Q29-54E - Organic Chemistry (Mcmurry)

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?

Step-by-Step Solution

Answer

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

Step 1: 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

Step 2: Addition of hydride ion

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