Enzyme catalytic cycle: serine protease template

Catalytic cycle of a serine protease (chymotrypsin): Ser-195 adds to the amide C=O (tetrahedral intermediate), the amine leaves (acyl-enzyme), water adds and the acid leaves, regenerating the enzyme; Michaelis–Menten kinetics in the key points; key step: Ser–His acylation with curly arrows.

What the template shows

• Chymotrypsin hydrolyses an amide through an acyl-enzyme: E + S ⇌ ES → E–acyl → E + P
• Overall: R–CO–NH–R′ + H₂O → R–CO₂H + H₂N–R′ (enzyme E; E–CH₂OH = Ser-195)
• Free enzyme (Ser-195 OH)
• triad Ser-195, His-57, Asp-102
• Tetrahedral intermediate 1
• O⁻ in the oxyanion hole
• Acyl-enzyme (ester)
• Tetrahedral intermediate 2
• His-57 shuttles each proton
• Michaelis–Menten: v = kcat[E]₀[S] / (Kₘ + [S]); at high [S] the rate levels off at Vmax = kcat[E]₀.
• The enzyme is regenerated unchanged each turn: kcat is the number of turnovers per second.
• Two half-reactions: acylation (amine P₁ leaves) then deacylation (water in, acid P₂ leaves).
• The oxyanion hole stabilises the tetrahedral intermediates and the transition states that lead to them.
• Key step: acylation by the catalytic triad
• His-57 (held by Asp-102) takes the serine proton as the Ser-195 oxygen adds to the amide carbonyl; the C=O π electrons move onto oxygen, stabilised by N–H bonds of the oxyanion hole.
• tetrahedral intermediate
• E = enzyme; E–CH₂OH is the Ser-195 side chain · every part of this figure is editable

Drawn with 53 atoms, 8 arrows, 67 shapes – every one editable in MolDraw.

Sources

• Hedstrom, Chem. Rev. 2002, 102, 4501 Review
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook