Enzymes

• Tells the ratio of products to reactants after a reaction has reached equilibrium
• Does NOT tell how long it took to reach equilibrium

Gproducts – G reactants

• Determines the reaction rate, refers to activation energy.(v)
• Slow reaction - large activation energy

Small molecule required for enzyme activity

• An organic cofactor that is loosely bound to the enzyme
• Examples: NADP(H), NAD(H), ATP

• Organic OR inorganic compound tightly bound to the enzyme
• Examples: Heme, iron-sulfur clusters, some metal ions

• Entropy reduction
• General Acid-Base Catalysis
• Covalent Catalysis

Substrates bound in the enzyme's active site are oriented to promote a reaction

Protons are accepted or donated by amino acids in the active site to promote a reaction

A temporary covalent bond forms between the enzyme and the substrate to increase the reactivity of the substrate

pH at which an enzyme will catalyze a reaction at its optimal rate (ACID-BASE catalysis only)

• Oxidoreductases
• Transferases
• Hydrolases
• Lyases
• Isomerases
• Ligase/synthetases

When [LDH1] >[LHD2] in the blood due to M.I.

• 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

• AST
• ALT

• ALP
• GGT
• If both are elevated, it's a bile tract issue

• Alteplase, tPA, PLAT
• Used to bust clots after MI

• Elspar
• Catalyzes conversion of Asn to Asp. Asn is an essential amino acid for ALL. Elspar starves the ALL cells

Catalyzes digestion of lactose in the gut of lactose intolerant folks

• Product inhibition
• Allosteric regulation
• Covalent modification
• Protein-protein regulation
• Zymogen cleavage
• Enzyme synthesis and degredation

• 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

• 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

• Inhibition/activation occurs when a molecule is covalently attached to the enzyme, causing a conformational change
• Example: phosphorylation, protein kinases, protein phosphatases

• 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.

• Irreversible activation of a zymogen (inactive enzyme precursor) by proteolytic cleavage
• Causes conformational changes that reveal the active site
• Example: Chymotrypsin

• If more enzyme is produced, reaction rate increases
• Takes time, hours to days
• Example: insulin

Describes relationship between v and [S]

Small change in [S] means significant change in v

Change in [S] has little effect on v

Maximum rate of a reaction, depending on [S]

• When 1/2 the substrate is bound to the enzyme
• When v = 1/2 vMax
• (always expressed as [S]

• Reversible competetive
• Reversible uncompetetive
• Reversible mixed (and pure noncompetetive)
• Irreversible

• 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)

• Atorvastatin
• Rivastigmine (Exelon)

• Will only bind to the enzyme while the substrate is also bound. Will not bind free enzyme. (rare)
• Decreases Km
• Lowers Vmax

• Mycophenelate (IMP dehydrogenase used in purine synthesis)
• Immune cells depent in purine synthesis pathway , so mycophenelate is used to supress immune system after transpalnts

• 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

• Caspofungin
• Foscarnet (Foscavir)

• 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

• Sarin Gas
• 5-Fu

• 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.