Subunit structure and amino acid composition of xylose isomerase from Streptomyces albus

The subunit structure and amino acid composition of xylose isomerase from S. albus have been examined. A native molecular weight of 165,000 determined by sedimentation equilibrium was reduced to 43,000 when the protein was treated with 6 M guanidine hydrochloride. No further reduction in molecular weight was observed when potential disulfide bridges of xylose isomerase were reduced and alkylated, indicating that the protein was devoid of interchain disulfide bonds. NH₂ terminal analysis using [³H]dansyl chloride showed 0.86 residues of methionine per M(r) = 41,500 unit. Analysis of the native protein with an automated protein sequenator revealed the presence of only one degradable polypeptide chain. Fractionation of the soluble tryptic peptides of S [¹⁴C]carboxymethyl xylose isomerase by ion exchange chromatography and one dimensional paper electrophoresis yielded 37 to 43 peptides. When the acid insoluble tryptic peptides were dissolved and analyzed using gel filtration techniques, an additional four peptides were found. A unique radioactive tryptic peptide containing S carboxymethylcysteine was found among the soluble peptides, confirming cysteine as the limiting amino acid residue in the amino acid composition of xylose isomerase. On the basis of its lysine and arginine content, the number of tryptic peptides is consistent with the hypothesis that the native xylose isomerase is a tetramer of four very similar or identical subunits of M(r) = 41,500 associated by noncovalent bonds.