Guanylate kinase
= Guanylate kinase
<Guanylate kinase> is an <enzyme> that transfers a phosphoryl group from <ATP> to <guanosine monophosphate>, producing <ADP> and <guanosine diphosphate>:
$$
\mathrm{ATP+GMP\rightleftharpoons ADP+GDP}.
$$
Its official classification is EC 2.7.4.8. The enzyme can also use the corresponding deoxy-nucleotide acceptor and deoxy-ATP donor.<Enzyme Nomenclature by the IUBMB>[EC 2.7.4.8, Reaction, Systematic name and Comments]{ref}
A constructed mixture containing $3\,\mathrm{\mu mol}$ <ATP> and $2\,\mathrm{\mu mol}$ <guanosine monophosphate>, initially without products, permits at most $2\,\mathrm{\mu mol}$ of each product through the forward reaction. At least $1\,\mathrm{\mu mol}$ ATP remains if no other reactions consume it. Per turnover, the phosphate inventory is conserved: $3+1=2+2$.
This ceiling concerns stoichiometry. Reversible <chemical equilibrium>, substrate concentrations and assay conditions determine the actual conversion; the inventory alone does not specify an enzyme rate.
In 1992, <M. Konrad> isolated and characterized the yeast <GUK1> gene encoding guanylate kinase, connecting a chemically defined activity with a molecularly identifiable catalyst.<Cloning and expression of the essential gene for guanylate kinase from yeast by M. Konrad>[Abstract, reversible phosphoryl transfer and gene identification]{ref} The kinase reaction should also be distinguished from <guanylate cyclase>, which converts GTP to a cyclic nucleotide rather than transferring ATP's terminal phosphoryl group to GMP.