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MCAT - Enzymes

Kaplan MCAT Biology Chapter 2

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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