MCAT Bio: Enzymes

Question	Answer
Enzymes	Enzymes lower the activation E of a rxn making it easier for the substrate to reach the transition state. - Enzymes increase the rate of a rxn (how fast equilib is reached), but do not alter the position of the equilibrium; - Do not affect the overall free E or enthalpy of a rxn; - Are not altered or consumed in the course of a rxn; thus, small amt required. - Enzymes are pH and T sensitive; ranges of optimal activity.
Enzyme Specificity	Enzymes are designed to work only on a specific substrate or group of closely related substrates;
Enzyme prefix	End in -ase
Active site	Active sitelocation on the enzyme where the substrate is held via weak chemical bonds
Lock and Key Theory	One of two theories about enzymes (the other is 'Induced Fit'); says that the spatial structure of an enzyme's active site (lock) is exactly complimentary to the spatial structure of the substrate (key); this theory has been largely discredited.
Induced Fit Theory	This is the more widely accepted theory; it describes the active site as having flexibility of shape, when the appropriate substrate comes in contact with the active site, the conformation of the active stie changes to fit the substrate. - release of substrate from active site is exothermic process
Cofactors	A nonprotein molecule or ion that is required for the proper functioning of an enzyme. Cofactors can be permanently bound to the active site or may bind loosely with the substrate during catalysis.
Coenzymes	Organic molecules serving as cofactors (required for the proper functioning of an enzyme). - Vitamins and ATP function as coenzymes in metabolic reactions.
Prosthetic groups	Covalently or non-covalently (yet tightly) bound non-protein groups; Function as co-enzymes or are integral to protein function. Usually either metal ions or small organic molecules. Frequently function as electron acceptors because none of the amino acid side chains are good electron acceptors.
Optimal Enzyme T and pH in body	T = 37 C pH around 7.4 but varies pH 7.3 termed acidosis
pepsin pH	pH = 2 (acidic in stomach)
pancreatic enzymes pH	pH = 8.5 (alkaline in small intestine)
apoenzyme	'a poor enzyme', non-functioning enzymes without their cofactors.
holoenzyme	'a whole enzyme', an enzyme complete with its apoenzyme and cofactors.
Enzyme Kinetics	- depends on concentration of the enzyme and the substrate - depends on environmental variables (temperatures, pH)
How to determine which is a better enzyme?	A better enzyme has a smaller K-m and larger V-max.
substrate concentration and rate	Velocity of Enzyme reactions depends on [S]. At low [S], any increase will increase velocity of reaction. At high [S], when enzyme saturated, further increase in [S] will have minimal effect
high enzyme to subrate ratio	fast reaction, reach equilibrium quickly.
Vmax	maximum enzyme functioning velocity; enzyme active sites are fully saturated. Decreases w/ non-competitive inhibitors.
Km	Indicates the substrate concentration at which the reaction rate is at half-maximum (Km=[S]=(1/2)Vmax) and is a measure of the substrate's affinity for the enzyme. A small Km indicates high affinity, meaning that the rate will approach Vmax more quickly.
[S] less than Km	changes in [S] will affect rate significantly.
[S] more than Km	increased [S] hardly affects rate as we approach Vmax.
Until optimum T is reached, enzyme-catalyzed rxns tend to double in rate for every increase in ____________.	10 degrees C - Following optimal pH, denatures enzymes and activities decline.
Allosteric Enzymes	enzymes that have multiple binding sites (at least one active site and one regulatory site) and alternate between an active and an inactive form.
Allosteric Site	A specific site on the enzyme where an allostatic inhibitor or activator binds quickly; the binding causes a conformational change in the enzyme that either inhibits the enzyme or stimulates it to catalyze a reaction. - This site is separate from the activate site. - May alter the affinity of the enzyme for its substrate.
Feedback Inhibition	A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway;aka negative feedback.
Reversible Inhibition	occurs when an inhibitor forms weak chemical bonds with the enzyme; - can be competitive or noncompetitive inhibition.
Competitive Inhibition	this occurs when a molecule that resembles substrate bonds to the active site, thereby blocking the substrate; this occurs with weak bonds; the higher the concentration of the inhibitor, the slower the reaction increasing Km (less affininty) ; overcome by increase in [S].
Noncompetitive Inhibition	form of enzyme inhibition where the inhibitor binds to an enzyme at a location other than the active site (allosteric site/ regulatory site); while at this site, the enzyme shape changes, the inhibitor is unable to bind to its substrate, and no product forms. - Km remains unchanged (no change in [S] while help) but Vmax decreases.
Irreversible Inhibition	Inhibitor similar in structure to the substrate binds covalently the active site, alters substrate binding site irreversibly; - Increasing substrate concentration does not reverse inhibition; - Inhibition continues after drug is removed; - Km is unaffected, but Vmax is decreased; - To overcome must make new enzymes.
Zymogen	aka a proenzyme, an inactive form of an enzyme. - When specific peptide bonds on zymogens are cleaved, the zymogen becomes irreversibly activated; - Activation of zymogens may be instigated by other enzymes, or by a change in environment; Ex: pepsinogen (notice the "-ogen" at the end indicating zymogen status) is the zymogen of pepsin and is activated by low pH.

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