Enzyme Kinetics
1. Recognize the significance of enzyme specificity and active sites
2. Discuss the impact of cofactors and coenzymes on enzyme functioning
3. Experimentally investigate the optimal conditions for a selected enzyme
4. Compare the different mechanisms of enzyme inhibition |
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Characteristics of enzymes
1. they are globular proteins
with projections, grooves, charged areas, hydrophobic areas, hydrophilic
areas usually have small non-protein parts associated with the folded polypeptides
· metallic ions (Cu, Mn, Zn, Fe, etc)
· coenzymes = small organic molecules
- derived from vitamins (Bs, etc)
- participate in reaction & may leave enzyme with a small product of
reaction and go to another enzyme
- thus, coenzymes link one reaction to another
2. very specific - a specific
enzyme will only work on a certain substrate
3. reusable
4. very efficient catalysts
- a catalyst speeds up a chemical reaction without becoming a part of the
reaction; one catalase molecule can catalyze 40 million reactions per second
5. since reactions need a certain
amount of activiation energy to occur (think of a roller coaster having
to get over that first big hill), enzymes can lower the amount of activiation
energy needed
Cofactors & Coenzymes
- Cofactor
- A non-protein enzyme component (i.e. metal ion, organic compound) is
called a cofactor (or coenzyme).
- Cofactors are found in active sites and play important roles in catalysis.
- Essential trace minerals are usually employed as cofactors.
- An enzyme that requires a coenzyme for activity is known as an apoenzyme.
- Coenzymes
- Coenzymes are cofactors that are organic compounds.
- Vitamins often serve as coenzymes.
How do enzymes catalyze reactions?
¨ enzyme + reactants
-> enzyme-substrate complex -> products
¨ substrates
collide with enzyme and enter active site
¨ enzyme hold
substrates in a specific area of enzyme called the active site
¨ binding of
substrates to enzyme causes enzyme to fold around the substrates which
stabilizes the reactants
· weakens old bonds so the reaction is easier
· orients reactants correctly and very closely to help with the
formation of new bonds
¨ the kinetic
energy of the vibrating enzyme-substrate complex is sufficient for products
to be made
¨ the products
have a different molecular shape and do not fit the active site very well;
so they move away and the enzyme can then catalyze another set of reactants.
Factors which affect the rate of enzymatic reactions
- factors which denature enzymes:
- pH (there's an optimal pH for each reaction)
- salts
- temperature (higher temps -> faster reactions)
- factors which change collision
rates:
- enzyme concentration (more enzyme -> faster reactions)
- substrate concentration (more substrate -> slower reactions)
- molecules which bind to enzymes,
altering their shape and function:
1. inhibitors - decrease rate
a) competitive inhibitors - ex: enzyme in photosynthesis should grab
CO2 , sometimes grabs an O2
b) non-competitive inhibitors can act toxic to enzymes
c) allosteric inhibitors
- allosteric enzymes = enzymes with 2 shapes
these are regulated enzymes that have at least 2 binding sites:
1) an active site for reactants and 2) a regulatory site
- allosteric inhibitor = small molecule which attaches to an allosteric
enzyme at its regulatory site and inactivates it
2. activators = small molecules which attaches to an allosteric enzyme
and activates it, which increases rates
Naming enzymes
- names end in "ase'
- many enzyme names have 3 parts: [substrate name][type of
reaction][ase]
Mechanisms of Enzyme Inhibition
- Enzyme inhibition may be irreversible
or reversible.
- Enzymes may be irreversibly inactivated
by heat, or chemical reagents.
- There are three basic types of reversible
inhibition known. There also may be mixed inhibition (a combination
of these).
1. Competitive inhibition
- Competitive inhibition occurs when a compound has a similar chemical
structure to the enzyme substrate.
- The inhibitor binds to the active site and competes with the substrate
for binding.
2. Noncompetitive
inhibition
- The inhibitor binds to another site on the enzyme and inactivates the
enzyme molecule.
3. Uncompetitive
inhibition
- Uncompetitive inhibitors bind to both free enzyme and enzyme-substrate
complex
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