The enzyme (the large folded blob with a cleft) binds substrate
molecules (orange) in its active site, converts them to product
(blue) using a shuttling cofactor (small green token, e.g. NAD(P)H),
and releases both — then repeats. Enzymes are proteins, so their
activity is bell-shaped in both temperature and pH: too far from the
optimum and the protein starts to unfold ("denature"), permanently
losing activity.
v = kcat·[E]·[S] / (Km + [S]) (Michaelis–Menten)
activity(T,pH) = activity_max · exp(−((T−Topt)/w)²) · exp(−((pH−pHopt)/w2)²)
- Enzyme variant — directed-evolution "engineered" variants often shift the optimum, widen substrate tolerance, or raise selectivity versus the wild-type.
- Temperature — speeds catalysis near the optimum but denatures the protein above it, permanently dropping activity.
- pH — active-site residues need specific protonation states; far from the optimum, binding and catalysis both weaken.
- Substrate concentration — raises rate following saturation kinetics until every active site is occupied (rate plateaus at high [S]).
Because enzymes are chiral catalysts built from chiral amino acids,
they routinely deliver very high enantiomeric excess — a major reason
biocatalysis is prized in pharmaceutical synthesis over classical
chemical catalysts.