Affinity labeling of chicken liver fatty acid synthase with chloroacetyl-CoA and bromopyruvate

Add time:07/20/2019    Source:sciencedirect.com

Fatty acid synthase of chicken liver is inactivated rapidly and irreversibly by incubation with chloroacetyl-CoA or with bromopyruvate. Inactivation by both reagents follows saturation kinetics, indicating the formation of an E ⋯ I complex (dissociation constants of 0.36 μM for chloroacetyl-CoA and 31 μM for bromopyruvate) prior to alkylation. The limiting rate constants are 0.15 s−1 for bromopyruvate and 0.041 s−1 for chloroacetyl-CoA. Inactivation by both reagents is protected by NADPH and 200 mM KCl, and by saturating amounts of thioester substrates which reduced the limiting rate constants 6.5–30-fold. Active-site-directed reaction of chloroacetyl-CoA is supported by the ability of this compound to form a kinetically viable complex with the enzyme as competitive inhibitor of acetyl-CoA. Chloroacetyl-CoA interacts initially at the CoA binding pocket, since the nucleotide afforded competitive protection of inactivation and caused a large decrease in its affinity. Subsequently, the phosphopantetheine prosthetic group is alkylated. Evidence is presented to show that bromopyruvate competes with chloroacetyl-CoA for the same target site.

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