Properties of Enzymes Flashcards
Catalysts
Increase the rate that equilibrium is reached by lowering the activation energy for a process
Substrate
What an enzyme acts on
Oxidoreductases
Class of enzymes
Catalyze oxidation-reduction reactions
Usually require a coenzyme like NAD+ or NADP+
Coenzyme
Molecules that supply enzymes with additional functional groups needed for reactions
Transferases
Class of enzymes
Catalyze functional group-transfer reactions
May require coenzymes
Hydrolases
Class of enzymes
Catalyze hydrolysis of bonds
Lyases
Class of enzymes
Catalyze lysis of a substrate (elimination reactions)
Synthases
Lyases that catalyze addition reactions
Isomerases
Class of enzymes
Catalyze interconversion between isomers
Ligases
Class of enzymes
Catalyze joining of 2 substrates
Often require energy from ATP to drive reaction
Initial velocity of enzymatic reaction
Vo= k2 [ES]
Steady-state approximation
Period of time (usually at the beginning of a reaction) where the ES complex is formed at the same rate as it decomposes, so that there is no net change in the [ES]
Michaelis-Menten equation
Vo=k2[ES]=(Vmax[S])/(Km+[S])
kcat (catalytic constant)
kcat= (moles of substrate -> product)/[(second)(moles of enzyme)] Vmax= kcat[E total]
Km (Michaelis constant)
Measure of affinity of the enzyme for the substrate
Km=[S]=0.5Vmax
Catalytic proficiency
Effectiveness of an enzyme
Lineweaver-Burke plot
Plot 1/Vo (y-axis) vs. 1/[S] (x-axis)
Y-intercept is 1/Vmax
X-intercept is -1/Km
Inhibitor
Compound that binds to an enzyme and interferes with its activity
Reversible inhibitor
Binds enzyme via noncovalent forces
Irreversible inhibitor
Binds enzyme covalently
“Kills” enzyme
Example: organophosphorus inhibitors (sarin: nerve gas)
Competitive inhibitor
Most common mode of enzyme inhibition Binds free enzyme molecule only Competes with substrate for binding Doesn't affect Vmax Raises Km
Uncompetitive inhibitor
Binds ES complex (not free enzyme)
Usually only occurs with multisubstrate reactions
Lowers both Vmax and Km
Noncompetitive inhibitor
Can bind to enzyme or ES complex
Don’t bind at same site as substrate
Decreases Vmax
Doesn’t affect Km
Affinity labels
Irreversible inhibitors with affinity for an enzyme’s active site
Useful in determining which residues are critical for enzymatic activity or are proximal to substrate binding
Allosteric enzymes
Don’t follow Michaelis-Menten kinetics (sigmoidal plot instead of line)
Enzymatic activities are changed by metabolic activators and inhibitors (molecules that don’t resemble substrates or products)
Exhibit cooperativity
Regulation by covalent modification
Method of controlling enzyme activity through phosphorylation
More permanent that allosteric modification, but can be reversible
6 categories of enzymes
Oxidoreductases Transferases Hydrolases Lyases Isomerases Ligases
