Lecture 10

Question 1

What is covalent catalysis?

Answer 1

covalent catalysis involves the formation of a transient covalent bond between the substrate and a residue in the enzyme active site (or sometimes with a cofactor at the enzyme active site). In such catalytic mechanisms, an additional covalent intermediate is added to the reaction

Question 2

What allows covalent bonds to form?

Answer 2

Usually, the covalent bonds are able to be formed as the result of an attack by a nucleophilic group on the enzyme, with an electrophilic group on the substrate. Covalent catalysis and acid-base catalysis often occur together.

Question 3

What are the three stages in covalent catalysis

Answer 3

Nucleophilic reaction between the enzyme and the substrate
• Electrophilic withdrawal of electrons from the substrate
• Elimination reaction (this is the reverse of the first step

Question 4

What is acid base catalysis?

Answer 4

Acid-base catalysis involves the partial proton transfer from an acid to a base or vice versa. By donating or accepting electrons, the free energy of the reactions transition state in lowered.

Question 5

What things are involved in acid-base catalysis?

Answer 5

Ionisable amino acids are commonly involved in acid-base catalysis. Histidine is also particularly susceptible to acid-base catalysis as it can both accept and donate protons, depending on the pH of the environment.

Question 6

What are serine proteases?

Answer 6

Serine proteases are enzymes that cleave peptide bonds in proteins, in which serine serves as the nucleophilic amino acid at the active site

Question 7

What are the structural and functional similarities between serine proteases such as chymotrypsin, trypsin, and elastase

Answer 7

DNA level where their genes are very similar; the location and order of e.g. catalytic residues is the same.

Question 8

What is chymotrypsin?

Answer 8

Chymotrypsin is a digestive enzyme which hydrolyses specific peptide bonds in dietary proteins.The enzyme cleaves the peptide bond next to aromatic, hydrophobic residues.

Question 9

Structure of chymotrypsin active site?

Answer 9

The active site of chymotrypsin contains a ‘catalytic triad’; three amino acids in the active site work together to catalyse the reaction. In chymotrypsin, the catalytic triad comprises a serine, histidine, and aspartate

Question 10

What is the role of serine in the chymotrypsin triad

Answer 10

acts as the nucleophile that will break the peptide bond

Question 11

What is the role of Histidine in the chymotrypsin triad

Answer 11

acts as the electrophile, accepting a proton from serine and thus making serine a
stronger nucleophile

Question 12

What is the role of Aspartate in the chymotrypsin triad

Answer 12

forms a strong hydrogen bond with the histidine. Its main function in doing this is to
hold histidine in the orientation and state required to accept the proton from serine.

Question 13

Steps in the chymotrypsin mechanism?

Answer 13

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