Glucose-6-phosphate dehydrogenase (G6PD) is a regulatory enzyme catalyzing the first step of the pentose phosphate pathway: oxidation of glucose-6-phosphate using NADP+ and/or NAD+.
The G6PD of L. mesenteroides has been studied in great detail because of its unique dual-coenzyme specificity. Most dehydrogenases prefer either NAD+ or NADP+, but L. mesenteroides G6PD can use either.
History
The bacterial genus Leuconostoc first gained interest in the late 1920s and early 1930s as a means of studying lactic fermentation. Pederson determined that glucose fermentation yields equal quantities of lactate, ethanol, and CO2, which was later confirmed by Friedemann (Pederson 1929, and Friedemann 1939).
In 1951, De Moss et al. first isolated glucose-6-phosphate dehydrogenase from L. mesenteroides. De Moss et al. also demonstrated its dual nucleotide specificity in 1953, and in 1955 showed that fermentation by L. mesenteroides proceeds via a mechanism differing from the classical Embden-Meyerhof glycolytic scheme.
In the 1960s, Kemp and Rose demonstrated that the two reduced coenzyme products have different metabolic roles in vivo: NADPH is used in biosynthetic reactions and NADH is used in ATP-generating reactions in the production of ethanol and lactate (Kemp and Rose 1964, and White and Levy 1987).
In the late 1980s and into the 1990s, critical amino acid residues were identified, and it was shown that the kinetic mechanisms of NAD- and NADP-linked reactions differ (Levy 1989). In 1991, Lee et al. published the complete amino acid sequence of the monomer.
Current research has shown that in the case of G6PD from L. mesenteroides the enzyme is active as both a homodimer and monomer, whereas the enzyme from yeast is active only in the native, homodimeric form (Ravera et al. 2010). G6PD has also been coupled to other enzymatic reactions, where it functions to regenerate coenzyme (NADH) (Ohno et al. 2008). The stability of the enzyme and its resistance to inactivation while bound to substrates also has recently been investigated (Duggleby 2007).
Molecular Characteristics
The L. mesenteroides G6PD gene encodes a polypeptide of 485 amino acids. G6PD is unique in that it contains no cysteine residues. The sequences of human and E. coli, which use only NADP+ under physiological conditions, have been determined. The sequence of Z. mobili, which like L. mesenteroides can use either NADP+ or NAD+, has also been determined. There is approximately 30-36% sequence identity between any two of these species. A conserved sequence begins at Arg175, and contains a lysine residue thought to function in glucose-6-phosphate binding (Lee et al. 1991).
