Enzyme Rate Enhancement How do enzymes work to catalyze reactions?

Enzyme Classes by Reaction Type

Oxidation-Reduction Reaction: Oxidoreductase

Functional Group Transfer: Transferase

Cutting with Water: Hydrolysis

Group Elimination with Double Bond Formation: Lyase

Compound Conversion to an Isomeric Form: Isomerase Triose phosphate isomerase

Bond Formation with ATP Hydrolysis: Ligase DNA Ligase -- nucleotide base coupling with loss of PPi

Chymotrypsin Catalyzes Peptide Hydrolysis Enzyme specificity in terms of: Site of hydrolysis and Substrate reactivity How can a reaction be pushed in the forward direction?

Reactions that are Spontaneous in the Forward and Reverse Direction How do enzymes affect the energy diagram of a reaction?

Enzymes Decrease the Activation Energy How can enzymes lower the transition state? Is external energy required?

Mechanisms of Enzyme Catalysis Acid-base, covalent, metal ion, orientation and proximity

Ketone-Enol Conversion What mechanism of enzyme catalysis is operative here?

Amino Acid Residues that Function in Acid-Base Catalysis

Conventions for Depicting Reaction Mechanisms Curved arrow indicates electron rearrangement during a reaction

Covalent Catalysis A formal positive charge favors electron flow to the nitrogen group

Covalent Catalysis

Amino Acids that can Participate in Covalent Catalysis

Metal Ion Catalysis

Components that Facilitate Enzyme Catalysis What is the cofactor in ATP hydrolysis? What is the co-substrate in an oxidation-reduction reaction?

Enzymes Decrease the Activation Energy

Mechanisms of Enzyme Catalysis Acid-Base Catalysis Covalent Catalysis Metal Ion Catalysis Orientation/Proximity Effects Preferential Transition- State Binding

Proximity and Orientation Effects Facilitate Catalysis

Chymotrypsin Specificity Cleaves peptides on the C-terminus side of hydrophobic residues (e.g. Phe, Tyr and Try)

Active Site Mapping via Irreversible Inhibitors Diisopropylphosphofluoridate (DIPF) inhibits chymotrypsin by modifying 1 of 28 serine residues

Active Site Mapping via Irreversible Inhibitors

Covalent Catalysis for Chymotrypsin: a Two Step Process Enzyme acylation with leaving group departure Enzyme deacylation What is the leaving group with Chymotrypsin?

Chymotrypsin Catalysis Proceeds via a Two-Step Mechanism Chromogenic substrate for kinetic studies Why is this compound not an ideal substrate mimic?

Chymotrypsin Catalytic Triad Catalytic triad serves as the site of catalysis Aspartate and histidine contribute serine’s basicity Serine serves as a nucleophile in covalent catalysis What type of catalysis occurs?

Mechanism of Peptide Hydrolysis in Chymotrypsin Substrate binding via nucleophilic attack

Mechanism of Peptide Hydrolysis in Chymotrypsin Polypeptide original C-side serves as leaving group

Mechanism of Peptide Hydrolysis in Chymotrypsin Water attacks original N-side of polypeptide

Mechanism of Peptide Hydrolysis in Chymotrypsin Polypeptide original N-side serves as leaving group and enzyme is regenerated

Chymotrypsin Hydrolysis

Tetrahedral-Intermediate Stabilization in Chymotrypsin H-bonds ideally positioned in the oxyanion hole stabilize the sp3 transition state

Chymotrypsin Specificity Pocket Large structural pocket lined with hydrophobic amino acids favors bulky hydrophobic residues

Serine Proteases Differ in Little Except Their Specificity Pockets Chymotrypsin Trypsin Elastase

Substrate Specificity Observed with each Proteolytic Enzyme Papain cleave peptides non-selectively Trypsin cleaves carboxyl side of bulky + charged R- groups Chymotrypsin cleaves carboxyl side of bulky aromatic R-groups Thrombin

Divergent Evolution Percent Sequence Identity among Three Serine Proteases Bovine trypsin 100% Bovine chymotrypsinogen 53% Porcine elatase 48% Common ancestor with retention of overall structure and catalytic mechanism

Convergent Evolution Bovine versus bacterial serine protease No sequence or structural similarity but the same catalytic triad and oxyanion hole in the active site

Enzyme-Substrate Binding Critical for Catalysis Lock and Key Model Enzyme Active Site 3-D cleft or crevice Small part of enzyme Unique micro-environment Substrate binding by weak forces Induced Fit Model

Substrate-Induced Enzyme Conformational Change

Inhibition by Transition State Analogs Pyrrolidine the natural substrate binds 160 less tightly than pyrrole a transition state analog. What is the favored enzyme binding geometry?

Rate of Enzyme Catalysis Explain why enzyme activity increases with temperature and then precipitously drops off

RNAas A Digestive Enzyme Cleaving Mechanism Why does ribonuclease catalyzes the hydrolysis of RNA but not DNA

Conversion of Adenosine to Inosine What does the much greater binding affinity of 1,6-dihydropurine ribonucleoside than the substrate indicate about the enzyme mechanism?

Chapter 6 Problems: 1, 3, 7, 9, 11, 15, 20, 23, 25 and 37