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