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Equilibrium constant (Keq)
- Tells the ratio of products to reactants after a reaction has reached equilibrium
- Does NOT tell how long it took to reach equilibrium
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DGrxn
Gproducts – G reactants
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ΔGact
- Determines the reaction rate, refers to activation energy.(v)
- Slow reaction - large activation energy
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Enzyme cofactors
Small molecule required for enzyme activity
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Coenzyme
- An organic cofactor that is loosely bound to the enzyme
- Examples: NADP(H), NAD(H), ATP
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Prostethic group
- Organic OR inorganic compound tightly bound to the enzyme
- Examples: Heme, iron-sulfur clusters, some metal ions
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Structural mechanisms for enzyme catalysis (3)
- Entropy reduction
- General Acid-Base Catalysis
- Covalent Catalysis
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Entropy Reduction
Substrates bound in the enzyme's active site are oriented to promote a reaction
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General acid-base catalysis
Protons are accepted or donated by amino acids in the active site to promote a reaction
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Covalent catalysis
A temporary covalent bond forms between the enzyme and the substrate to increase the reactivity of the substrate
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pH Optima
pH at which an enzyme will catalyze a reaction at its optimal rate (ACID-BASE catalysis only)
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Classes of enzyme reactions (6)
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligase/synthetases
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Flipped LDH
When [LDH1] >[LHD2] in the blood due to M.I.
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Diagnosis of heart attack using CK
- CK-MB represents ~50% of total CK in heart muscle
- If [CK-MB] is elevated in blood, supports diagnosis of heart attack due to likelihood that damaged heart muscle is the source of extra CK-MB
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Enzymes used to diagnose liver disease
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Enzymes used to diagnose bile-tract problems
- ALP
- GGT
- If both are elevated, it's a bile tract issue
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Streptokinase
- Alteplase, tPA, PLAT
- Used to bust clots after MI
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Asparaginase
- Elspar
- Catalyzes conversion of Asn to Asp. Asn is an essential amino acid for ALL. Elspar starves the ALL cells
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Lactase
Catalyzes digestion of lactose in the gut of lactose intolerant folks
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Mechanisms of enzyme regulation (6)
- Product inhibition
- Allosteric regulation
- Covalent modification
- Protein-protein regulation
- Zymogen cleavage
- Enzyme synthesis and degredation
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Product inhibition
- Reversible inhibition of an enzyme when the product competes with the substrate for binding at the active site
- Example:The first step in the metabolism of glucose, catalyzed by a hexokinase enzyme, can be inhibited by the product, glucose-6-phosphate
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Allosteric regulation
- Reversible inhibition/activation that occurs when a molecule causes a conformational change by binding somewhere other than the active site
- If increased - positive
- If inhibited - negative
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Covalent modification
- Inhibition/activation occurs when a molecule is covalently attached to the enzyme, causing a conformational change
- Example: phosphorylation, protein kinases, protein phosphatases
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Protein-protein interactions
- reversible activation/inhibition of an enzyme that occurs when the enzyme is bound by another protein to form a complex
- Example: calmodulin is a protein that binds and activates to several cellular enzymes, including protein kinases.
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Zymogen cleavage
- Irreversible activation of a zymogen (inactive enzyme precursor) by proteolytic cleavage
- Causes conformational changes that reveal the active site
- Example: Chymotrypsin
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Enxyme synthesis
- If more enzyme is produced, reaction rate increases
- Takes time, hours to days
- Example: insulin
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Michaelis-Menton Equation
Describes relationship between v and [S]
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First order kinetics
Small change in [S] means significant change in v
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Second order kinetics
Change in [S] has little effect on v
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Vmax
Maximum rate of a reaction, depending on [S]
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Km meanings (2)
- When 1/2 the substrate is bound to the enzyme
- When v = 1/2 vMax
- (always expressed as [S]
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Enzyme Inhibition Types (4)
- Reversible competetive
- Reversible uncompetetive
- Reversible mixed (and pure noncompetetive)
- Irreversible
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Competetive inhibition
- Inhibitor competes wiht the substrate for the active site of the enzyme
- Increases Km
- Does not affect Vmax (eventually enough substrate can be added to reach Vmax)
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Reversible Competetive Inhibitor Examples (2)
- Atorvastatin
- Rivastigmine (Exelon)
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Uncompetetive inhibition
- Will only bind to the enzyme while the substrate is also bound. Will not bind free enzyme. (rare)
- Decreases Km
- Lowers Vmax
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Uncompetetive Inhibitor Example
- Mycophenelate (IMP dehydrogenase used in purine synthesis)
- Immune cells depent in purine synthesis pathway , so mycophenelate is used to supress immune system after transpalnts
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Mixed & Noncompetetive Inhibition
- Can bind with free enzyme or enzyme/substrate complex
- More [E] affinity - increases Km
- More [ES] affinity - decreases Km
- Both lower Vmax
- If affinities are equal, noncpompetetive, Km unchanged but Vmax lowered
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Mixed/Noncompetetive Inhibition examples (2)
- Caspofungin
- Foscarnet (Foscavir)
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Irreversible Inhibition
- Enzyme is poisoned by inhibitor, rendering it useless
- If [I] < [E], Km won't be affected but Vmax will be lowered
- If [I] > [E], reaction won't go
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Irreversable Inhibitors Examples (2)
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Kinetics of allosterically regulated enzymes
- A substrate vs. rate curve for an allosterically-regulated enzyme is sigmoidal rather than hyperbolic and does not fit the Michaelis-Menton equation.
- For these enzymes, “K0.5” is used instead of “Km” to indicate the substrate concentration at which half maximal velocity/half-saturation is achieved.
- An allosteric inhibitor shifts the curve to the right and increases K0.5.
- An allosteric activator shifts the curve to the left and decreases K0.5.
- An allosteric activator or inhibitor may increase or decrease Vmax, or leave it unchanged.
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