Hyaluronic acid (HA), a linear polysaccharide composed ofalternating glucuronic acid
andN-acetylglucosamine residues, is an essential molecule ofhigher vertebrates. The fowl cholera pathogen
Pasteurella multocida Carter Type A also produces HA in theform of an extracellular capsule in orderto evade host defenses. HA synthase activity could be obtainedfrom cell-free membrane preparations of
P.
multocida. The enzyme utilized UDP-sugarprecursors of HA in the presence of Mg
2+ orMn
2+ atneutral pH. Mn
2+ at 1 mM stimulated ~2-fold moreincorporation than Mg
2+ at 10 mM. On theotherh
and, the analogous enzyme from group A
Streptococcus, HasA,is stimulated more by Mg
2+ thanMn
2+.The apparent Michaelis constants, K
M, of the
P.
multocida HA synthase forUDP-
N-acetylglucosamine
and UDP-glucuronic acid were estimated to be ~75
and ~20
M,respectively, in the presence of Mg
2+,which suggests that the substrates are bound with 2-3-fold higheraffinity than by the HasA enzyme.The rate enhancement observed with Mn
2+ isapparently not due to better binding of the sugarnucleotideprecursors complexed to Mn ion because the
KMvalue, a measure of substrate affinity, increases by25-50% in comparison to Mg
2+. In summary, the HAsynthase from
P.
multocida, aGram-negativebacterium, has kinetic optima distinct from those of HasA, the analogfrom the Gram-positive group A
Streptococcus.