Transcript Enzymeregulation

Enzyme Regulation
Chemical Reactions
 Thousands of chemical reactions occur in
living organisms every second.
 Energy is required to start each reaction=
activation energy.
 Enzymes reduce the amount of activation
energy required to start a reaction.
ENZYMES
 All enzymes are very large proteins made of
chains of amino acids linked together by peptide
bonds.
 Function of Enzymes:
 Enzymes are catalysts.
 Catalysts speed up chemical reactions by lowering
the activation energy.
 Catalysts are not changed in the reaction.
 Each enzyme is very specific= only reacts with a
few types of molecules (substrate
Lock and Key Model:
 The enzyme and substrate fit together.
 The enzyme changes the substrate into
products.
 The enzyme releases the products and is
able to bond with the next substrate.
The activity of enzymes is strongly
affected by
 Changes in pH
 Changes in temperature
 Amount of time
 Concentration of reactants
 Concentration of enzyme
Many enzymes require
cofactors
1. some enzymes have tightly bound helpers
called coenzymes or cofactors
2. Cofactors can be single metal ions (Mg, Zn,
Co, Mn, etc)
3. Cofactors can be small organic molecule
called
Inhibitors
 slow down rate of reaction of enzyme
when necessary
 I. Competitive Inhibitors
 Compete with substrate for active site
 Shape similar to substrates / prevents
access when bonded
[EXAMPLE]
 Methanol Poisoning
 Methanol CH3OH is a competitive inhibitor
 CH3OH can bind to dehydrogenase whose
true substrate is C2H5OH
 A person who has accidentally swallowed
methanol is treated by being given large
doses of C2H5OH
 C2H5OH competes with CH3OH for the active
site
Non-competitive Inhibitors
 Chemical does not have to resemble
the substrate
 Binds to enzyme other than at active
site
 This changes the enzyme's active site
and prevents access to it
Irreversible Inhibition
 Chemical permanently binds to the enzyme
or massively denatures the enzyme
 Nerve gas permanently blocks pathways
involved in nerve message transmission,
resulting in death
 Penicillin, the first of "wonder drug"
antibiotics, permanently blocks pathways
certain bacteria use to assemble their cell
wall component (peptidoglycan)
1. Consider Regulatory problem of cell:
thousands of enzymes, each with a "mind
of its own". Yet cell needs overall stability.
2. Example: synthesis of a certain amino
acid. Reaction scheme looks like this:
3. Suppose supply of E in cell increases (e.g.
eat a meal rich in E). How to shut down
synthesis of E?
1.
2.
3.
Suppose supply of E in cell increases (e.g. eat a
meal rich in E). How to shut down synthesis of
E?
Cell's answer: Enzyme 1 is reversibly inhibited
by E. Note that E is not the substrate, and
chemically so different that it cannot bind to
active site. How does E shut down Enzyme 1?
Enz 1 is a special type of enzyme called an
allosteric enzyme. It causes feedback
inhibition. Allosteric enzymes contains two
distinct subunits, one with active site (binds
substrate A and catalyzes reaction), one with
allosteric site (binds E).
Allosteric Inhibition
 simulation
1. When E binds, causes shape change in the
enzyme, this is transmitted to block
activity of active site.
2. Net result: whole pathway is turned on or
off as a unit by end-product. Called
Feedback inhibition. Crucial to cell
regulation.