Purification and Properties of Mycobacterial GDP–Mannose Pyrophosphorylase

The enzyme that catalyzes the formation of GDP-d-mannose from GTP and α-d-mannose-1-P was purified about 2300-fold to near homogeneity from the soluble fraction ofMycobacterium smegmatis.At the final stage of purification, a major protein band of 37 kDa was observed and this band was specifically labeled, and in a concentration-dependent manner, by the photoaffinity probe 8-N3-GDP[32P]-d-mannose. The purified enzyme was stable for several months when kept in the frozen state. The 37-kDa band was subjected to protein sequencing and one peptide sequence of 25 amino acids showed over 80% identity to GDP–mannose pyrophosphorylases of pig liver andSaccharomyces cerevesiae.In contrast to some other bacterial GDP–mannose pyrophosphorylases, the mycobacterial enzyme was not multifunctional and did not have phosphomannose isomerase or phosphoglucose isomerase activity. Also, in contrast to the pig liver enzyme which uses mannose-1-P or glucose-1-P plus GTP to synthesize either GDP–mannose or GDP–glucose, the mycobacterial enzyme was specific for mannose-1-P as the sugar phosphate substrate. The enzyme was also relatively specific for GTP as the nucleoside triphosphate substrate. ITP was about 18% as effective as GTP, but ATP, CTP, and UTP were inactive. The activity of the enzyme was inhibited by GDP–glucose and glucose-1-P, although neither was a substrate for this enzyme. The pH optimum for the enzyme was 8.0, and Mg2+was the best cation with optimum activity at about 5 mM. This enzyme is important for producing the activated form of mannose for formation of cell wall lipoarabinomannan and various mannose-containing glycolipids and polysaccharides.