Examine the image depicting the active site of malate dehydrogenase with bound m

Question

Examine the image depicting the active site of malate dehydrogenase with bound malate and pick the best answer to describe the role of enzyme in this catalysis:

Select one:

a. The aromatic side chains from tryptophan act as electron donors facilitating the hydride transfer from the sp3 carbon to aspartate side chain

b. The Asp residue acts as a nucleophilic catalyst performing SN2 chemistry while two Trp residues hydrogen bond to the leaving chloride stabilizing the transition state

c. The Zn2+ ion helps to deprotonate alcohol and holds the electron-rich alcoxide in a good position for the hydride transfer to NAD+

d. The cysteine residues in the active site act as covalent catalyst by forming a bond with the benzylic carbon; the Zn atom facilitates the departure of the hydroxide group by stabilizing its negative charge

e. The positively charged side chains of Arg and His increase the acidity of hydrogen on C2 so that it can leave as a proton to the negatively charged thiamine cofactor

f. The Arg and His residues tightly hold the substrate in a position where the hydride transfer from C2 to NAD+ can readily take place

Explanation / Answer

f) The Arg and His residues tightly hold the substrate in a position where the hydride transfer from C2 to NAD+ can readily take place

because

The active site of malate dehydrogenase is a hydrophobic cavity within the protein complex that has specific binding sites for the substrate and its coenzyme, NAD+. In its active state, MDH undergoes a conformational change that encloses the substrate to minimize solvent exposure and to position key residues in closer proximity to the substrate.[6] The three residues in particular that comprise a catalytic triad are histidine (His-195), aspartate (Asp-168), both of which work together as a proton transfer system, and arginines (Arg-102, Arg-109, Arg-171), which secure the substrate

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