Fatty acid synthesis- lecture 2 (Acetyl co A carboxylase)

The fatty acid synthesis starts with the carboxylation of acetyl CoA to malonyl CoA. This irreversible reaction is the committed step in fatty acid synthesis (figure 1).

Figure- 1- Showing the formation of Malonyl co A from Acetyl co A

Bicarbonate as a source of CO₂ is required in the initial reaction for the carboxylation of acetyl-CoA to malonyl-CoA in the presence of ATP and acetyl-CoA carboxylase.

Components of Acetyl co A carboxylase

The enzyme is a multienzyme protein containing a variable number of identical subunits, each containing-

1) Biotin

2) Biotin carboxylase,

3) Biotin carboxyl carrier protein

4) Transcarboxylase,

5) A regulatory allosteric site.

The reaction involved in the conversion of Acetyl Co-A to Malonyl Co -A  takes place in two steps:

(1) carboxylation of biotin involving ATP (figure 2) and

(2) transfer of the carboxyl to acetyl-CoA to form malonyl-CoA (Figure-3)

Figure-2- showing the attachment of Biotin to the enzyme and the formation of carboxy biotin. Biotin is the first acceptor of the carboxyl group

Figure-3 – showing the role of biotin in the carboxylation of Acetyl co A

The first reaction which includes the carboxylation of biotin to form carboxybiotin is catalyzed with the biotin subunit of acetyl-CoA carboxylase.  This portion of the mechanism is ATP dependent; also, the bicarbonate provides the CO2.  The second step of this mechanism requires that the carboxyl group be transferred from the biotin to the acetyl-CoA to form malonyl-CoA.  This reaction thermodynamically should not be spontaneous, but because it is coupled with the hydrolysis of ATP to ADP this reaction goes on.  This hydrolysis of ATP also is one main reason this reaction is the committed step of this metabolic cycle.  This mechanism is very much like the pyruvate kinase that is a major committed step in glycolysis that converts phosphoenolpyruvate to pyruvate.

Regulation of Acetyl co A carboxylase

Acetyl co A carboxylase is the rate-controlling enzyme in the pathway of lipogenesis. It is regulated by-

• Allosteric modification-Acetyl-CoA carboxylase is an allosteric enzyme and is activated by citrate, which increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl-CoA. Citrate converts the enzyme from an inactive dimer to an active polymeric form, with a molecular mass of several million. Inactivation is promoted by long-chain acyl-CoA molecules (figure-4).

Figure-4 showing the regulation of Acetyl Co-A carboxylase by Citrate and long-chain fatty acyl Co-A.

• Feedback Inhibition- The enzyme is inhibited by Malonyl co A and Palmitoyl co A, an example of negative feedback inhibition by a product of a reaction (figure-4). Thus, if acyl-CoA accumulates because it is not esterified quickly enough or because of increased lipolysis or an influx of free fatty acids into the tissue, it will automatically reduce the synthesis of new fatty acid. Acyl-CoA also inhibits the mitochondrial tricarboxylate transporter, thus preventing activation of the enzyme by egress of citrate from the mitochondria into the cytosol.
• Covalent Modification-Acetyl-CoA carboxylase is also regulated by hormones such as glucagon, epinephrine, and insulin via changes in its phosphorylation state (figure-5 and 6)

Figure-5-Regulation of acetyl-CoA carboxylase by hormones.

Figure-6- The hormonal regulation is mediated by phosphorylation/dephosphorylation, the enzyme is inactivated by phosphorylation by AMP-activated protein kinase (AMPK), which in turn is phosphorylated and activated by AMP-activated protein kinase kinase (AMPKK). Glucagon (and epinephrine) increase cAMP and thus activate this latter enzyme via cAMP-dependent protein kinase. The kinase kinase enzyme is also believed to be activated by acyl-CoA. Insulin activates acetyl-CoA carboxylase, probably through an “activator” protein and an insulin-stimulated protein kinase.

• Induction and Repression-Insulin is an important hormone causing gene expression and induction of enzyme biosynthesis, and glucagon (via cAMP) antagonizes this effect. Feeding fats containing polyunsaturated fatty acids coordinately regulate the inhibition of expression of key enzymes of glycolysis and lipogenesis. These mechanisms for longer-term regulation of lipogenesis take several days to become fully manifested and augment the direct and immediate effect of free fatty acids and hormones such as insulin and glucagon.