Succinyl-CoA synthetase (also called succinate thiokinase) catalyzes a key step

Question

Succinyl-CoA synthetase (also called succinate thiokinase) catalyzes a key step in the citric acid cycle that is summarized in the following reaction. Look at the reaction and then answer the questions that follow (18 points total).

            a.         This enzyme is called a synthetase but it is an example of a larger class of enzymes called ligases. Why should this particular enzyme be placed in this class? (2 points)

            b.         This reaction can be written as the sum of two separate reactions:

            succinyl-CoA + H2O -----> succinate + Coenzyme A          DG'o = - 36 kJ/mol

            GDP + Pi                    -----> GTP + H2O                              DG'o = + 30.5 kJ/mol

                        What is the overall free energy change for the reaction? What does this indicate about the directionality of the reaction? (2 points)

            c.         The substrate of this reaction (succinyl-CoA) is an important regulator of the citric acid cycle. Which enzyme(s) are affected by this compound? Does accumulation of succinyl-CoA increase or decrease the flow of compounds through the citric acid cycle? (2 points)

            d.         Succinyl-CoA synthetase also is the precursor of several key compounds including porphyrins. If succinyl-CoA is removed prior to the succinyl-CoA synthetase reaction for porphyrin synthesis, how does a cell respond in order to continue its metabolism? (2 points)

Explanation / Answer

A. Ligases area class of enzymes that carry out joining function. To put it more clearly, ligages join molecules to biomolecules parts of biomolecules to each other. For example, DNA ligase joins two strans of DNA. Since Succinyl coA joins phosphate to Adenosine diphosphate (ADP) or Guanosine diphosphate (GDP) to form Adenosine or Guanosine triphosphate (ATP or GTP), the enzyme is classified under ligases.

B. The overall free energy change for the reaction is -5.5 kj/mol. Free energy change equals to DG'o of reaction 2 (+30.5kJ/mol) + DG'o of reaction 1 (-36 kJ/mol) = -5.5 kj/mol.

If DG'o <0, the reaction proceeds from left to right spontaneously. Therefore, succinyl-CoA + H2O -----> succinate + Coenzyme A (DG'o = - 36 kJ/mol ) will proceed from succinyl-CoA + H2O to succinate + Coenzyme A.  If DG'o >0, the reaction proceeds from right to left. So,  GDP + Pi -----> GTP + H2O ( DG'o = + 30.5 kJ/mol) will proceed from GTP + H2O to GDP + Pi. But since combined free energy of both reactions is <0, the reaction will proceed GTP + H2O to GDP + Pi and succinyl-CoA + H2O to succinate + Coenzyme A, that is in the forward direction. Thus, GTP will be formed from GDP with help of enzyme.

C. Succinyl CoA regulates alpha-ketoglutarate enzyme. Accumulation of succinyl CoA will inhibit this enzyme and decrease flow of compounds through citric acid cycle.

D. It depends on the type of cell. For example,  in plants, algae, most bacteriaand archaea, glutamic acid is used to produce porphyrins using glutamyl-tRNA and glutamate-1-semialdehyde, with help of enzymes - glutamyl-tRNA synthetase, glutamyl-tRNA reductase, and glutamate-1-semialdehyde 2,1-aminomutase.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.