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MCAT - Enzymes
Kaplan MCAT Biology Chapter 2
| Question | Answer |
|---|---|
| Enzymes | Protein catalysts that accelerate reactions by reducing the initial energy (activation energy) |
| Are enzyme reactions usually reversible? | yes |
| How are enzyme reactions reversed? | The product synthesized by an enzyme can be decomposed by the same enzyme |
| Delta G | overall change in energy of a reaction |
| Enzymes affect ______ not _____ | rate; delta G |
| Substrate | Molecule upon which an enzyme acts |
| Enzyme-substrate complex | Substrate bound to the active site of an enzyme |
| 2 models that describe formation of an enzyme-substrate complex | The lock and Key theory; The induced fit hypothesis |
| The lock and key theory | The spatial structure of an enzymes active site is exactly complementary to the spatial structure of its substrate |
| The induced fit hypothesis | Active site of enzyme has flexibility and will be induced to change in shape to fit around the substrate; the most widely accepted theory |
| Cofactors | nonprotein molecules that are required by many enzymes to become catalytically active |
| How are cofactors used by enzymes | cofactors either aid in binding the substrate to the enzyme or stabilize the enzyme in an active conformation |
| Apoenzyme | An enzyme that is lacking its needed cofactor |
| Holoenzyme | An enzyme containing its cofactor |
| How do cofactors bond to enzymes | by weak noncovalent interactions or by strong covalent bonds |
| Prosthetic groups | tightly bound cofactors |
| 2 types of cofactors | Metal cations; coenzymes |
| How are coenzymes obtained | Through the diet as vitamin derivatives - most are not made by the body |
| What effects the rate of enzyme catalyzed reactions? | Concentrations of enzyme and substrate; temperature; pH |
| If the concentration of the substrate is low, what will the effect be on the reaction rate | slow |
| When will increases in substrate concentration not increase reaction rate | A maximal velocity (Vmax); b/c all active sites of enzymes are occupied |
| Michaelis-Menton Model | describes the relationship between the rates of enzyme-substrate complex formation, dissociation, product formation |
| Enzyme-substrate complex formed at rate | k1 |
| Enzyme-substrate complex can dissociate into E and S at rate | k2 |
| Enzyme-substrate complex can form product at rate | k3 |
| Michaelis constant | km = (k2+k3)/k1 |
| When the reaction rate is half of Vmax... | half of the enzyme active sites are filled and Km = [S] |
| When [S] is less than Km... | changes in substrate concentration greatly affect the reaction rate |
| Reaction rate of enzyme catalyzed reactions tend to double for every __ increase until optimal temperature is reached | 10 degree Celsius |
| Most enzymes in the body optimize at | 37 degrees Celsius |
| Maximal activity of many human enzymes is around pH _________ | 7.4 +/- 0.05 |
| Plueral fluid standard pH | 7.6 |
| Pepsin works in pH | 2 (stomach) |
| Pancreatic enzymes maximal activity at pH | 8.5 |
| How is enzymatic activity mostly regulated | allosteric effects and inhibition |
| Allosteric enzymes have | at least one active site (catalytic site) and one separate regulatory site |
| Regulators bind to allosteric enzymes and stabilize what? | either the active state or the inactive state |
| What are the 2 types of regulators | allosteric inhibitors and allosteric activators |
| What can increase the affinity of an enzyme for its substrate | the binding of a regulator or a substrate binding to an active site which stimulates other active sites on the enzyme |
| What are the 3 types of inhibition? | Feedback inhibition; competitive inhibition; noncompetitive inhibition |
| Feedback inhibition | end product becomes an allosteric inhibitor (negative feedback) |
| competitive inhibitors | compete with substrate and bid to active site of enzyme; reversible with increased conc of substrate |
| noncompetitive inhibitors | substances that form strong covalent bonds with enzyme either at, near or remote from the active site; irreversible |
| How can noncompetitive inhibitors be overcome? | increasing the concentration of the enzyme |
| Zymogen | enzyme secreted in an inactive form that is cleaved under certain physiological conditions into the active form |
| Examples of zymogens | Pepsinogen cleaved into pepsin; trypsinogen cleaved into trypsin; chymotrypsinogen cleaved into chymotrypsin |
Created by:
susanivey
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