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

Transcribed Image Text:

CH3ČSCOA Acetyl CoA O An acetyl group is HSACP HCOS", ATP O Acetyl CoA is carboxylated to give malonyl CoA. transferred from CoA to HSCOA ADP + P + H ACP (acyl carrior protein). DČCH2ČSCOA CH3CSACP Acetyl ACP Malonyl CoA 8 The acetyl group is transferred again, from ACP to a synthase enzyme. O The malonyl group is HS-Synthase HSACP transferred from CoA to ACP. HSACP HSCOA -оссн,сsACP CH3CS-Synthase Acetyl synthase Malonyl ACP 6 Claisen-like condensation of malonyl ACP with acetyl synthase occurs, followed by decarboxylation to yield acetoacetyl ACP, a B-keto S-Synthuse + CO2 thioester. CHасснCSACP Acetoacetyl ACP NADPHH O Reduction of the ketone by NADPH yields the corresponding 8-hydroxy NADP thioester. OH CHяCн сH,CSACP B-Hydroxybutyryl ACP O Dehydration of B-hydroxybutyryl ACP gives crotonyl ACP, an aB-unsaturated thioester. CH3CH=CHCSACP Crotonyl ACP NADPHIH O Reduction of the double NADP+ bond yields the saturated, chain elongated butyryl ACP. CH3CH2CH3ČSACP Butyryl ACP н он NADPH NADP+ Нас" Нас SACP SACP н Acetoacetyl ACP B-Hydroxybutyryl ACP