PC

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Introduction

Pyruvate carboxylase (PC) is a biotin-containing enzyme responsible for catalyzing HCO^3- and MgATP-dependent pyruvate carboxylation to form oxaloacetate. This is an anaplerotic reaction, supplementing oxaloacetate for various key biochemical pathways. Therefore, PC is an enzyme that is very important for intermediary metabolism, controlling fuel partitioning, adipogenesis, gluconeogenesis, and insulin secretion.

The enzyme was first discovered in 1959. After continuous efforts, great progress has been made in understanding its structure and function. In most organisms, PC is a tetrameric protein that is allosterically regulated by acetyl-CoA and aspartate.

Recently, high-resolution crystal structures of holoenzymes that bind various ligands have been obtained, revealing various details of biotin carboxylase, including carboxyltransferase and biotin carboxyl carrier domain and also allosteric effector domain. The combined use of some classical biochemical and genetic methods has investigated the bona fide role of PC in non-gluconeogenic tissues. The first cloning of the PC gene promoter in mammals and subsequent transcription studies revealed some key cognate transcription factors that regulate tissue-specific expression.

The role of PC in Microorganisms

PC exists in many prokaryotes, including Pseudomonas, Bacillus, Rhizobium, Mycobacterium, Staphylococcus, and archaebacteria, but does not exist in enteric bacteria that use phosphoenolpyruvate carboxylase (PEPC) to directly convert phosphoenolpyruvate (PEP) into oxaloacetate. In yeast, two metabolic pathways that produce oxaloacetate are the PC- catalyzed reaction and the glyoxylate cycle. When yeast grow on acetate, PC- catalyzed oxaloacetate formation is inhibited, but the glyoxylate cycle is active, and vice versa if grow on glucose minimal media. In E. coli, PEPC replaces the function of PC. Overexpression of PC can increase the production of recombinant protein in this organism. It is speculated that this is due to improved oxaloacetate production enhancing fuel oxidation.

The Role of PC in Mammalian Tissues

• The role of PC in gluconeogenesis

PC is responsible for catalyzing the first key step in gluconeogenesis, and the resulting oxaloacetate is used for subsequent catalyzed conversion to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (PEPCK). PEPCK exists in mitochondria or cytoplasm in two different isoforms, and the relative proportions between species are different. Oxaloacetate is converted into PEP through mitochondrial PEPCK, and transported into the cytoplasm by the tricarboxylic acid anion carrier system, where it is converted into glucose through the reverse pathway of glycolysis and the catalysis of two other gluconeogenic enzymes (i.e., fructose-1,6-bisphosphatase and glucose-6-phosphatase). However, if the cytosolic NADH level is low and the mitochondrial NADH level is high (such as in starvation), alanine becomes the main source of pyruvate in the liver, and the oxaloacetate formed by PC can also be used as a carrier of reduction equivalent to the cytoplasm.

• The role of PC in adipogenesis

PC is highly expressed in adipose tissue. As the key enzyme of gluconeogenesis, glucose-6-phosphatase and fructose-1,6-bisphosphatase do not exist in adipose tissue, Ballard and Hanson proposed that PC participate in the de novo synthesis of fatty acids. Oxaloacetate provided by PC is converted into citrate and participates in this pathway. Citrate is exported from mitochondria and cleaved in the cytosol to form oxaloacetate and acetyl-CoA. Acetyl-CoA serves as the cornerstone of long-chain fatty acid synthesis.

• The role of PC in pancreatic islets

In addition to providing a large amount of oxaloacetate to the TCA cycle to make up for input, PC also participates in the formation of NADPH. The shuttle between mitochondria and cytoplasm called the 'pyruvate cycling' produces high levels of NADPH. The oxaloacetate formed by PC is converted into either malate or citrate, and leaves the mitochondria through pyruvate/malate or citrate/pyruvate shuttle. The malic enzyme (EC 1.1.1.40) catalyzes the production of NADPH in both shuttles. Due to these two shuttles, the amount of NADPH produced is much higher than that obtained from the pentose phosphate pathway.

Figure 1. Anaplerotic role of PC in various mammalian tissues (Jitrapakdee, S.; et al. 2010)