Guanylate cyclase

Guanylate cyclase is an enzyme that converts GTP into cyclic guanosine monophosphate, releasing diphosphate. The official classification is EC 4.6.1.2. The phosphate retained in the product forms a cyclic linkage involving the ribose 3' and 5' positions.^[ref]

The reaction changes nucleotide connectivity rather than making the guanine-containing part from scratch. In an ideal closed assay with no competing reactions, forming $10\mathrm{nmol}$ of cyclic guanosine monophosphate consumes $10\mathrm{nmol}$ of GTP and produces $10\mathrm{nmol}$ of diphosphate. The enzyme's name specifies a chemical transformation; it does not identify a unique concentration or cellular rate.

Synthesis and accumulation also differ. If a phosphodiesterase simultaneously removes the cyclic product, its pool obeys a material balance,with rates expressed in compatible concentration-per-time units. For constructed rates of $2\mathrm{nM}{\mathrm{s}}^{-1}$ for both processes, the product concentration is constant even though both reactions are active.

Consequently, an assay of guanylate cyclase should distinguish product formation from net product accumulation, using defined conditions or an independent estimate of removal. A constant cyclic-nucleotide signal alone does not establish that synthesis has stopped.