Studies on the Structure of Yeast Mannan

The mode of action of the extracellular a-mannosidase from Arfhrobacfer GJM-1 has been studied with several yeast mannans as substrates. In vifro, the enzyme cleaves most of the cu-(1 + 2)and a-(1 + 3)-linked side chains from Saccharomyces cerevisine mannan, producing free mannose and leaving a resistant polymer containing mainly ar-(1 --f 6)-linkages. This material cannot be further degraded by the enzyme. The extent to which mannans from several Candida species are digested by the enzyme is inversely proportional to their phosphorus content. Kloeckera brevis mannan, although highly phosphorylated, is extensively digested by the enzyme. The reason for this discrepancy is unclear, but it may be related to the positions of the phosphate residues in the respective mannans. Growth of Arfhrobacfer GJM-1 on S. cerevisiae mannan results in the accumulation of an undigested residue in the cultural filtrate. This residue remains in the supematant after ammonium sulfate precipitation of the a-mannosidase. It has been purified by gel filtration and precipitation with Fehling’s solution. The structure of this residue has been studied by acetolysis, proton magnetic resonance spectrometry, and methylation. These studies indicate that the product is exclusively a-(1 ---) 6)-linked and represents the mannan backbone. The data suggest that the cu-mannosidase from Arfhrobacfer GJM-1 is an exoglycosidase which acts by splitting off single mannose residues from the nonreducing ends of the side chains of the mannan molecule. Sodium borotritide reduction of the mannan residue from the cultural filtrate (RCF) of the Arfhrobacfer results in the incorporation of tritium into the polymer. Hydrolysis of the labeled RCF yields radioactive mannitol. A comparison of the amount of label incorporated into RCF with that incorporated into mannotetraose under the same conditions allows a calculation of the molecular weight of RCF, the value being 7100. The methylation end group data suggest a value of 6500, while a figure of 8100 was obtained by high speed sedimentation equilibrium.