Figure 3
From: Amino acid fermentation at the origin of the genetic code
The Stickland reaction. In the genus Clostridium, amino acid fermentation requires two steps. (A) Oxidative deamination of the amino acid with the higher oxidation state yields an α-keto acid, ammonium, and two protons. For example, if R1 is a methyl group (-CH3), then the donor amino acid is an alanine, and the product of the oxidative deamination would be pyruvate, a central compounds of the intermediary metabolism. (B) Inorganic phosphate attaches at the carboxyl terminus. The second proton takes the place of the amino radical. The product of this reaction is an acyl-phosphate. For example when R2 is a hydrogen (thus the acceptor amino acid is a glycine) this product corresponds to acetyl-phosphate. The two phosphate compounds from A and B are substrates for the synthesis of ATP from ADP. Naturally, in Clostridium spp. all these reactions occur with the aid of hydrogenases (for the electron donors) and reductases (for the electron acceptors). ATP synthesis is catalysed by kinases. However, in chemo-autotrophic systems, the acetyl-phosphate is readily the energy carrier (see text).