Fig. 3 The biosynthetic pathway of yeast mannoprotein
(Glu—glucose;G6-P—glucose-6-phosphat;PGM1—Phosphoglucomutase;G1-P—glucose-1-phosphat;UGP1—uridine diphosphate glucose pyrophosphatase;UDP-Glu—uridine diphosphate glucose;PGI1—phosphoglucose isomerase;F6-P—fructose-6-phosphate;M6-P—mannose-6-phosphate;PMI40—mannose phosphate isomerase;SEC53—phosphomannomutase;M1-P—mannose-1-phosphate;PSA1—guanosine diphosphate mannose pyrophosphatase;GDP-Man—guanosine diphosphate mannose;UDP-GlcNAc—uridine diphosphate-N-acetylglucosamine;Dol-P—dolichol phosphate;Alg—glycosyltransferase;OST—oligosaccharides transferase;G-Ⅰ/G-Ⅱ—α- glucosidase;ERMan1—Endoplasmic reticulum mannosidase I;MGAT—mannosyl-glycoprotein-N-acetylglucosaminyltransferase;DPM—dolichol phosphate mannose synthase;PMT—mannosyltransferase)

酵母甘露糖蛋白生物合成途径是一个复杂的过程，涉及大约1200个基因［59］，与糖酵解、鸟苷二磷酸（GDP）-甘露糖合成、内质网（ER）和高尔基体有关，如图3所示。甘露糖蛋白与支链葡聚糖结合存在［60］。由GDP-甘露糖合成的甘露糖蛋白有两种糖基化类型（N-糖基化和O-甘露糖基化）［61， 62］。GDP-甘露糖由糖酵解中间体果糖-6-磷酸合成［63］，该过程涉及几个酶促步骤，首先果糖-6-磷酸通过甘露糖-6-磷酸异构酶（PMI40）转化为甘露糖-6-磷酸。磷酸甘露糖变位酶（SEC53）催化甘露糖-6-磷酸转化为甘露糖-1-磷酸。最后通过GDP-甘露糖焦磷酸化酶（PSA1）将甘露糖-1-磷酸合成为GDP-甘露糖［63］。GDP-甘露糖进入ER并作为糖供体［64］。