Glycine reductase protein

Glycine reductase protein A C. sticklandii Turner and Stadtman (1973)... 

StadtmanTC. 1989. Clostridial glycine reductase protein C, the acetyl group acceptor, catalyzes the arsenate-dependent decomposition of acetyl phosphate. Proc Natl Acad Sci USA 86 7853-6. 

StadtmanTC, Davis JN. 1991. Glycine reductase protein C. Properties and characterization of its role in the reductive cleavage of Se-carboxymethyl-selenoprotein A. J Biol Chem 266 22147-53. 

Meyer M, Granderath K, Andreesen JR. 1995. Purification and characterization of protein PB of betaine reductase and its relationship to the corresponding proteins glycine reductase and sarcosine reductase from Eubacterium acidaminophilum. Eur J Biochem 234 184-91. 

Tanaka H, Stadtman TC. 1979. Selenium-dependent clostridial glycine reductase. Purification and characterization of the two membrane-associated protein components. J Biol Chem 254 447-52. 

Turner DC, Stadtman TC. 1973. Purification of protein components of the clostridial glycine reductase system and characterization of protein A as a selenoprotein. Arch Biochem Biophys 154 366-81. 

A very interesting aspect of biological selenium compounds is the occurrence of selenocysteine in a protein component (selenoprotein A) of glycine reductase, in formate dehydrogenase and in glutathione peroxidase (see below). As pointed out by Stadt-man , the incorporation of this rather unusual amino acid into proteins in a very specific way poses something of a mystery and at present the mechanism by which this occurs is not known. 

It is important to remember that only minute amounts of selenium are required for the synthesis of these proteins and whatever mechanism(s) accounts for the incorporation of the selenium, there must be an extremely effective discrimination between it and sulfur. Otherwise sulfur, being so much more abundant, in any kind of competitive mechanism, would completely overwhelm the selenium incorporation. In this regard it is interesting to note that even when bacteria are cultured in the presence of a tremendous molar excess of sulfur, clostridial glycine reductase is synthesized normally, containing the single selenocysteine in the same polypeptide chain as two cysteine and three methionine residues ... 

Eubacterium acidaminophilum not only reductively deaminates glycine to ammonia and acetate but also expresses enzymes capable of reductively deaminating sarcosine and betaine when cells are cultured in the presence of formate (Hormann and Andreesen 1989). One would expect that the enzymes catalyzing the latter two reactions might be similar to GR. In fact, the substrate-specific protein B for sarcosine reductase was purified and found to be similar to GR protein B from C. sticklandii (Meyer et al. 1995). Apparently, this organism has evolved means to use different amino acids as electron acceptors and preferentially expresses each in response to conditions in the environment. 

Stadtman TC. 1966. Glycine reduction to acetate and ammonia identification of ferredoxin and another low molecular weight acidic protein as components of the reductase system. Arch Biochem Biophys 113 9-19. 

Figure 1 Reactions involved in the glycine reduction pathway hy Clostridia. Pa, Pb, and Pc denote the components of the glycine reduction complex where Pb is specific for the substrate (e.g. glycine, betaine, sarcosine). The reduction of oxidized protein Pa occurs via a redox chain involving thioredoxin reductase (TR) and thioredoxin (TRX). The involvement of ketene as an intermediate has not yet been proven...

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