Carboxybiotin, fatty acid

FIGURE 25.2 (a) The acetyl-CoA carboxylase reaction produces malonyl-CoA for fatty acid synthesis, (b) A mechanism for the acetyl-CoA carboxylase reaction. Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonylphosphate intermediate (Step 1). In a typical biotin-dependent reaction, nncleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2). 

One of the important points at which CO2 enters as a reagent carried by biotin is in fatty acid biosynthesis where C02 is transferred to the enol of acetyl CoA. A magnesium(II) ion is also required and we may imagine the reaction as a nucleophilic attack of the enol on the magnesium salt of carboxybiotin. Most of the C02 transfers we have met take place by mechanisms of this sort nucleophilic attack on a bound molecule of C02, usually involving a metal ion. 

The answer is d. (Murray, pp 627-661. Scrivcr, pp 3897-3964. Sack, pp 121—138. Wilson, pp 287-320.) The key enzymatic step of fatty acid synthesis is the carboxylation of acetyl CoA to form malonyl CoA. The carboxyl of biotin is covalently attached to an -amino acid group of a lysine residue of acetyl CoA carboxylase. The reaction occurs in two stages. In the first step, a carboxybiotin is formed ... 

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