such a person would face. What other biosynthesis would be affected by a lack of dietary methionine?

20.  Light effects. The adjoining graph shows the concentration of several free amino acids in light- and dark-adapted plants.



(a) Of the amino acids shown, which are most affected by light–dark adaptation?

(b) Suggest a plausible biochemical explanation for the difference observed.

(c) White asparagus, a culinary delicacy, is the result of growing asparagus plants in the dark. What chemical might you think enhances the taste of white asparagus?

21.  From sugar to amino acid. Write a balanced equation for the synthesis of alanine from glucose. ✓ 4

22.  Connections. How might increased synthesis of aspartate and glutamate affect energy production in a cell? How would the cell respond to such an effect? ✓ 4

23.  Comparing K_M. Glutamate dehydrogenase and glutamine synthetase are present in all organisms. Most bacteria also contain another enzyme, glutamate synthase, which catalyzes the reductive amination of α-ketoglutarate with the use of glutamine as the nitrogen donor:



The side-chain amide of glutamine is hydrolyzed to generate ammonia within the enzyme. When NH₄⁺ is limiting, most of the glutamate is made by the sequential action of glutamine synthetase and glutamate synthase. The sum of these reactions is



Note that this stoichiometry differs from that of the glutamate dehydrogenase reaction in that ATP is hydrolyzed. Why do bacteria sometimes use this more expensive pathway? (Hint: The K_M value for NH₄⁺ of glutamate dehydrogenase is higher than that of glutamine synthase.) ✓ 4