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CHAPTER 15 ENZYMES. Enzymes provide an alternative pathway for reaction, one with a significantly lower activation energy and, therefore, a faster rate. Terms in Enzyme Chem. Apoenzyme : the protein part of an enzyme.

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CHAPTER 15 ENZYMES

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Chapter 15 enzymes

CHAPTER 15

ENZYMES


Chapter 15 enzymes

  • Enzymes provide an alternative pathway for reaction, one with a significantly lower activation energy and, therefore, a faster rate.


Terms in enzyme chem

Terms in Enzyme Chem

  • Apoenzyme: the protein part of an enzyme.

  • Cofactor: a nonprotein portion of an enzyme that is necessary for catalytic function; examples are metallic ions such as Zn2+ and Mg2+.

  • Coenzyme: a nonprotein organic molecule, frequently a B vitamin, that acts as a cofactor.

  • Substrate: the compound or compounds whose reaction an enzyme catalyzes.

  • Active site: the specific portion of the enzyme to which a substrate binds during reaction.


Specificity

SPECIFICITY

  • How selective an enzyme is for a substrate or reaction.

  • High Specificity – ie. Thrombin

  • Low Specificity – ie. Papain


Chapter 15 enzymes

Thrombin clips two peptides from fibrinogen. This produces the fibrin monomer with exposed polymerization sites that can bind to other fibrin monomers. The monomers polymerize to form a loose clot.


Chapter 15 enzymes

  • Used as meat tenderizer

  • Treatment for jellyfish, bee and wasp stings

  • Used in contact lens solutions

  • Breaks down the protein in toxins, venoms and bacteria


Mechanism of action

Mechanism of Action

  • Lock-and-key model of enzyme mechanism.

    • The enzyme is a rigid three-dimensional body.

    • The enzyme surface contains the active site.


Mechanism of action1

Mechanism of Action

  • Induced-fit model

    • The active site becomes modified to accommodate the substrate.


Enzyme activity

Enzyme Activity

  • Enzyme activity: a measure of how much a reaction rate is increased.

  • We examine how the rate of an enzyme-catalyzed reaction is effected by:

    • enzyme concentration

    • substrate concentration

    • temperature

    • pH


Enzyme activity1

Enzyme Activity

  • The effect of enzyme concentration on the rate of an enzyme-catalyzed reaction. Substrate concentration, temperature, and pH are constant.


Enzyme activity2

Enzyme Activity

  • The effect of substrate concentration on the rate of an enzyme-catalyzed reaction. Enzyme concentration, temperature, and pH are constant.


Enzyme activity3

Enzyme Activity

  • The effect of temperature on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and pH are constant.


Enzyme activity4

Enzyme Activity

  • The effect of pH on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and temperature are constant.


Chapter 15 enzymes

1

2

6

4

5

3

Feedback – Regulation by end product of a reaction on an earlier step


Enzyme regulation

Enzyme Regulation

  • Allosterism: enzyme regulation based on an event occurring at a place other than the active site but that creates a change in the active site.

    • An enzyme regulated by this mechanism is called an allosteric enzyme.

    • Allosteric enzymes often have multiple polypeptide chains.

    • Negative modulation: inhibition of an allosteric enzyme.

    • Positive modulation: stimulation of an allosteric enzyme.

    • Regulator: a substance that binds to an allosteric enzyme.


The allosteric effect

The Allosteric Effect

  • The allosteric effect. Binding of the regulator to a site other than the active site changes the shape of the active site.


Chapter 15 enzymes

ZYMOGENS/PROENZYMES

  • Enzyme made in inactive form

  • Change to the enzyme makes it active

  • Digestion and blood coagulation


Chapter 15 enzymes

Thrombin clips two peptides from fibrinogen. This produces the fibrin monomer with exposed polymerization sites that can bind to other fibrin monomers. The monomers polymerize to form a loose clot.


Enzyme inhibition

ENZYME INHIBITION

  • Reversible

    • Noncompetitive – Inhibitor does not bind at the active site (ieallosteric control)

    • Competitive – Inhibitor binds at the active site


Chapter 15 enzymes

Competitive Inhibition


Chapter 15 enzymes

Treatment of HIV

With AZT


Chapter 15 enzymes

Competitive Inhibition

Noncompetitive Inhibition


Irreversible inhibition

Irreversible Inhibition

  • Inhibitor binds at active site and forms covalent bond with amino acid side groups involved in catalysis.

  • Most toxins and

    poisons

  • Many pharmaceuticals


Chapter 15 enzymes

Heavy Metal Poisoning

(Hg and Pb)

-CH2-SH

+ Hg or Pb

-CH2-S-Hg

Pb

Lead poisoning may have caused years of chronic illness in composer Ludwig van Beethoven and may have contributed to his death. Chemical analysis experts found unusually high levels of lead in strands of Beethoven's hair.


Chapter 15 enzymes

Lead in the Environment

  • Homes built prior to 1977 are likely to contain lead-based paint.

  • Soil near heavily-used streets and roads may contain lead because lead was used in gasoline until the 1970’s.

  • Lead may get into drinking water from materials used in plumbing, such as pipes.

  • Lead-glazed or lead-painted pottery, leaded crystal, and some inks


Chapter 15 enzymes

Tar Creek Site


Chapter 15 enzymes

Mercury in the Environment

Predator fish including tuna, mackeral, shark and swordfish

“Mad as a Hatter” was a common expression in the 1800’s.


Chapter 15 enzymes

-CH2-OH

Nerve Gases

+ Nerve Gas (X)

-CH2-O-X

Binds to the active site of acetylcholinesterase so acetylcholine is not broken down to acetyl CoA and choline, acetylcholine builds up in the synapse and jams nerves in the "on" position so they constantly signal muscles to contract.


Chapter 15 enzymes

-CH2-OH

+

-CH2-O-


Chapter 15 enzymes

Antibiotics

All penicillin derivatives inhibit bacterial cell wall synthesis. If cell walls are improperly made it allows water to flow into the cell causing it to burst.

The structure of a complex between penicillin G and the Streptomyces transpeptidase.


Genetic control

Genetic Control

Best for adapting to environmental

or developmental changes


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