Question

MCAT Biology Tutorial: Introduction to Enzyme Catalysis

Answer 1

\({\bf{Basic~Terminology:}}\)
- catabolism: breaking down molecules; produce ATP; negative Gibbs free energy
- anabolism: making new molecules; use ATP; positive Gibbs free energy
- Gibbs Free Energy: energy available to do work
- activation energy: energy required in order for the reaction to proceed
- enzyme: catalysts that lower activation energy; are not altered or used up
> bind with the reactant (substrate) to form enzyme-substrate complex
- cofactor: nonprotein molecule/ion needed to activate an enzyme
> apoenzyme: enzyme without cofactor
> holoenzyme: enzyme + cofactor
> prosthetic group: cofactor that is permanently bound
> coenzyme: cofactor that is reversibly bound, also called second substrates



\({\bf{Methods~of~Catalysis:}}\)
1. add strain to the substrate bonds so they can break more readily
2. changes the physical orientation of the substrate so they can react more readily
3. changes the enviornment, like pH or temperature
4. forms temporary bonds that can help the substrate react more readily

\({\bf{Enzymatic~Models:}}\)
- active site: enzyme has a fixed structure designed to fit the substrate exactly
- induced fit: enzyme changes its structure upon bonding to facilitate binding
- specificity: the range of substrates to which an enzyme can bind

Answer 2

\({\bf{Enzyme~Kinetics:}}\)

general reaction pattern: enzyme + substrate --> enzyme-substrate complex --> enzyme + product

enzymatic activity increases w/ substrate conc. up to a certain point, then levels off (Vmax) depending on active site availability

Michaelis-Menten:
V = reaction rate = Vmax*S/{Km+S} where Km



Michaelis-Menten constant Km: measure of enzymatic activity; lower Km means higher binding affinity for substrate and higher enzymatic reaction rate
also the substrate conc. where V = (1/2)Vmax

- every enzyme has a temp. pH range, going too far outside the range will inhibit enzyme activity or even cause the enzyme to denature

Answer 3

\({\bf{Enzymatic~Regulation:}}\) activators increase enzymatic activity, inhibitors decrease it; can be reversible or irreversible, with most naturally occurring enzymatic regulators being inhibitory + reversible

- competitive inhibitors: bind to the same sites as the substrate
- uncompetitive inhibitors: bind to allosteric sites and lower Vmax + Km, occurs more often at high S concentrations
- noncompetitive inhibitors: inhibitor binds to the allosteric sites regardless of substrate concentration; does not change Km
- mixed inhibitors: lowers Vmax and increases Km

- feedback regulation: the process by which the products of an enzyme reaction either increase or decrease further activity of that enzyme; inhibition much more common
- cooperativity: process by which substrate regulates the binding of additional substrate, can be positive or negative
- allosteric enzymes: have many binding sites besides the active site; do not follow michaelis menten rules, have a sigmoidal reaction diagram
- covalent modification: phosphorylation/dephosphorylation, cleavage, etc.
- zymogen: a precursor enzyme that is inactive

Answer 4

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT