Enzymes Flashcards
Enzymes and their substrates
- most enzymes end in -ase
- molecule upon which an enzyme acts is known as substrate
- when meet and greet is called the enzyme-substrate complex
- active site = location within the enzyme where the substrate is held during chem rxn
Thermodynamics and Kinematics
-thermodynamics relate relative energy states of a reaction in terms of products and reactants
-endothermic rxn = requires energy input; endo means in
~~exothermic rxn diagram shows initial energy higher and final energy state lower, activation energy required for a catalyzed reaction is lower that than of an uncatalyzed reaction while the change in free energy (G) and change of H remains the same
-exothermic rxn = energy is given off; exo means out; change of G is negative
-enzymes do NOT alter the overall free energy change for a reaction nor the equilibrium of a reaction
— will affect the RATE (kinetics) at which the reaction occurs -> affect how quickly a reaction gets to equilibrium (the hump in the curve where the free energy changes direction) but not the actual equilibrium state itself
- enzymes are catalysts that LOWER the activation energy and make it EASIER for the substrate to reach its equilibrium/transition state, are unchanged by the reaction which means that far fewer enzymes are required for the overall amount of substrate
Lock and Key Theory
- enzymes active site (lock) is already in appropriate confirmation for substrate (key) to bind
- no alteration of structure is needed to bind substrate and enzyme
Induced Fit Theory
- induced fit model starts with a substrate and an enzyme active site that dont quite fit together
- once substrate present and ready, molecules find that induced form or transition state is mor confortable for both of them
- shape of the active site becomes truly complementary only after the substrate binds to the enzyme —> ONLY the enzyme changes shape to accommodate the substrate
- enzyme will go back to its prior state once the substrate leaves (like a foam ball)
Key features of enzymes
- lower activation energy
- increase rate of rxn
- Do not alter the eq constant
- Are not changed or consumed in rxn (appears in both products and reactants)
- Are ph and temp sensitive, with optimal activity at specific ph and temp ranges
- Do not affect the overall free energy (G) of the rxn
- Are specific for a particular rxn or class of rxns
Cofactors
- enzymes require non protein molecules called cofactors
- enzymes w/o cofactors called apoenzymes
- enzymes w/ cofactors called holoenzymes
- are attached variety of ways: tightly bound factors known as prosthetic groups
- 2 important types of cofactors: small metal ions and small organic groups called coenzymes (vitamins)
Enzyme kinetics - effects of concentration
The concentration of substrate and enzyme affects how quickly a rxn will occur
- as substrate concentration increases, less enzymes become available and soon all enzymes are occupied and the rate of rxn cannot go any faster AKA SATURATION; at this rate the enzyme is working at max velocity
- at lower substrate concentration small changes in the concentration drastically change the reaction rate
- when at 1/2 velocity, half of the enzymes active sites are full
- a low concentration of substrate that results in 1/2 the velocity reflects that the enzymes has a high affinity for the substrate (IT TAKES LESS SUBSTRATE TO GET TO HALF THE MAX VELOCITY)
- but when a high concentration of substrate is needed to get to 1/2the velocity, then the enzymes has a lower affinity for the substrate (IT TAKES MORE SUBSTRATE TO GET TO HALF THE MAX VELOCITY)
Effects of Temperature on enzyme kinetics
- enzyme cat rxns double in rate for every 10 degrees Celsius increase in temp of the enzymes until the optimum temp reached
- for human body this is 37 degrees Celsius
- after this optimum temp is reached rate of activity of the enzyme falls off sharply as the enzyme will denature at higher temps
Effects of pH on enzyme kinetics
- Most enzymes depend on a ph to function properly
- in humans the ph is 7.4 -> so a ph of 7.3 is acidosis
- pepsin has max activity at ph of 2 in gi tract
Allosteric Effects
- enzymes with this have multiple binding sites
- active site is present as well as one other that can regulate the availability of active site (aka ALLOSTERIC SITE)
- allosteric enzymes alternate between active and inactive form, inactive is incapable of rxn
- binding to allosteric site may consist of either allosteric activators or allosteric inhibitors –> either may cause a conformational shift in the protein OR the affinity of the enzyme for its substrate
- activators results in shift makes active site more available for binding to substrate
- -inhibitor will make it less available
Feedback inhibition of enzymes
The product may bind to an enzyme that acted earlier in its bio synthetic pathway which results in making that enzyme unavailable to use by other substrates
Once have enough of product, why make more?
Common in hormone pathways
reversible inhibition
- competitive inhibition: occupancy of the active site; substrates cant access binding sites; can be overcome by adding more substrate so ratio is higher
- noncompetitive inhibition: inhibitor binding to allosteric site instead of the active site - induces a change in enzyme conformation; cannot be overcome by adding more substrate because once enzymes conformation is altered no amount of extra substrate will be able to form the omplex
- irreversible inhibition: active site is made permanently unavailable or te enzyme is permanently altered
inactive enzymes
can be dangerous
to avoid danger, these enzymes are secreted as inactive zymogens -> have a catalytic (active) domain and a regulatory domain
-regulatory domain must either be removed or altered to expose the active site
