Metal cofactors, polypeptide-bound

Frequently, metal ions are associated with the prosthetic group or cofactor. Heme rings usually contain a chelated iron atom. Occasionally, however, these metals are merely bound within folded polypeptide regions with no additional organic constituents required. Many metal ions are known to participate in enzymatic activity. One or more of the ions of Na, K, Ca, Zn, Cu, Mg, Mn, as well as Co and Mo are often required by enzymes to maintain activity. 

FeMoco can be extracted from the MoFe protein into A(-methylfor-mamide (NMF) solution 32) and has been analyzed extensively using a wide range of spectroscopic techniques both bound to the protein and in solution after extraction from it (33). The extracted FeMoco can be combined with the MoFe protein polypeptides, isolated from strains unable to synthesize the cofactor, to generate active protein. The structure of the FeMoco is now agreed 4, 5, 7) as MoFeTSg homocitrate as in Fig. 4. FeMoco is bound to the a subunit through residues Cys 275, to the terminal tetrahedral iron atom, and His 442 to the molybdenum atom (residue numbers refer to A. vinelandii). A number of other residues in its environment are hydrogen bonded to FeMoco and are essential to its activity (see Section V,E,2). The metal... 

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

The molecular weight is determined by the amino acid sequence of the protein, any post-translational modifications such as glycosylation, and the presence of bound nonpeptide groups such as metals and cofactors. While the exact weight of a polypeptide chain can be calculated from its amino acid sequence, a rule-of-thumb calculation, based on the proportion of different amino acids found in most proteins, gives the molecular weight as the number of residues multiplied by 110. 

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