Properties As Protein A Molecular Weight

The XYLA genes in the various microorganisms code for a variety of glucose isomerase subunits that can roughly be divided into two groups based on the number of amino acids and derived molecular weights (Table 4).

Group I includes the enzymes from actinomycetes like Actinoplanes, Streptomyces, and the like, and can be regarded as "small" enzymes with an average of 390 ± 4 amino acids. This corresponds to calculated subunit molecular weights ranging from 42,675 to 43,900 daltons. Group II includes the "large" enzymes from Bacillus, Escherichia, Clostridium, Lactobacilli, and others. The subunits of these enzymes have some 442 ± 7 amino acids, resulting in calculated molecular weights ranging from 49,700 to 50,893 daltons.

Almost all glucose isomerases are tetramers, built up of four identical subunits as was concluded from experimental techniques and 3D structure determinations. Therefore, the native molecular weights are in the order of 170,000 and 200,000 daltons for the group I and group II enzymes, respectively. Only the glucose isomerase from Streptomyces olivochromo-genus was found to be a dimer (38).

Experimentally, the molecular weights can be determined under either non-dissociating or dissociating conditions. Subunit molecular weights are obtained under dissociating conditions using techniques such as polyacrylamide gel electrophoresis (PAGE) in the presence of the denaturant sodium dodecyl sulfate. Native molecular weights are obtained under non-dissociating conditions such as gel exclusion chroma-tography, sedimentation coefficients, or density gradient centrifugation. It was found that the experimental molecular weights for various glucose isomerases correspond reasonably well with the calculated values mentioned.

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