Pyruvate carboxylase

• OMIM: 608 786
• UniProt: P11498

Pyruvate carboxylase (PC) is that of the enzyme in all living organisms ( other than the plant ), which catalyzes the addition of carbon dioxide to pyruvate. This reaction represents the first step of gluconeogenesis, and also serves as anaplerotic reaction for the citric acid cycle. The enzyme is localized in the mitochondria and is allosterically regulated by the concentration of acetyl -CoA. The enzyme function is critically dependent on this regulation, so that in the absence of acetyl -CoA is virtually no activity detectable. Mutations on the PC gene in humans can cause rare PC deficiency.

Structure

Mainly in the active form, the enzyme as a ( hetero) tetramer is present, which communicates with the dimers and monomers in the formation equilibrium. However, the tetrameric state is not necessary for the basic function of the enzyme, so that the di-, and the monomers have an activity. The molar mass of a single monomer is 130 kDa.

The functionally most interesting sections of the protein are the N- and C-terminal tails. The first 300 to 350 N-terminal amino acids form an ATP binding domain ( ATP-grasp domain) and the outermost 80 C-terminal amino acids, the biotin- binding domain, in which the biotin is covalently linked through an amide bond with the- amino group of a lysine.

Reaction mechanism

The exact reaction mechanism of pyruvate carboxylase involves three steps:

1 ) Activation of CO2 (which in aqueous solution as bicarbonate anion HCO3- present ) to Carboxyphosphat:

2) addition of the Carboxyphosphats to the biotin ( N1 atom ):

3) transfer of the activated carboxyl group on the pyruvate:

By the use of biotin as a coenzyme, the pyruvate carboxylase as any other biotin- dependent enzyme is susceptible to inhibition by avidin, as it as an avidin ( biotin) 4 complex irreversibly binds biotin.