A carboxypeptidase is a metalloenyme (its active site contains one or more metal ions essential for the function) that catalyzes the hydrolysis of the peptide bond of the terminal amino acid of a polypeptide chain (where the free carboxyl group occurs). The binding of an L-alanyl-L-tyrosine peptide substrate in the active site of the enzyme is represented in the scheme below: Glu ++ Zn -COO™ H3C OH CH₂ HC NH C C H O Poche apolaire C O +N; H H H -HO H C NH₂ H Arg 145 Tyr 248 NB: This scheme gives a planar representation of the spatial structure of the active site where indicated contacts (hatched lines) are supposed to occur in the 3D structure of the enzyme. 1- Describe the interactions that occur between the ligand and amino acid residues of the active site. 2- What would be the impact on the Km value if we replace L-alanyl-L-tyrosine by the following substrates: L-alanyl-L-phenylalanine; L-alanyl-L-aspartate; L-aspartyl-L- tyrosine.

Many enzymes are switched "on" by attachment of a phosphate group at a specific serine somewhere on the protein (phosphorylation). The basic reaction is: E + ATP2 Ep + ADP Po SERINE PHOSPHO SERINC (Note the "squiggles" before the backone amide and carbonyl indicate the polypeptide chain continues on either side of the serine). For phosphorylation to have this effect, there has to be some equilibrium between inactive and active forms conformations of the enzyme: [Eactive] [Einactive] Einactive 2 Eactive; K* The same basic equilibrium must exist for the phosphorylated protein: [Ep,active] [Ep,inactive] EP,inactive 2 Ep,active; Kp = (a) If phosphorylation increases the measured activity of the enzyme, is K* or K larger? Why? (b) Does the phosphorylation site need to be near the site where the enzyme binds its substrate (e.g. the reactant whose chemistry it catalyzes)? Why or why not?

the structures of chymotrypsin and other serine proteases revealed that the active sites of these enzymes shared a particular sterochemical arrangement of residues crucial to their activity. This came to be known as the catalytic triad, as it consisted of the oponyomous serine (Ser) residue, along with a histidine (His) and an aspartate (Asp) residue. Which of the following would result in the greatest decrease in function of the catalytic triad found in serine proteases? O Mutation to Ser, Cys, Asp O Mutation to Ser, His, Asp O Mutation to Cys, His, Asp O Mutation to Ser, His, Glu

Which of the following statements are descriptions of metal ion catalysis or examples of metal ion catalysis? Choose all correct answers a Zn²+ cofactor may properly orient the substrate in the active site through ionic interactions. a covalent bond forms between enzyme and substrate lowers the energy or stabilizes the transition state or intermediate catalyst retains its original form after reaction occurs catalysts may participate in oxidation-reduction reactions by changes in the oxidation state