The enzyme 3-ketobutanoyl thioester reductase catalyzes the reduction of the ketone carbonyl group of the following compound with NADPH to give the R stereoisomer of the product.

(This is an important reaction in the biosynthesis of fatty acids.)

(a) Give the structure of the product, including stereochemistry.

(b) The pro-(R) hydrogen of NADPH is transferred in the reaction. Show the relative positions of the NADPH and the 3-ketobutanoyl thioester molecules in the transition state of the reaction.

(c) In several different variants of this enzyme, the ¬OH groups of both tyrosine (Tyr) and serine (Ser) residues are positioned near the ketone carbonyl group. What is the likely role of these groups?

Add these amino acid side chains to the diagram you drew in part (b).

Transcribed Image Text:

O O H₂C-C-CH₂-C-S-protein CH₂ 3-ketobutanoyl thioester