58 by: Yannick Martens
Student – Yannick Martens
Enzyme – Pyruvate Carboxylase
EC# – 6.4.1.1
Pyruvate carboxylase is found in most organisms. In mammalian cells, it is found both the mitochondria and cytoplasm, with proportions in either differing amongst species1. As it partakes in gluconeogenesis, it is found in especially in the liver, kidneys and small intestines. Pyruvate carboxylase occurs in two different forms, α4 or α4β4, depending on the organism. This video looks at the α4 form, which is made up of 4 identical α subunit, and is found in most organisms1. Each subunit is ~120-130 kDa in size, and has a biotin molecule attached to the ε-amino group2 of a lysine residue approximately 35 residues from the C-terminus of the subunit1. Biotin acts as a cofactor3. Pyruvate carboxylase has two active sites, a Biotin Carboxylation (BC) domain, at the n-terminus, and a Carboxyltransferase (CT) domain. Bicarbonate is first activated by ATP, converting it into more activated CO2 through an intermediate carboxyphosphate. CO2 then is used to carboxylate biotin into carboxybiotin2. Carboxybiotin then transports CO2, as a carboxyl group, from the BC domain to the CT domain1 on an adjacent subunit2, where it is released from biotin. Pyruvate enters the CT domain, and is enolated by biotin. The pyruvate enolate then reacts with CO2 to form oxaloacetate. This mechanism functions through acid-base catalysis, as the interactions of biotin with either CO2 or pyruvate through the loss and gain of protons.
The majority of α4 forms of pyruvate carboxylase are regulated through allosteric regulation, where acetyl-CoA acts as an activator and L-aspartate acts as an inhibitor.
References
1. Jitrapakdee, S., St Maurice, M., Rayment, I., Cleland, W. W., Wallace, J. C., & Attwood, P. (2008). Structure, mechanism, and regulation of pyruvate carboxylase. Biochemical Journal, 413(3), 369-387. doi: 10.1042/BJ20080709.
2. Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry. (7th ed.). Macmillan Learning.
3. St Maurice, M., Reinhardt, L., Surinya, K. H., Attwood, P. V., Wallace, J. C., Cleland, W. W., & Rayment, I. (2007). Domain Architecture of Pyruvate Carboxylase, a Biotin-Dependent Multifunctional Enzyme. Science, 317(5841), 1076-1079. doi: 10.1126/science.1144504
