Download B. Enzymes have four features

Enzymes
Enzymes Help With Energy Hills
A. Enzymes are catalytic molecules that alter the rate of a
chemical reaction without being used in them.
1. there names usually reflect their function and end
in “ase”
B. Enzymes have four features:
1. Enzymes speed up reactions.
2. Enzymes can be reused.
3. Enzymes, at least some of them, can recognize both
reactants and products in order to catalyze a reaction
in both directions.
4. Enzymes are very selective about the substrates to
which they will bind and thereby bring about change
C. Enzymes increase the rate of a reaction by lowering the
activation energy (the amount of energy needed to get a
reaction going).
D. they don’t “force” reactions, they just speed up reactions
that already occur
E. enzymes are specific
1. most catalyze only a few closely related chemical
reactions; many only 1
How Do Enzymes Lower Energy Hills?
A. The Active Site
1. Enzymes increase the rate of reactions by creating
a microenvironment that is energetically more
favorable for the reaction.
2. Each enzyme molecule has an active site where the
substrate binds to the enzyme during a reaction
B. Transition at the Top of the Hill
1. Activation energy brings the reactive chemical
groups into alignment so that chemical bonds can be
broken, created, and rearranged.
2. The substrate is brought to its transition state, the
point when a reaction can occur.
3. enzyme + substrate enzyme-substrate complex
(intermediate) enzyme + product
C. How Enzymes Work
1. Binding energy helps bring about the transition
state by four mechanisms:
a. Helping substrates get together;
b. Orienting substrates in positions favoring
reaction;
c. Shutting out water;
d. Inducing changes in enzyme shape (inducedfit model).
-induce strain in the substrate
D. About Those Cofactors
-Some enzymes have 2 parts; a protein called the apoenzyme and
an additional cofactor
1. Cofactors are nonprotein groups that bind to many enzymes
and make them more reactive. Without the cofactors, the enzyme
doesn’t function properly
a. coenzymes – organic, nonpolypeptide compounds that
seve as cofactors
-they are not permanently bonded to the enzyme
-they also act as carrier molecules that transfer
electrons or part of a substrate between molecules
b. prosthetic groups – permanently bonded to the enzyme
2. Inorganic metal ions such as Fe++ also serve as cofactors when
assisting membrane cytochrome proteins in their electron
transfers in chloroplasts and mitochondria.
E. Why Are Enzymes So Big?
1. A large molecule affords structural stability.
2. The extensive folding of the polypeptide chains puts
amino acids and functional groups in locations and
orientations that favor interaction with water and
substrate.
Enzymes Don't Work in a Vacuum
A. How Is Enzyme Activity Controlled?
1. Some controls regulate the number of enzyme
molecules available by speeding up/slowing down their
synthesis.
a. remember, genes direct the synthesis of each
type of enzyme and genes can be switched on
and off; therefore controlling the amount of
enzyme present in the cell
2. we refer to the series of chemical reactions in which
the product of one reaction is the substrate of the next
as a metabolic pathway
3. inhibitors – substances that bind to an enzyme and slow
down the reaction
a. some inhibitors occur naturally, some are artificial
b. reversible vs irreversible
-reversible inhibitors can leave the enzyme and
the enzyme will return to normal function
-irreversible inhibitors never allow the enzyme
to return to normal function
c. competitive vs noncompetitive
-competitive inhibitors bond to the enzymes in their
active site so the substrate cannot; they are in
direct competition with the substrate for the active site
-noncompetitive inhibitors bond to the enzyme
somewhere other than the active site; but, their
bonding causes a conformational change in the
enzyme which changes the active site so that it no
longer accepts the substrate
4. feedback inhibition – type of enzyme regulation in which
the formation of a product inhibits an earlier reaction
5. Allosteric enzymes have (in addition to active sites)
regulatory sites where control substances can bind to alter
enzyme activity; if this control substance is the end product in
the enzyme’s metabolic pathway, feedback inhibition can
occur.
a. allosteric regulators bind to a site on the enzyme
that is not the active site
b. negative regulators – act as inhibitors; when they
are in place the enzyme is not active
c. positive regulators – act as activators; when they
are in place the enzyme can function
B. Do Temperature and pH Affect Enzymes?
1. Because enzymes operate best within defined
temperature ranges, high temperatures decrease
reaction rate by disrupting the bonds that maintain
three-dimensional shape (denaturation occurs); cold
temperatures decrease the movement of particles and
therefore the reaction. Some archaebacteria can
survive extreme temperatures.
2. Most enzymes function best at a pH near 7 (pepsin
in the stomach is an exception); higher or lower values
disrupt enzyme shape and halt function.
C. Substrate Concentration can affect reaction rate
1. adding more substrate will increase the reaction
rate to a certain point, after which the enzymes are
working at a maximum rate and all of the active sites
are filled; therefore, adding more substrate will no
increase the rate
Bozeman biology enzymes
http://www.youtube.com/watch?v=ok9esggzN18