The Figure also shows that the mechanism of catalysis of this enzyme involves a direct transfer of an electron from the side chain of Cys184 to the peptide substrate, which results in the cleavage of the peptide bond in the substrate. 

What would be the most likely effect of a missense mutation resulting in a Cys184His substitution on the enzyme's activity?

Individuals

who have brain creatine deficiency display intellectual disability. Sequencing

of exons in such individuals revealed that the brain creatine deficiency is

linked to an inborn genetic defect: such individuals are homozygous for a G→A

transition at nucleotide

Which of

the following options provides the best explanation for why this mutation

causes disease?

During exercise, a substance called creatine phosphate is used to

replenish ATP in the muscle. An enzyme called arginine:glycine amidinotransferase

(AGAT) is responsible for the first step in the biosynthesis of creatine. This

enzyme catalyses the transfer of an amidino group from arginine to glycine to

produce ornithine and guanidinoacetic acid.

Analysis of the structure of this enzyme bound to one of its

substrates, arginine, revealed the location of the active site and interactions

that stabilise the arginine molecule bound in the active site. The figure below

shows schematic representations of these interactions, and the impact of

different amino acid substitutions on the enzymatic activity.

You work in the drug development division of a large pharmaceutical company. The current commercially available medicine for a heart condition works as an inhibitor of a beta receptor, which it binds to with the dissociation constant (Kd) of 10 nM. You designed a new drug candidate. To measure the binding affinity between the receptor and the drug candidate (ligand), you incubated the receptor with the ligand at various concentrations, and measured the fraction of receptor bound. The results are presented in the graph below. What is the value of the dissociation constant (Kd) for the new drug candidate, and did you succeed in designing a candidate that binds stronger than the available drug?