Biosynthesis of KDN (2-Keto-3-deoxy-d-glycero -d-galacto -nononic acid)

Abstract

Although the deaminoneuraminic acid or KDN glycotope (2-keto-3-deoxy-D-glycero-D-galacto-nononic acid)is expressed in glycoconjugates that range in evolutionary diversity from bacteria to man, there is little information as to how this novel sugar is synthesized. Accordingly, biosynthetic studies were initiated in trout testis, an organ rich in KDN, to determine how this sialic acid is formed. These studies have shown that the pathway consists of the following three sequential reactions: 1) Man ؉ ATP 3 Man-6-P ؉ ADP; 2) Man-6-P ؉ PEP 3 KDN-9-P ؉ Pi; 3) KDN-9-P 3 KDN ؉ Pi. Reaction 1, catalyzed by a hexokinase, is the 6-O-phosphorylation of mannose to form D-mannose 6-phosphate (Man-6-P). Reaction 2, catalyzed by KDN-9-phosphate (KDN-9-P) synthetase, condenses Man-6-P and phosphoenolpyruvate (PEP) to form KDN-9-P. Reaction 3, catalyzed by a phosphatase, is the dephosphorylation of KDN-9-P to yield free KDN. It is not known if a kinase specific for Man (Reaction 1) and a phosphatase specific for KDN-9-P (Reaction 3) may exist in tissues actively synthesizing KDN. In this study, the KDN-9-P synthetase, an enzyme that has not been previously described, was identified as at least one key enzyme that is specific for the KDN biosynthetic pathway. This enzyme was purified 50-fold from rainbow trout testis and characterized. The molecular weight of the enzyme was estimated to be about 80,000, and activity was maximum at neutral pH in the presence of Mn 2؉ . N-Acetylneuraminic acid 9-phosphate (Neu5Ac-9-P) synthetase, which catalyzes the condensation of N-acetyl-D-mannosamine 6-phosphate and phosphoenol-pyruvate to produce Neu5Ac-9-P, was co-purified with the KDN-9-P synthetase. Substrate competition experiments revealed, however, that syntheses of KDN-9-P and Neu5Ac-9-P were catalyzed by two separate synthetase activities. The significance of these studies takes on added importance with the recent discovery that the level of free KDN is elevated in human fetal cord but not matched adult red blood cells and (25), , and KDN residue-cleaving sialidases (29 -31), have been identified and partially characterized, and their similarity and dissimilarity to the metabolism of Neu5Acyl residues have been described. In marked contrast, nothing is known concerning how the KDN monosaccharide is synthesized. Two pathways are possible for synthesis of the KDN monomer. First, it could be formed directly from Neu5Ac by deacylation and deamination. Alternatively, it could arise by de novo synthesis via enzymes different from those required for synthesis of Neu5Ac. However, no evidence for the direct conversion of Neu5Ac to KDN has been obtained in any tissue homogenates examined to date. Thus, de novo synthesis by a separate pathway appears to be plausible by analogy with synthesis of other ulosonic acids. For example, 3-deoxy-D-arabino-heptulosonic acid 7-phosphate, a biosynthetic precursor of aromatic amino acids (32), 3-deoxy-D-manno-octulosonic acid, a constituent of bacterial lipopolysaccharides (33, 34) and Neu5Ac (35) are synthesi...

Cloning and Expression of the HumanN-Acetylneuraminic Acid Phosphate Synthase Gene with 2-Keto-3-deoxy-d-glycero- d-galacto-nononic Acid Biosynthetic Ability

“…To our knowledge, this is the first report of the cloning of a eukaryotic sialic acid phosphate synthase gene. Despite the importance of sialic acids in many biological recognition phenomena, sialic acid phosphate synthase genes have not been cloned because the enzymes they encode are unstable and difficult to purify (12,19). Even the E. coli sialic acid synthase enzyme, whose sequence is known, has low specific activity and is unstable (9).…”

“…In Vitro Activity-In vitro activity assays were based on the procedure of Angata et al (19). Lysates were prepared from A35 and AcSAS infected and uninfected Sf-9 cells cultured in T-75 flasks.…”

“…6A), consistent with phosphorylated sugars. In previous studies, phosphorylated KDN was detected as DMB-KDN after AP treatment and DMB derivatization (19). Similarly, the peak eluting at 5.5 min was exchanged for one that eluted at the same time as authentic Neu5Ac following AP treatment (Fig.…”

“…In order to determine which substrates are used by the human enzyme, in vitro assays were performed using lysates of infected Sf-9 cells and protein purified from the prokaryotic expression system. Lysates or purified protein plus PEP and MnCl 2 (19) were incubated with Man, Man-6-P, ManNAc, or ManNAc-6-P followed by DMB labeling and HPLC analysis.…”

“…Less is known about KDN synthesis than Neu5Ac synthesis, although recent enzymatic studies in fish indicate that mannose (Man) and PEP are the substrates (17). KDN is produced through a multienzyme pathway involving phosphate intermediates similar to those of the mammalian Neu5Ac synthesis pathway (19).…”