Ch. 6: Enzymes & Regulation Flashcards
6 major classes of enzymes? (IUBMB System)
1) oxidoreductases
2) transferases
3) hydrolases
4) lyases
5) isomerases
6) ligases
What are oxidoreductases?
Oxidation-reduction reactions
What are transferases?
Transfer of C, N, or P containing groups
What are hydrolases?
Cleavage of bonds via addition of water
What are lyases?
Cleavage of C-C, C-S, & certain C-N bonds
What are isomerases?
Racemization of optical/geometric isomers
What are ligases?
Formation of bonds bet. C and O, S,N coupled to hydrolysis of high-energy phosphates
Reactions occurring in mitochondria?
1) TCA cycle
2) Fatty acid oxidation
3) Decarboxylation of pyruvate
Reactions occurring in cytosol?
1) glycolysis
2) hexose monophosphate shunt (HMP pathway)
3) fatty acid synthesis
Reactions occurring in nucleus?
DNA & RNA synthesis
Reactions occurring in lysosomes?
Degradation of complex macromolecules
2 factors governing rate of enzyme-catalyzed reactions?
1) temperature
2) pH
Curve shape of enzymes following Michaelis-Menten kinetics?
Hyperbolic
Curve shape of allosteric enzymes?
Sigmoid
Equation for Michaelis-Menten kinetics?
Vo = Vmax * [S] / Km + [S]
What does a high Km mean?
Enzyme has ⬇️ affinity for substrate
What does a low Km mean?
Enzyme has ⬆️ affinity for substrate
At 1/2 Vmax, what is Km?
Km = [S]
What is a first order reaction?
[S] «_space;Km
[S]⬆️, v ⬆️
What is a zero order reaction?
[S]»_space; Km
v constant & equal to Vmax, independent of [S]
What is turnover number?
substrate converted to product/enzyme/unit time
Equation for Lineweaver-Burke plot?
X-axis?
Y-axis?
Slope?
1/v = Km / Vmax * [S] + 1 / Vmax
X-axis = -1 / Km Y-axis = 1 / Vmax Slope = Km / Vmax
What is a reversible inhibitor?
Types?
Binds enzyme via non-covalent bonds
1) competitive
2) noncompetitive
3) uncompetitive
What is competitive inhibition?
Structural analog of substrate binds to active site on enzyme
Km ⬆️, Vmax stays same
4 circumstances for competitive inhibition?
1) analogs compete w/S to bind @ active site on E
2) in 2S E catalyzed rxn, ⬆️ [2nd S] competes w/first for binding
3) in freely reversible rxn, P functions as inhibitor
4) in rxn w/metal ion cofactors, similar metal ions compete for same binding site on E
Describe Michaels-Menton & Line weaver-Burke plot graphs for competitive inhibition?
Km ⬆️, Vmax stays same
Curve slopes up at lower X value (MM)
Straight line slopes higher, through same Y-intercept, different X-intercept (LB)
What is noncompetitive inhibition?
Inhibitor not structurally similar to S
Inhibitor binds to different site from S binding site on E (inactivates via binding to free E or ES complex)
Vmax ⬇️, Km stays same
Describe Michaels-Menton & Line weaver-Burke plot graphs for noncompetitive inhibition?
Vmax ⬇️, Km stays same
Curve flattens at lower Y value (MM)
Straight line slopes higher, through same X-intercept, different Y-intercept (LB)
What is uncompetitive inhibition?
Inhibitor binds to ES complex ➡️ forms ESI complex
Km and Vmax change
Describe Michaels-Menton & Line weaver-Burke plot graphs for uncompetitive inhibition?
Vmax & Km change
Curve slopes up at lower X value, flattens at lower Y value (MM)
Parallel lines, smaller X intercept, larger Y intercept (LB)
What is a irreversible inhibitor?
Reacts act/near active site of enzyme, covalently modifies active site OR binds so tightly that dilution of EI complex doesn’t provide recovery of E activity
2 examples of irreversible inhibition?
AchE: inhibited by organophosphorus compounds (sarin gas)
Cytochrome oxidase: inhibited by cyanide (CN-)
How is cytochrome oxidase reactivated?
Nitrites (NaNO2), makes MetHb ➡️ higher affinity for CN- than cytochrome oxidase
2 types of regulation?
Regulatory enzymes: catalyze rate-limiting step or committed step
Feedback inhibition by end product
What are allosteric enzymes?
Regulated by non-covalent interactions of compounds at sites besides the catalytic site
Rate determining enzymes
What are the ligands that regulate at allosteric sites?
Effectors, modulators, or modifiers
Type of graph for allosteric enzymes?
Sigmoidal (does not follow MM kinetics)
What is cooperativity?
How binding of ligand at allosteric site affects binding of same or another ligand to enzyme
Can be positive (activatory, ⬆️ affinity for S) or negative (inhibitory, ⬇️ affinity for S)
What are homotropic interactions in allosteric enzymes?
Same ligand positively influences cooperativity bet diff modulator sites
3 features of homotropic enzyme graph?
[S] = + modulator
K0.5 = 1/2Vmax (not Km)
Vmax at high [S]
What are heterotropic interactions in allosteric enzymes?
Effect of one ligand on binding of diff ligand
Features of heterotropic graph: same Vmax, changed K0.5
+ modulator: ⬇️ K0.5
- modulator: ⬆️ K0.5
Features of heterotropic graph: changed Vmax, same K0.5
+ modulator: ⬆️ Vmax
- modulator: ⬇️ Vmax
2 examples of allosteric enzymes?
Asp transcarbamoylase (ATCase): CTP (- modulator), feedback inhibition; ATP (+ modulator)
Hemoglobin: homotropic = O2 binding, heterotropic = H+, CO2, 2,3-BPG
