1408 Ch5 B copy - Collin Collegeiws.collin.edu/cdoumen/1408/1408_Ch4_5/1408_Ch5_C.pdf10/16/14 2...
Transcript of 1408 Ch5 B copy - Collin Collegeiws.collin.edu/cdoumen/1408/1408_Ch4_5/1408_Ch5_C.pdf10/16/14 2...
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Enzymes � What is needed for chemical reactions to proceed in the body ( and thus metabolism to proceed) is the presence of specific proteins called ENZYMES.
� Most chemical reactions do not proceed spontaneously and they need help from a catalyst.
� An energy barrier must be overcome before a chemical reaction can begin. This energy is called the activation energy (because it activates the reactants).
Enzymes
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Products
Reactant
Ener
gy
Activation energy barrier
Without enzyme
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Enzymes
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Reactant
Products
Enzyme
Ener
gy
Activation energy barrier reduced by enzyme
With enzyme
Enzymes Lower the Ac1va1on Energy Barrier
Reactants
Progress of the reaction Products
a
b
c Ener
gy
The activation energy “a” is lowered to “b” when an enzyme is involved in this reaction. Thus the reaction will proceed much easier with an enzyme.
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A specific enzyme catalyzes each cellular reac1on � An enzyme
� is very selective in the reaction it catalyzes � has a shape that determines the enzyme’s specificity for the substrate
� produces a very specific product
� The specific reactant that an enzyme acts on is called the enzyme’s substrate.
� A substrate fits into a region of the enzyme called the active site.
A specific enzyme catalyzes each cellular reac1on � Enzymes are specific because only a specific substrate molecules fits into their active site and only a specific product is made
� The fact that enzymes are specific is also reflected in their names
� An enzyme is usually named after the substrate that it uses followed with -‐ase � SUCRASE : uses sucrose as a substrate � LACTASE : uses lactose as a substrate
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Enzyme (sucrase)
Active site
The enzyme available with an empty active site
1
Example of the action of an Enzyme
Enzyme (sucrase)
Substrate binds to enzyme with induced fit.
Substrate (sucrose)
Active site
The enzyme available with an empty active site
1
2
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The substrate is converted to products
H2O
Enzyme (sucrase)
Substrate binds to enzyme with induced fit.
Substrate (sucrose)
Active site
The enzyme available with an empty active site
1
2
3
Glucose
Fructose
The products are released
The substrate is converted to products
H2O
Enzyme (sucrase)
Substrate binds to enzyme with induced fit.
Substrate (sucrose)
Active site
The enzyme available with an empty active site
1
2
3
4
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Enzymes have op1mal condi1ons for func1oning � For every enzyme, there are optimal conditions under which it is most effective.
� Temperature affects molecular motion. � An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site.
� Most human enzymes work best at 35–40°C. � The optimal pH for most enzymes is near neutrality (pH ~ 7)
� The optimal pH and temperature tells us the environment where the enzyme functions best.
Enzymes have op1mal condi1ons for func1oning Since enzymes are proteins, they are vulnerable to denaturation. The 3-‐ dimensional framework of the enzyme falls apart and they lose efficiency. Examples are changes in Temperaure and pH.
Enzyme 1 will denature at pH 7 and Enzyme 2 will denature at pH 2.
1 2
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A specific enzyme catalyzes each cellular reac1on � Many enzymes require non-‐protein helpers called cofactors, which � bind to the active site and � function in catalysis.
� Some cofactors are inorganic, such as the ions of zinc, iron, magnesium, or copper.
� If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme.
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Enzyme inhibi1on can regulate enzyme ac1vity in a cell A chemical that interferes with an enzyme’s activity is called an inhibitor. � Competitive inhibitors
� block substrates from entering the active site and � reduce an enzyme’s productivity.
� Noncompetitive inhibitors � bind to the enzyme somewhere other than the active site,
� change the shape of the active site, and � prevent the substrate from binding.
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Substrate
Enzyme
Active site
Normal binding of substrate
Competitive inhibitor
Noncompetitive inhibitor
Enzyme inhibition
Enzyme inhibi1on can regulate enzyme ac1vity in a cell � Enzyme inhibitors are important in regulating cell metabolism.
� In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition.
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Feedback inhibition
Enzyme 1
Reaction 1 A
Starting molecule
Product
–
Enzyme 2
Reaction 2 B
Enzyme 3
Reaction 3 C D
Many drugs, pes1cides, and poisons are enzyme inhibitors � Many beneficial drugs act as enzyme inhibitors, including � ibuprofen, which inhibits an enzyme involved in the production of prostaglandins (messenger molecules that increase the sensation of pain and inflammation),
� some blood pressure medicines, � some antidepressants, � many antibiotics, and � protease inhibitors used to fight HIV.
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Many drugs, pes1cides, and poisons are enzyme inhibitors
� Enzyme inhibitors have also been developed as � pesticides and deadly poisons for chemical warfare.
� Natural occurring poisons also can function as enzyme inhibitors