PROBLEM SET 18A-Catabolism of Cysteine and Methionine



We are going to divide the metabolism of cysteine and methionine into two parts. Problem Set 18A deals with energy production (catabolism). Problem Set 18B, which will be posted next time,  deals with biochemistry that has nothing to do with energy production.

Catabolism of these amino acids  follows that same principles we have been using for other amino acids, but we will encounter some new chemistry (see CBM Chapter 18, section 18.2).

Here are some rules:

Many sulfur groups are good nucleophiles

Many sulfur groups are good leaving groups

Sulfur can act as a reaction center


Given: Cysteine desulfurase (EC uses a cysteine residue to remove the sulfhydryl group from cysteine by nucleophilic substitution.

1. Write a general mechanism for the cysteine desulfurase reaction.

2. Cysteine desulfurase is a pyridoxal-phosphate protein. Why is PALP required. Write a mechanism that explains how alanine, rather than pyruvate, is formed. Be careful: Don’t mix up tautomers and resonance forms.

3. What is the biochemical function of the cysteine desulfurase  reaction.

4. Transamination also occurs. Predict the product.

5. The sulfhydryl group of 3-mercptopyuvate cannot be removed by beta-elimination. Write a mechanism for its removal and predict the product(s).

6. Predict the products of the methionine-glyoxylate transaminase reaction.

7.  The thiomethyl group of methionine can be removed by two different elimination reactions. Both require PALP.  Write the mechanisms and predict the product(s). Then, predict the product of the next reaction.



Leave a Comment