Zymogen (source code)
= Zymogen
A <zymogen> is a precursor of an <enzyme> that becomes substantially more active after a chemical activation step, commonly limited <proteolysis>. Regulating activation permits a <biological cell> or <organism> to produce a catalytic precursor before the full activity is needed. Low precursor activity need not be exactly zero in every assay.
<Prothrombin> provides a concrete example. Specific cleavage sites connect its precursor architecture to generation of <thrombin>. A 2013 structural study examined an engineered precursor lacking one domain and carrying an inactive catalytic-site substitution, supplemented by solution measurements. It related exposure of activation sites to conformational organization.<Crystal Structure of Prothrombin Reveals Conformational Flexibility and Mechanism of Activation by Nicola Pozzi and others>[Abstract and Fig.1: precursor construct, cleavage sites and structural interpretation]{ref} A <zymogen> can therefore be a folded, regulated <protein> rather than an unfinished chain awaiting <protein folding>.
Consider a synthetic assay in which 20 percent of a $1\,\mu\mathrm M$ precursor pool becomes active <enzyme>, one molecule for one molecule. Active <enzyme> is then $0.2\,\mu\mathrm M$. At saturating <substrate> and an illustrative <turnover number> of $10\,\mathrm{s^{-1}}$, the predicted initial <reaction rate> is $2\,\mu\mathrm{M\,s^{-1}}$. Measuring total precursor-plus-product abundance would not reveal that active fraction.
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