In step 7 of fatty-acid biosynthesis (Figure), dehydration of a ?-hydroxy thioester occurs to give trans-crotonyl ACP. Is the dehydration a syn elimination or an anti elimination?

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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 NADPH/H O Reduction of the double NADP+ bond yields the saturated, chain elongated butyryl ACP. CH3CH2CH2ČSACP Butyryl ACP H20 HgC SACP H3C SACP нн trans-Crotonyl ACP