Glutaredoxin-like protein

Jordan, A., slund, F., Pontis, E., Reichard, P., and Holmgren, A., 1997, Characterization of Escherichia coli NrdH. A glutaredoxin-like protein with a thioredoxin-like activity profile. J. Biol. Chem. 272 18044918050. 

Concerning the general topic of ferredoxin and redox proteins, it is important to note the existence of several of these in various methanogens. Ferredoxin has been reported in M barkeri [iQ2], M. thermophila [249,303], and Methanococcus thermolithotrophicus [304,305]. In one case, ferredoxin is required in electron transfer from carbon monoxide dehydrogenase to a membrane-bound hydrogenase in M. thermophila [306]. Another redox protein, referred to as a glutaredoxin-like protein , has been isolated from M. thermoautotrophicum [307], but has no known function. As mentioned above, MVH gene clusters contain a sequence for a polyferredoxin of unknown function. 

Deponte, M., Becker, K., and Rahlfs, S. (2005). Plasmodium falciparum glutaredoxin-like proteins. Biol. Chem. 386,33-40. 

A highly thermostable protein disulfide oxidoreductase was first isolated from Sulfolobus solfataricus. From its ability to catalyze the reduction of insulin disulfides in the presence of dithiothreitol (DTT), the protein was considered a thioredoxin. The protein showed an unusually high molecular mass of 25 kDa and from amino acid composition analysis contained four cysteine residues. A homologous protein was subsequently purified from Pyrococcus furiosus. From its amino acid sequence, which showed two distinct CXXC motifs, and from its thioltransferase activity the protein was considered to be a glutaredoxin-like protein. 

Little information is availble on protein disulfide oxidoreductases in Archaea. A glut iredoxin-like protein, isolated from Methanobacterium thermoautotroph-icum, was shown to have a molecular mass of 9 kDa, and a low sequence identity (<20%) to known glutaredoxin, and not to function as glutaredoxin-dependent enzyme. 

Unlike thioredoxin, glutaredoxin, and DsbA, which possess only one thiore-doxin-like motif, P/PDO is the first protein disulfide oxidoreductase whose three-dimensional structure has been shown to contain two thioredoxin fold motifs with two active sites. Thus, P/PDO shows structural resemblance to PDI and PDI-like protein s. This structural feature suggests that P/PDO is probably not just a simple protein disulfide reductant like thioredoxin as described previously. It may belong to the growing family of PDI-like proteins. From a structural point of view, P/PDO may represent the simplest form of PDI. 

Glutaredoxin is another small ubiquitous protein with a different dithiol-active center which catalyzes GSH-disulfide transhydrogenase reactions. It is GSH-specific and cannot be reduced by thioredoxin reductase. It uses GSH and an NADPH-coupled glutaredoxin reductase to catalyze the reduction of a variety of disulfide substrates, including 2-hydroxyethyl-disulfide and ribonucleotide reductase [281]. Since GSSG inhibits the latter reaction, a high ratio of GSH to GSSG will promote the synthesis of deoxyribonucleotides, which is a likely control mechanism of DNA synthesis. 

As shown in Fig. 1, the enzyme catalyzes the reduction of ribonucleoside diphosphates (Fig. lA) by dithiothreitol (Fig. IB). K values for CDP and DTT are 70 x Af and 20 mAf, respectively. The requirement for a dithiol suggests that, as for other class II RNRs, such as the extensively studied enzyme from Lactobacillus leichmannii, the hydrogen donor is very likely to be a dithiol protein such as thioredoxin or glutaredoxin. However, there is still no experimental evidence that an archaeal thioredoxin operates as an electron source for RNRs. The enzyme also requires AdoCbl for which a value of 1 pAf has been obtained (Fig. 1C). Finally, the reaction has an optimal temperature of 80° (Fig. ID), with very little activity at 30°. How AdoCbl resists such a high temperature and how the enzyme controls Co-C bond homolysis required for catalysis in thermophilic AdoCbl-dependent enzymes is an intriguing question. These properties are shared by other isolated thermophilic class II RNRs (Table II). 

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