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Guanylate kinase is an enzyme that transfers a phosphoryl group from ATP to guanosine monophosphate, producing ADP and guanosine diphosphate:Its official classification is EC 2.7.4.8. The enzyme can also use the corresponding deoxy-nucleotide acceptor and deoxy-ATP donor.^[ref]

A constructed mixture containing $3\mu \mathrm{mol}$ ATP and $2\mu \mathrm{mol}$ guanosine monophosphate, initially without products, permits at most $2\mu \mathrm{mol}$ of each product through the forward reaction. At least $1\mu \mathrm{mol}$ 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.^[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.

GUK1 is the symbol for a gene encoding guanylate kinase. In the human annotation, GUK1 is the protein-coding gene named guanylate kinase 1, identified by NCBI GeneID2987. The gene is a DNA locus; the encoded enzyme catalyses chemistry after its expression.^[ref]

The enzyme transfers a phosphoryl group between ATP and guanosine monophosphate, giving ADP and guanosine diphosphate. A compact biochemical notation isPhosphorus bookkeeping gives $3+1=2+2$. The equation describes the protein's activity, not a chemical reaction performed directly by the DNA.

A 1992 study cloned and characterized the yeast gene for guanylate kinase, connecting an inherited sequence with a measured enzyme activity.^[ref] The organism must be stated when identifying a gene: the same functional symbol in yeast and humans does not mean that both organisms carry one identical DNA sequence. Likewise, a change in transcript abundance is not by itself a measurement of catalytic rate.