Lecture 9

enzyme lowers the activation energy required to achieve the transition state

• transition state is a state where it is energetically favorable to form the products

delta G — free energy of reaction

• difference in energy between reactants and products

• does not change whether there is an enzyme there or not

enzymes do not cause reactions to occur that would not eventually occur anyway; only speech up existing reactions

• many enzymes increase reaction rates by several million times

• some enzymes increase reaction rates by several trillion times

• ex: 2H2O2 → 2H2O + O2

  • platinum (inorganic catalyst) decreases Ea by 1/3rd

  • catalase (enzyme) decrease Ea by almost 90%

enzymes bind substrates with extremely high specificity into their active sites (usually just a few amino acids)

• enzymes will most likely cause some conformational change in the substrate molecules(s), but they themselves usually change shape upon binding substrate

  • called induced fit

• if the substrate doesn’t “fit”, the enzyme will reject it

• the active site is only a small part of the enzyme

how does substrate binding to active site decrease Ea?

• acting as a template for substrate orientation (induced fit)

  • there’s only one way substrates can fit

• stressing the substrate(s) and stabilizing the transition state

• providing a favorable microenvironment (pH, etc.)

• participating directly in the catalytic reaction

  • mediator

  • accepts a proton, etc. just to give it to the second substrate

  • not every enzyme does all this

if an enzyme accepts a group from a substrate, it must in turn donate that group to help form a product

enzymes are (ultimately) unchanged by the reactions they catalyze)

• they might change in the course of the reaction, but have to come out unchanged

enzymes do not change the equilibrium of reactions, they only make it easier (and therefore faster) to reach that equilibrium

• if Ea is lowered for forward reaction, Ea is lowered the same amount for the reverse reaction

enzymes decrease Ea by the same amount in both directions

because most enzymes are proteins, it follows that conditions that affect protein stability also affect enzyme activity

• enzymes have temperature and pH optimums

• most human enzymes tend to be near body temperature (37°C) and neutral pH (7.0)

Enzyme inhibition

• E + S → [ES] → E+P

• E + I → [EI] -/->

• can either be reversible or irreversible

• reversible inhibition can be competitive or noncompetitive

irreversible inhibitors

• permanently bind to or modify active site; changing concentration of natural substrate or inhibitor has no effect

  • nerve agents like sarin gas are irreversible inhibitors of acetylcholinesterase, which catalyzes termination of nerve impulses

• tend to be molecules not typically encountered by that particular cell

• irreversible inhibition is a demonstration of the important point that enzymes must ultimately be unchanged if they are to be used over and over

in competitive inhibition, the inhibitor molecule physically resembles the natural substrate, and occupies active site

• enzyme can’t use inhibitor as substrate — no products are forms

• can be “flooded out” by increasing concentration of natural substrate

• decreasing concentration of inhibitor also reduces probability of inhibitor finding active sit

• ex: in bacteria, DHPS catalyzes the conversion of p-aminobenzoic acid into folic acid

  • sulfa drugs like sulfanilamide are inhibitors of DHPS; bacteria die

in noncompetitive inhibition, the inhibitor molecule binds to the enzyme in a place other than the active site

• if change in enzyme completely prevents substrate binding, increasing substrate concentration has no effect

• reversible because inhibitor can become unbound

V_max and K_M

• V_max (products/second) — the maximum velocity products can be created

• K_M — the time it takes to reach ½ (V_max)

• competitive inhibition

  • V_max stays the same

  • K_M takes longer

• noncompetitive inhibition

  • V_max is lowered

  • K_M stays the same