= Hyaluronan synthase
<Hyaluronan synthase> builds <hyaluronic acid> by alternately transferring <glucuronate> and <N-acetylglucosamine> residues from activated sugar donors. The donors are <UDP-glucuronic acid> and <UDP-N-acetylglucosamine>; alternating transfers create the polymer's two-sugar repeat rather than a random mixture of the two sugars.<Hyaluronan synthase in IUBMB Enzyme Nomenclature>[EC 2.4.1.212, reactions 1 and 2]{ref}
For an idealized chain extension of $n$ complete repeats, the enzyme incorporates $n$ residues from each donor. Supplying only one donor cannot indefinitely produce the normal alternating chain. This makes donor omission a useful control in an <enzyme assay>.
In 1996, <P. L. DeAngelis> measured synthesis by cell-free membrane preparations from Pasteurella multocida. Activity required the sugar donors and suitable divalent ions. At the reported concentrations, $1\,\mathrm{mM}$ manganese produced approximately twice the incorporation observed with $10\,\mathrm{mM}$ magnesium. An analogous streptococcal enzyme favored magnesium instead.<Enzymological characterization of the Pasteurella multocida hyaluronic acid synthase by P. L. DeAngelis>[Abstract, ion concentrations and comparative incorporation]{ref}
The comparison demonstrates why an enzyme name does not completely specify experimental conditions. Enzymes making the same polysaccharide can differ in ion dependence and kinetics, so an activity measured for one organism cannot automatically be assigned to every hyaluronan-producing system.
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