Pyruvate carboxylation in Gluconeogenesis

The first roadblock to overcome in the synthesis of glucose from pyruvate is the irreversible conversion in glycolysis of PEP to pyruvate by pyruvate kinase (PK). In gluconeogenesis, pyruvate is carboxylated by pyruvate carboxylase (PC) to OAA, which is converted to PEP by PEPcarboxykinase (PEPCK) (Fig. 1).

Figure 1: PEP synthesis in the cytosol. [Note: The process moves nicotinamide adenine dinucleotide (NADH) reducing equivalents required for gluconeogenesis out of mitochondria into the cytosol.] MDm and MDc = mitochondrial and cytosolic isozymes of malate dehydrogenase; GTP and GDP = guanosine tri- and diphosphates; ADP = adenosine diphosphate.
1. Biotin: PC requires the coenzyme biotin  covalently bound to the ε-amino group of a lysine residue in the enzyme (see Fig. 1). ATP hydrolysis drives formation of an enzyme–biotin–carbon dioxide (CO2) intermediate, which then carboxylates pyruvate to form OAA. [Note: HCO3− provides the CO2.] The PC reaction occurs in the mitochondria of liver and kidney cells and has two purposes: to allow production of PEP, an important substrate for gluconeogenesis, and to provide OAA that can replenish the TCA cycle intermediates that may become depleted.
Muscle cells also contain PC but use the OAA product only for the replenishment (anaplerotic) purpose and do not synthesize glucose. [Note: Pyruvate carrier protein moves pyruvate from the cytosol into mitochondria.]
PC is one of several carboxylases that require biotin. Others include acetyl CoA carboxylase , propionyl CoA carboxylase , and methylcrotonyl CoA carboxylase .
2. Allosteric regulation: PC is allosterically activated by acetyl CoA. Elevated levels of acetyl CoA in mitochondria signal a metabolic state in which increased synthesis of OAA is required. For example, this occurs during fasting, when OAA is used for gluconeogenesis in the liver and kidneys. Conversely, at low levels of acetyl CoA, PC is largely inactive, and pyruvate is primarily oxidized by the PDHC to acetyl CoA that can
be further oxidized by the TCA cycle .