Amino acids produced by hydrolysis of tissue proteins are the major sources of glucose during a fast. Their metabolism generates α-keto acids, such as pyruvate that is converted to glucose, or α-ketoglutarate that can enter the tricarboxylic acid (TCA) cycle and form oxaloacetate (OAA), a direct precursor of phosphoenolpyruvate (PEP). [Note: Acetyl coenzyme A (CoA) and compounds that give rise only to acetyl CoA (for example, acetoacetate, lysine, and leucine) cannot give rise to a net synthesis of glucose. This is because of the irreversible nature of the pyruvate dehydrogenase complex (PDHC), which converts pyruvate to acetyl CoA (see p. 109). These compounds give rise instead to ketone bodies (see p.
195) and are termed ketogenic.]
III. REACTIONS
Seven glycolytic reactions are reversible and are used in the synthesis of glucose from lactate or pyruvate. However, three glycolytic reactions are irreversible and must be circumvented by four alternate reactions that energetically favor the synthesis of glucose. These irreversible reactions, which together are unique to gluconeogenesis, are described below.
A. Pyruvate carboxylation
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. 10.3).
mitochondrial and cytosolic isozymes of malate dehydrogenase; GTP and GDP = guanosine tri-and diphosphates; ADP = adenosine diphosphate.
1. Biotin: PC requires the coenzyme biotin (see p. 385) covalently bound to the ε-amino group of a lysine residue in the enzyme (see Fig. 10.3). 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 (p. 183), propionyl CoA carboxylase (p. 194), and methylcrotonyl CoA carboxylase (p. 266).
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 (see p. 109).
B. Oxaloacetate transport to the cytosol