Lecture 7: Enzymes Flashcards
Enzyme
Biocatalyst which speeds up a reaction without being changed. Has specificity and high catalytic power, accelerating reactions by a factor of 10^6 or more.
Cofactor
Small nonprotein molecule that binds and facilitates enzyme activity
Holoenzyme
Enzyme with its cofactor; active state
Apoenzyme
Enzyme without its cofactor; inactive state
What are examples of inorganic cofactors?
Metal ions (Mg2+, Zn2+, K+, etc.)
What are the types of organic cofactors?
Organic cofactors (aka coenzymes):
1. Co-substrates
2. Prosthetic groups
Co-substrate
Type of organic cofactor which binds loosely and is changed by the reaction (e.g. NAD+/NADH)
Prosthetic group
Organic cofactor which binds tightly and covalently, is not changed by the reaction (e.g. biotin)
Enzyme specificity factors and degrees
Enzymes catalyze 1 reaction or a set of closely related reactions, differing in degree of substrate specificity. Specificity is due to precise substrate interaction with the enzyme’s 3D structure.
How do enzymes function?
Enzymes lower activation energy of a reaction, providing an alternative kinetically advantageous path.
Where on the enzyme does catalysis occur? How?
The active site brings substrates/cofactors/enzyme close together in favorable orientations.
Equation for free energy in nonstandard conditions
ΔG = ΔGo’ + RT ln([products]/[reactants])
How does ΔG describe spontaneity?
ΔG < 0 for spontaneous, exergonic reactions
ΔG = 0 at equilibrium
ΔG > 0 for non-spontaneous endergonic reactions
Induced fit model
Induced fit says that the enzyme and substrate induce small conformational changes in each other to create an ideal catalytic fit. This explains both specificity and transition state stabilization, unlike the older rigid lock and key model.
Catalytic strategies for enzymes
- Binding energy of ES complex
- Covalent catalysis
- General acid-base catalysis
- Catalysis by approximation
- Metal ion catalysis