Enzymes• important functions of proteins > catalysts

(substance that enhances the rate of chemical reaction but is not permanently altered by the reaction).

• enzyme contains a unique, intricately shaped binding surface called an active site.

> a small cleft on a large protein where substrates bind to

• Active site also takes part in catalysis –side chains of aa line with active site

•Enzymes work by the Key Lock Model (1890)

•Each enzyme binds to a single type of substrate > both have complementary structure

•substrate overall shape and charge distribution allow it to enter and interact with the enzymes active site.

E + S > ES > E+ P

Another model by which enzymes work is the INDUCED KEY MODEL. - the substrate does not fit the into the active site, but binding of the substrate induces a conformational: change in the enzyme that causes the active site to fit around the substrate in a lock and key manner

• Some enzymes require non-protein components for their activities.

• Cofactors- may be ions, such as Mg2+ or Zn2+ or complex organic molecules, referred to as co-enzymes

• A protein component of an enzyme lacking essential co-factor is called an apoenzyme.

• Intact enzymes with their bound co-factor are refereed to as holoenzymes

The six major enzyme categories are 1. Oxidoreductases -catalyses oxidation-reduction

reactions

A– + B → A + B–

2. Transferases – catalyses reactions that involve the transfer of groups from one molecule to another. E.g. of such groups include amino, carboxyl methyl etc..

A–X + B → A + B–X

3. Hydrolases- catalyze reactions in which the cleavage of bonds is accomplished by adding water. E.g. esterases, phosphatases and peptidases

A–B + H2O → A–OH + B–H

4. Lyases- catalyse reactions in which groups (e.g. H2O, CO2 and NH3) are removed to form a double bond or are added to a double bond. E.g. Decarboxylases

5. Isomerases- heterogeneous group of enzymes that catalyze intramolecular rearrangements

6. Ligases- catalyzes bond formation b/w two substrate molecules. Energy for these reactions is always supplied by ATP hydrolysis.

Enzyme Activity

The properties of enzymes related to their tertiary structure.The effects of change in

temperature,pH,substrate concentration,and competitive and non-competitive inhibition on

the rate of enzyme action

HOW ENZYMES WORK

• Enzymes are ORGANIC CATALYSTS. A CATALYST is anything that speeds up a chemical reaction that is occurring slowly. How does a catalyst work?

• The explanation of what happens lies in the fact that most chemical reactions that RELEASE ENERGY (exothermic reactions) require an INPUT of some energy to get them going. The initial input of energy is called the ACTIVATION ENERGY

Enzyme catalyzed reactions occur in two phases:

Substrate binds to active site of Enzyme; an intermediate ES forms. During the transition state (intermediate between S and P) S forms P. After brief time product leaves enzyme .

The ES complex is stabilized in the transition state by non-covalent interactions between substrate the the aa inthe active site.glove around a hand.

Where k+1, k-1 and k+2 are the respective rate constants which quantifies the speed of a chemical reaction

Properties of Enzymes relating to their tertiary structure.

• The activity of enzymes is strongly affected by changes in pH and temperature. Each enzyme works best at a certain pH and temperature,its activity decreasing at values above and below that point. This is because of the importance of tertiary structure (i.e. shape) in enzyme function and forces, e.g., ionic interactions and hydrogen bonds, in determining that shape.

The effects of change in temperature.

Temperature: enzymes work best at an optimum temperature.

Below this, an increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate will too.

Above the optimum temperature, and the enzymes are denatured. Bonds holding the structure together will be broken and the active site loses its shape and will no longer work

The effect of change in pH.

• pH: as with temperature, enzymes have an optimum pH. If the pH changes much from the optimum, the chemical nature of the amino acids can change.

This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be

denatured.

The effect of change in concentration

Enzyme concentration:• at low enzyme concentration there is great competition for the active sites

and the rate of reaction is low.

• As the enzyme concentration increases, there are more active sites and the reaction can proceed at a faster rate.

• Eventually, increasing the enzyme concentration beyond a certain point has no effect because the substrate concentration becomes the limiting factor.

Substrate concentration: • at a low substrate concentration there are many active sites that are not

occupied. This means that the reaction rate is low.

• When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases.

• Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed.

Competitive and non-competitive inhibition

• Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable

• Reversible inhibitors:• Inhibitors• Competitive reversible inhibitors: these molecules have

a similar structure to the actual substrate and so will bind temporarily with the active site. The rate of reaction will be closer to the maximum when there is more ‘real’ substrate, (e.g. arabinose competes with glucose for the active sites on glucose oxidase enzyme).

Reversible inhibitors of 2 kinds affects Vmax and Km differently

• Competitive Inhibitors •Inhibitor resembles S structurally so binds to the same active site as S would.

•Form a EI complex rather than ES

• since substrate and inhibitor compete can over come inhibition by

- diluting the inhibitor or - increasing the [S]

• Non-competitive reversible inhibitors:

• these molecules are not necessarily anything like the substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the enzyme though, so the reaction rate decreases.

• Non-competitive Inhibitors

Inhibitor binds elsewhere from active side

Binding alters the enzyme 3-D configuration and block the reaction

Inhibition cannot be reversed by increasing [S]

Irreversible inhibitors:These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction, for example, cyanide irreversibly inhibits the enzyme cytochrome oxidase found in the electron transport chain used in respiration. If this cannot be used, death will occur

Study Guide

1. Why do you think enzymes are important for chemical reactions in a living cell?

2. Describe the SIX classes of enzymes?

3. What is the general role of enzymes in the cells?

4. What is activation energy?

5. Explain in terms of energy changes how enzymes are able to speed up reactions?

6. What is meant by the term “enzyme substrate complex”?

7. Compare and contrast the “Lock and Key Model” with the “Induced Fit Model”

8. An enzyme that hydrolyses protein will not act on starch. Why do you think this happens?

9. What property of enzyme does this statement depict?

10. Why is this property important for enzymes?

11. What part of an enzyme enables it to be substrate specific? What aspects of this “parts” structure enable it to be so specific?

13. Why do you suppose most enzymes are so large relative to their substrate size?

14. What are co-enzymes?

15. Why are co-enzymes important in enzyme functioning?

16. Describe FOUR methods enzymes use to lower the energy of activation.

17. Distinguish competitive, uncompetitive and non competitive inhibitors.

19. Why would a change in pH affect the rate of an enzyme reaction?

20. How would the following temperature changes affect the activity of an enzyme extracted from human cells: to 80 C; to 42 C; to 4 C

21. Explain why i) Refrigeration preserves food ii) Boiling can be used to sterilize instruments