Test 2 - Enzymes Flashcards
Suffix that refers to enzymes?
-ase
What type of reaction is catalyzed by Oxidoreductases?
Transfer of electrons (Hydride ions or H atoms)
What type of reaction is catalyzed by Transferases?
Group transfer reactions
What type of reaction is catalyzed by Hydrolases?
Hydrolysis reactions (transfer of functional groups to water).
What type of reaction is catalyzed by Lyases?
Cleavage of C-C, C-O, C-N, or other bonds by elimination, leaving double bonds or rings, or addition of groups to double bonds.
What type of reaction is catalyzed by Isomerases?
Transfer of groups within molecules to yield isomeric forms.
What type of reaction is catalyzed by Ligases?
Formation of C-C, C-S, C-O, and C-N bonds by condensation reactions coupled to cleavage of ATP or similar cofactor.
What enzyme catalyzes the transfer of a phosphoryl group from ATP to glucose?
D-hexose 6-phosphotransferase
Definition of Apoenzyme:
Inactive enzyme without its factor
Definition of Holoenzyme?
Complete active enzyme with the factor.
Define Prosthetic Groups
Tightly and stably incorporated into a protein’s structure by covalent or noncovalent forces.
Define Cofactors, examples
Bind in a transient, dissociable manner either to the enzyme or to a substrate such as ATP.
Example: Metal Ions
Universal Electron Carriers
FAD and FMN
FAD states
FAD
FADH+
FADH2 (Fully reduced)
Universal Mobile electron carriers?
NAD+ and NADP+
What causes pellagra?
Niacin deficiancy
Why enzymes over inorganic catalysts?
Work at physiological pH and temp.
High reaction rate and capaciy for regulation
High specificty for substrates at the active site.
Two models of enzyme-substrate interactions that have been proposed?
Lock and key
Induced fit
Describe the Lock and Key model:
High specificity due to complementary nature of binding site and the ligand.
Complementary in: Size, Shape, Charge, Hydrophobic/Hydrophilic character (SSCHH).
Describe the Induced Fit Model
Conformational changes may occur upon ligand binding.
This adaptation is called the induced fit.
Both the ligand and the protein can change their conformations.
Mechanisms of Enzyme Catalysis
Catalysis by Proxmity
Acid-base Catalysis
Catalysis by Strain
Covalent Catalysis
Example of Covalent Catalysis compounds.
Serine protease, chymotrypsin.
What is Enzyme Kinetics?
Kinetics is the sutdy of the rate at which compounds react.
What affects the rate of enzymatic reaction?
Enzyme, substrate, effectors, temperature.
Explain Catalysis by Proximity
Effective molarity (concentration) and orientation of substrate molecules in the active site of enzymes will enhance the rate of reactions.
Substrate channeling by multi-enzyme complexes.
Explain Acid-Base Catalysis
Ionizable functional groups of amino acid side chains and prosthetic groups contribute to catalysis by acting as acids or bases.
Catalysis by Strain
Enzymes bind their substrates in an unfavorable conformation to weaken the bond that will undergo cleavage.
Example: Stickase model
Explain Covalent Catalysis
Formation of a covalent bond between the enzyme and one or more substrates to create a more reactive enzyme.
Example: Group transfer reactions
Catalysis by Fructose-2,6-Bisphosphatase role and mechanism?
Catalyzes the hydrolytic release of the phosphate on carbon 2 of fructose-2,6-bisphosphate.
Positively charged Lys and Arg residues stabilize the negative charge of the substrate.
Enzymes use the binding energy of substrates to organize the reactants into what?
Stable transition state (TA).
How is the Stable Transition State (TA) formed?
Positioning of acid-base groups to transfer protons to or from the developing transition state intermediate.
Imposing steric strain on substrates so that their geometry approaches to that of the transition state.
Do enzymes affect Keq?
No they do not.
Equilibrium constant (Keq) of an enzymatic reaction?
Keq = [P][E] / [S][E]
Keq = [P] / [S]
Effect of Enzymes on equilibrium?
They cannot change the equilibrium constant, but they can speed the onset to equilibrium.
Zero order kinetics definition
Rate of reaction is independent of substrate concentration [S], but proportional to enzyme concentration [E].
First order kinetics definition
Rate is proportional to concentration of substrate.
S -> P
v = k [S]
k is a rate constant.
Second order reaction definition
Rate proportional to concentration of 2 substrates.
S1 + S2 -> P
v = k [S1] [S2}
Pseudo First order reaction
It is a second order reaction, but [S2] is very high and [S1] is very low causing the rate to appear to depend on concentration of only one substrate.
v = k [S1]
Rate is proportional to [S1} only, since S1 is the rate limiting substrate.
What are the assumptions of the Michaelis - Menten Equation?
Assumes the formation of an enzyme - substrate complex (ES)
Enzymatic reactions show saturation with their substrates [S].
ES complex is in rapid equilibrium with free enzyme [E]
Breakdown of ES to form products is assumed to be slower than the formation of ES and breakdown of ES to E and S.
Michaelis - Menten Equation
v = Vmax [S] / Km + [S]
What is Km in Enzyme Kinetics
Km is the substrate concentration that corresponds to half the Vmax and it can be roughly estimated from initial velocity (v) vs [S] plot.
Define First order kinetics
V0 to Km
Collision of substrate with enzyme is rate limiting due to low [S].
