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Aspartate carbamoyltransferase
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<p>Aspartate carbamoyltransferase (also known as aspartate transcarbamoylase or ATCase) <a href="/facts/Catalyze/LPBS7TQC">catalyzes</a> the first step in the <a href="/facts/Pyrimidine_biosynthetic_pathway/tItapHSg">pyrimidine biosynthetic pathway</a> (<a href="/facts/Enzyme_Commission_number/Y5GNLRKX">EC</a> <a href="https://enzyme.expasy.org/EC/2.1.3.2">2.1.3.2</a>). </p><p>In <i><a href="/facts/E._coli/nR4Gvl56">E. coli</a></i>, the enzyme is a multi-<a href="/facts/Protein_subunit/HhuET9UP">subunit</a> <a href="/facts/Protein/Jxmagu3E">protein</a> complex composed of 12 subunits (300 kDa in total). The composition of the subunits is C6R6, forming 2 <a href="/facts/Trimer_(chemistry)/ws3JkjuQ">trimers</a> of catalytic subunits (34 kDa) and 3 <a href="/facts/Protein_dimer/c8LwFJPG">dimers</a> of regulatory subunits (17 kDa). The particular arrangement of catalytic and regulatory subunits in this enzyme affords the complex with strongly <a href="/facts/Allosteric/alMRuyt9">allosteric</a> behaviour with respect to its substrates. The enzyme is an archetypal example of allosteric modulation of fine control of metabolic enzyme reactions. </p><p>ATCase does not follow <a href="/facts/Michaelis%25E2%2580%2593Menten_kinetics/lYFykW3p">Michaelis–Menten kinetics</a>. Instead, it lies between its low-activity, low-affinity "tense" and its high-activity, high-affinity "relaxed" states. The binding of substrate to the catalytic subunits results in an equilibrium shift towards the R state, whereas binding of <a href="/facts/Cytidine_triphosphate/WSfrrdH2">CTP</a> to the regulatory subunits results in an equilibrium shift towards the T state. Binding of ATP to the regulatory subunits results in an equilibrium shift towards the R state. </p>