Cystathioninuria can be caused by two different mutations in the enzyme cystathionase. Cystathioninuria caused by mutation 1 can be overcome by providing cells with increasing concentrations of the coenzyme pyridoxal phosphate. Which of the following statements describe the most likely changes in the binding affinities of the two mutant enzymes? A. Both mutant enzymes have lost the ability to bind the substrates.
B. The enzyme with mutation 1 has lost the ability to bind to the substrates, whereas the enzyme with mutation 2 has lost the ability to bind to pyridoxal phosphate.
C. The enzyme with mutation 1 has decreased affinity for pyridoxal phosphate, whereas the enzyme with mutation 2 has lost the ability to bind to the substrates.
D. Both mutant enzymes have lost the ability to bind pyridoxal phosphate.

C. The enzyme with mutation 1 has decreased affinity for pyridoxal phosphate, whereas the enzyme with mutation 2 has lost the ability to bind to the substrates.

Explanation:

A coenzyme is an organic cofactor that binds with an enzyme in order to initiate or aid the function of the enzyme. A coenzyme binds to the active site of the enzyme (where the reaction occurs), thereby triggering its activation by modifying protein structure during the reaction. Some examples of coenzymes include Coenzyme A and Adenosine triphosphate (ATP). Pyridoxal phosphate is a coenzyme (it is the active form of vitamin B6) that is required for the function of cystathionase. Moreover, cystathionase is an enzyme that enables cells the synthesis of cysteine from methionine (transsulfuration pathway). The binding of pyridoxal phosphate to the enzyme increases the binding affinity of the enzyme for the substrate, thereby influencing its activity. In this case, it is expected that mutation 1 reduces the binding affinity of the enzyme to the cofactor, and thereby the cofactor is required at a higher concentration to restore normal enzyme activity.