Activities | Families | Sequences | Fold types | References | Help
B6db activities:
Description GDP-4-dehydro-6-deoxy-D-mannose-3-dehydratase
Alternative names GDP-4-oxo-6-deoxy-D-mannose-3-dehydratase;
ColD (gene name).
Catalyzed reaction GDP-4-dehydro-alpha-D-rhamnose + L-glutamate = GDP-4-dehydro-3,6-dideoxy-alpha-D-mannose + 2-oxoglutarate + + NH(3)
Cofactor Pyridoxal-phosphate
Comments This enzyme, involved in beta-L-colitose biosynthesis, is a unique vitamin-B6-dependent enzyme.
In the first step of catalysis, the bound pyridoxal phosphate (PLP) cafactor is transaminated to the pyridoxamine 5'-phosphate (PMP) form of vitamin B(6), using L-glutamate as the amino group donor.
The PMP cofactor then forms a Schiff base with the sugar substrate and the resulting adduct undergoes a 1,4-dehydration to eliminate the 3-OH group.
Hydrolysis of the product from the enzyme restores the PLP cofactor and results in the release of an unstable enamine intermediate.
This intermediate tautomerizes to form an imine form, which hydrolyzes spontaneously, releasing ammonia and forming the final product.
The overall reaction proposedly includes (i) a half-transamination, in which glutamate reacts with PLP, generating oxoglutarate and PMP. (ii) Condensation of PMP with GDP-4-keto-6-deoxy-D-mannose, to yield the respective ketoimine. (iii) elimination of the 3-hydroxy group from the mannose moyety, with formation of the external aldimine of GDP-6-deoxy-4-amino-delta(3,4)-mannooseen (iv) hydrolysis of the aldimine linkage (regenerating PLP) and hydration of the double bond in the mannoseen ring, releasing ammonia and yielding the final carbohydrate product, GDP-4-keto-3,6-dideoxy-D-mannose
Colitose, the final product of the pathway, is a 3,6-dideoxysugar that has been isolated from the O-antigen of certain Gram-negative bacteria such as Escherichia coli, Yersinia pseudotuberculosis, Salmonella enterica, Vibrio cholerae, and in marine bacteria such as Pseudoalteromonas sp.
This enzyme is related to the E1 protein of CDP-4-dehydro-6-deoxyglucose reductase (EC and shares with the latter the absence of an active site lysine - meaning that PLP is not covalently bound to the enzyme through an internal aldimine linkage.
Organisms -Eubacteria

Family (0)
Links Enzyme (activities)
BRENDA (activities)
KEGG (pathways)
PLPMDB (PLP mutants)
References Articles on
last changed 2018/01/11 12:25

B6db activities