[2-KGA metabolism coupling respiratory chain in Ketogulonigenium vulgare--a review]

2-keto-L-gulonate (2-KGA) is the key intermediate of vitamin C, which can be biosynthesized by Ketogulonigenium vulgare. There are five reactions related to 2-KGA metabolism, including: (1) Oxidation of D-sorbitol to L-sorbose; (2) Oxidation of L-sorbose to L-sorbosone; (3) Oxidation of L-sorbosone (Pyranose form) to 2-KGA; (4) Oxidation of L-sorbosone (Furanose form) to vitamin C, and (5) Reduction of 2-KGA to L-idonate. L-sorbose/L-sorbosone dehydrogenase (SSDH) is responsible for the reaction of 1 through 3, L-sorbose dehydrogenase (SDH) is responsible for the reaction of 2 and 3, L-sorbosone dehydrogenase (SNDH) is responsible for the reaction of 3 and 4, aldehyde dehydrogenase (ALDH) is responsible for the reaction of 3, 2-KGA reductase (2-KGR) is responsible for the reaction of 5. Enzymes of SDH, SSDH and ALDH belong to Quinoprotein Type I that uses PQQ as the only prosthetic group. SNDH belongs to Quinoprotein Type II that is quinohemoprotein assembling heme c and PQQ. They are all soluble in the periplasm and coupled with the respiratory chain. The substrate respiration to generate ATP directly on the outside cellular membrane means this strain can use the substrate quickly in the natural environment for the necessary bioenergy.