MODULE 6: Chapter 7.3 Flashcards
What is the primary function of enzymes in chemical reactions?
Enzymes create ideal conditions for chemical reactions, providing an optimized microenvironment for catalysis.
How do enzymes lower activation energy?
By stabilizing transition states or providing an alternative pathway to product formation.
What are the two core concepts in enzymology reinforced by the examples of chymotrypsin, enolase, and HMG-CoA reductase?
- Substrates bind to enzyme active sites through weak noncovalent interactions
- Enzymes use conventional catalytic reaction mechanisms that follow basic principles of organic chemistry.
What family of enzymes does chymotrypsin belong to?
Chymotrypsin is a member of the serine proteases family.
What is the structure of chymotrypsin?
Chymotrypsin consists of three polypeptide chains linked by disulfide bonds.
What are the key amino acids in the catalytic triad of chymotrypsin?
- Ser195
- His57
- Asp102
What is the role of Ser195 in chymotrypsin’s mechanism?
Ser195 forms an enzyme–substrate covalent intermediate.
What is the proposed chymotrypsin mechanism divided into?
Two phases: formation of a covalent acyl-enzyme intermediate and regeneration of the enzyme.
In step 1 of chymotrypsin’s reaction, what facilitates the binding of the substrate?
A productive interaction between the aromatic side chain of the polypeptide substrate and the specificity pocket.
What happens during step 2 of chymotrypsin’s reaction mechanism?
A proton is transferred from Ser195 to His57, allowing nucleophilic attack by Ser195.
What is the oxyanion hole in the chymotrypsin mechanism?
A region in the active site where the negatively charged oxygen (oxyanion) forms hydrogen bonds with the backbone NH groups.
What is the role of His57 in chymotrypsin’s mechanism?
His57 functions as an acid catalyst by donating a proton to facilitate cleavage of the peptide bond.
What is the significance of the substrate specificity pocket in serine proteases?
It imparts a degree of substrate specificity based on the size and charge of amino acids adjacent to the scissile bond.
What type of reaction does enolase catalyze?
The dehydration of 2-phosphoglycerate to form phosphoenolpyruvate.
What is unique about enolase’s active site?
Each active site contains two divalent metal ions, which are required for the reaction.
What catalytic strategies does enolase use?
- General acid–base catalysis
- Metal-ion catalysis
In the enolase mechanism, what role does Lys345 play?
Lys345 functions as a general base, removing a proton at the C-2 position of the substrate.
What role do Mg2⁺ ions play in the enolase reaction?
- Bind and orient the substrate
- Stabilize the negative charges on the intermediate
What is HMG-CoA reductase involved in?
The biosynthetic pathway for cholesterol and other isoprenoids.
What is the significance of HMG-CoA reductase in medicine?
It is an important pharmaceutical target for the treatment of hypercholesterolemia.
What is HMG-CoA reductase?
An enzyme in the biosynthetic pathway for cholesterol and other isoprenoids.
Why is HMG-CoA reductase an important pharmaceutical target?
It treats hypercholesterolemia by reducing serum cholesterol.
What is the direct effect of HMG-CoA reductase inhibition?
It reduces the amount of cholesterol produced in liver cells.
What indirect benefit arises from reduced intracellular cholesterol?
Increased production of cell surface cholesterol receptors.
How do cholesterol receptors affect serum cholesterol levels?
They bind cholesterol-containing particles in the serum, decreasing total cholesterol.
What is the structure of HMG-CoA reductase?
A membrane-bound tetrameric enzyme with four active sites.
What substrates and cofactors does HMG-CoA reductase bind?
HMG-CoA and NADPH (or NADH for some organisms).
What is the product of HMG-CoA reduction?
Mevalonate and CoA.
How many NADPH molecules are used in the HMG-CoA reductase catalytic mechanism?
Two NADPH molecules.
What is the first step in the HMG-CoA reductase mechanism?
Hydride ion from NADPH attacks the carbonyl carbon of HMG-CoA.
What role does the glutamate residue play in the first step of the reaction?
Acts as a general acid to protonate the oxyanion.
What occurs during the second step of the HMG-CoA reductase mechanism?
NADP⁺ is exchanged for NADPH.
What happens in the third step of the mechanism?
The glutamate acts as a general base, deprotonating the hydroxyl group to produce an aldehyde.
What are the final products of the HMG-CoA reductase mechanism?
Reduced mevalonate species and NADP⁺.
What is a catalytic triad?
A set of three amino acids required for catalysis in serine proteases.
What is a serine protease?
An enzyme that cleaves the peptide backbone of proteins using serine as the nucleophile.
True or False: The chymotrypsin mechanism involves the formation of a covalent intermediate.
True.
What is the role of the oxyanion hole in enzyme reactions?
It accepts a negatively charged oxygen ion and forms hydrogen bonds with nearby groups.
Fill in the blank: HMG-CoA reductase catalyzes the four-electron reduction of HMG-CoA into _______.
mevalonate.
What are the four steps in the overall reaction mechanism of HMG-CoA reductase?
- Hydride transfer to HMG-CoA
- NADP⁺ exchange
- Hydroxyl deprotonation to aldehyde
- Hydride transfer to aldehyde.
What are two key residues involved in the HMG-CoA reductase mechanism?
Glutamate and lysine.
