1.4 ENZYMES

An enzyme is a biological

protein-based catalyst

Enzyme nomenclature Enzymes are divided into 6 groups based on the

chemical reactions they catalyze.1. Oxidoreductases

2. Transferases

3. Hydrolases

4. Lyases

5. Isomerases

6. Ligases (synthetases)

Enzymes online tutorial• Excellent animations:

• http://www.lewport.wnyric.org/jwanamaker/animations.htm

• http://www.northland.cc.mn.us/biology/Biology1111/animations/enzyme.html

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter8/animations.html#

Substrate: the reactant that an enzyme

acts on when it catalyzes a chemical reaction

The Induced-fit model of enzyme-substrate interaction describes a protein as a dynamic molecule

that changes its shape to better accommodate the substrate

Factors Affecting Enzyme Activity

1.Temperature

2. pH

3. Substrate Concentration

4.Enzyme Concentration

37C

is the optimum temperature in

most living systems

(at high/temperatures enzyme function is

altered ~denatured

enzyme)

- most enzymes work best at pH ~7 (narrow range) - however, some such as “pepsin” have a pH of ~3 - changing the pH will alter the enzymes 3D shape

pH

Substrate Concentration

• As concentration increases, the rate of reaction increases

• Maximum occurs when all the enzymes are working

cofactors

•Inorganic atoms (Zn, Ca, Fe,…) which bind temporarily to enzyme

coenzyme

•Organic molecules

•E.g.; vitamins

•Necessary to activate enzyme and allow it to bind to substrate

•Also can help weaken bonds in substrates

Controlling Enzyme Activity

• 1. Regulating transcription/translation .

– By regulating transcription/translation, production of enzymes can be turned off/on.

– Some end products can act as transcription factors and inhibit the transcription/translation of enzymes

• 2. Competitive Inhibition : A substrate mimic (molecule that has the same configuration as the substrate) can enter into ACTIVE SITE and block enzyme action.

• Some antibiotics are competitive inhibitors– E.g.; HIV (protease inhibitors)– Penecillin (inhibits a bacteria transpepsidase: cell

wall is not built properly)

• When bind permanently (poisons or toxins)

• When bind temporarily (can be “disloged” by increasing concentration of substrate)

Inhibitors

• 3. Allosteric Inhibition (Feedback inhibition). Allows an enzyme to be temporarily inactivated.

• Binding of an allosteric inhibitor changes the shape of the enzyme, inactivating it while the inhibitor is still bound.

• This mechanism is commonly employed in feedback inhibition. Often one of the products of a series of reactions act as an allosteric inhibitor and blocks the pathway.

• AKA non-competitive inhibition

3.

Regulation of Enzyme Action

4. Allosteric Activators

• Substances bind to an allosteric site on enzyme and increase efficiency

a) Substrate and enzyme

b) Competitive Inhibitor

c) Non competitive inhibitor

Enzymes are _________________________ catalysts, and as such they _________________________ a chemical reaction without being _________________________ in the process. Enzymes work by reducing the _________________________. The _________________________ is the reactant that an enzyme act on. This reactant binds to a particular spot on the enzyme known as the _________________________. Enzymes are very _________________________ for the reactant to which they bind. The names of enzymes usually end in _________________________. 

ANS: protein, speed up, consumed, activation energy, substrate, active site, specific, ase

Temperature and pH affect enzyme activity. As with all other reactions, enzyme-catalyzed reactions _________________________ in speed with an increase in temperature. However, as the temperature increases beyond a critical point, the protein structure begins to get disrupted , resulting in _________________________ and loss of enzyme function. Every enzyme has a(n) _________________________ temperature at which it works best and activity tends to decrease on either side of this temperature. Most human enzymes work best at around _________________________. Some enzymes require nonprotein _________________________, such as zinc and manganese ions. Other enzymes may require organic _________________________ such as NAD+ and NADP+. A variety of substances inhibit enzyme activity. ___________________________________ are so similar to the enzyme's substrate that they are able to enter he enzyme's active site and block the normal substrate from binding. This process is reversible and can be overcome be increasing the concentration of the enzyme's substrate. Another class of inhibitors does not affect an enzyme at its' active site, they are called ___________________________________ and their effect cannot be overcome by adding more substrate.

ANS: increase, denaturation, optimal, 37C, cofactors, coenzymes, Competitive inhibitors, non-competitive inhibitors

Practice!1. Define catalyst

• A chemical that speeds up the rate of reaction without being consumed in the reaction.

2. Draw a labeled free-energy diagram to illustrate the effect of an enzyme on the activation energy of a hypothetical reaction. (Assume it is an exergonic reaction.)

3. What is meant by the statement, “an enzyme cannot affect the free-energy change of a reaction”?

• 5. How does an enzyme lower the activation energy of a biochemical reaction?

The enzymes do this by bringing the substrates into the correct geometry and by putting stress on the necessary chemical bonds.

6. How do competitive enzyme inhibition and noncompetitive enzyme inhibition differ?

A competitive inhibitor binds to the active site of an enzyme, preventing the substrate(s) from binding. In this case, the inhibitor competes with the substrate for the active site. A noncompetitive inhibitor attaches to an enzyme at a binding site other than the active site. This causes a conformational change in the enzyme’s protein structure that causes a loss of affinity of the active site for its substrate.

8. What happens to an enzyme after it has catalyzed a reaction?

After an enzyme catalyzes a reaction, it will catalyze the same reaction again.

For an exothermic reaction, H is negative.For an endothermic reaction, H is positive.