Question

In step 6 of fatty-acid biosynthesis (Figure 29-5), acetoacetyl ACP is reduced stereospecifically by NADPH to yield an alcohol. Does hydride ion add to the Si face or the Re face of acetoacetyl ACP?

Short answer

The hydride ion added to the Re or Si face of acetoacetyl ACP can be explained.

Step-by-step solution

Prochiral concept of addition

There is an addition of hydride H that occurs to the Re face of CH₃COSCOA. This concept is called the prochiral concept. The carbonyl group ketone of acetoacetyl ACP can be reduced into alcohol mechanistically. The Keto thioester reductase and NADPH, a reducing enzyme which is closely related to NADH. The prochirality concept is involved in many biological additions

Addition of hydride ion

There is an addition of hydride ion to the face of Si to acetoacetyl ACP.