28 z e o ae rs a hi i Enzyme activity is often influenced by the presence of other and may be organic molecules (coenzymes) or inorganic ions chemicals. Some of these may enhance an enzyme's activity. (e.g, Ca2+, Zn2+). Enzymes may also be deactivated, temporarily Called cofactors, they are a nonprotein component of an enzyme or permanently, by chemicals called enzyme inhibitors.

Types of Enzyme

Nearly all enzymes are made of protein, although RNA has been demonstrated to have enzymatic properties, Some enzymes consist of just protein, while others require the additior1l of extra components to complete their catalytic properties. These may be permanently attached parts called prosthetic groups, or temporarily attached pieces (coenzymes) lhat detach after a reaction, and may participate with another enzyme in other reactions.

...

Activesile r-I Prosthetic group

is more or less permanently altached

Apoenzyme

Enzyme comprises Prosthetic ,group required only protein. Contains apoenzyme ,(protein)

enzyme

e,g.lysozyme

Reversible Enzyme Inhibitors

Competitive The subslrale binds 10 theinhibitor blocksSubstrate active sHe bulthe speedlhe aclive sile of the reaclion is slowed.

t Noncompetitive inhibitor

No inhibition Competitive Noncompetitive Inhibitor inhibitor

piUS a prosthetic group, e.g. flavoprotein + FAD

The subslrate cannot bind to

: the active site~ X,oil Active site ,., is distorted

"/

Noncompetillve inhibItor

Allosteric en2yme Inhibitor

Enzyme inhibitors may be reversible or irreversible, Reversible inhibitors are used to control enzyme activity. There is often an interaction between the substrate or end product and the enzymes controlling the reaction. Buildup of the end product or a lack of substrate may deactivate the enzyme, This deactivation may take the form of competitive (competes for the active site) or noncompetitive inhibition. While noncompetitive inhibitors have the effect of slowing down the rate of reaction, allosteric inhibitors block the active site altogether and prevent its functioning.

1. Describe the general role of cofactors in enzyme activity:

2. (a) List four heavy metals that are toxic to humans:

(b) Explain in general terms why these heavy metals are toxic to life:

Coenzyme becomes detached after the reaction

Apoenzyme

Coenzyme required Contains apoenzyme (protein) plus a coenzyme (non-prolein) e,g. dehydrogenases + NAD

Irreversible Inhibitors (Poisons)

... ,. Substrate cannot bind

Arsenic binds and alters Ihe

'¥' aClive site

Some heavy metals, such as arsenic (As), cadmium (Cd), and lead (Pb) act as irreversible inhibitors. They bind strongly 10 the sulfhydryl (-SH) groups of a protein and destroy catalytic activity. Most, including arsenic (above), act as noncompetitive inhibitors. Mercury (Hg) is an exception; it acts a competitive inhibitor, binding to the sulfhydryl group in the active site of the papain enzyme, Heavy metals are retained in the body and lost slowly.

3. There are many enzyme inhibitors that are not heavy metals (e.g, those found in some pesticides).

(a) Name a common poison that is an enzyme inhibitor, but not a heavy metal:

(b) Try to find out how this poison interferes with enzyme function. Briefly describe its eHect on a named enzyme:

4. Distinguish between competitive and noncompetitive inhibition:

5. Explain how allosteric inhibitors diHer from other noncompetitive inhibitors:

c

Reac -on alesE zy Enzymes are sensitive molecules. They often have a narrow Extremes in acidity (pH) can also cause the protein structure range of conditions under which they operate properly. For most at enzymes to denature. Pois0ns often work by denaturing of the enzymes associated with plant and animal metabolism, enzymes or occupying the enzyme's active site so that it does there is little activity at low temperatures. As the temperature not tunction. In some cases, enzymes will not fvnction without increases, so too does the enzyme activity, until the point is cofactors, such as vitamins or trace elements. In the four graphs reached where the temperature is high enough to damage below, the rate of reaction or degree of enzyme activity is plotted the enzyme's structure. At this point, the enzyme ceases to against each at four factors that affect enzyme per10rmance. function; a phenomenon called enzyme or protein denaturation. Answer the questions relating to each graph:

o .~

'"~

o <l> ro a:

With ample substrate and cofaolo~ prssent

With fixed amount of enzyme and ample cofactors present

c .~ u '"<l> E ~ c W

Enzyme concentration

Concentration of substrate

Optimum temperature for enzyme

40 50..10o

Temperature (OC)

1. Enzyme concentration (a) Describe the change in the rate of reaction when the enzyme concentration

is increased (assuming there is plenty of the substrate present):

(6) Suggest how a cell may vary the amount of enzyme present in a cell:

2. Substrate concentration (a) Describe the change in the rate of reaction when the substrate concentration

is increased (assuming a fixed amount of enzyme and ample cofactors):

(b) Explain why the rate changes the way it does: _

3, Temperature Higher temperatures speed up all reactions, but few enzymes can tolerate temperatures higher than 50-60°C. The rate at which enzymes are denatured (change their shape and become inactive) increases with higher temperatures.

(a) Describe what is meant by an optimum temperature for enzyme activity:

(b) Explain why most enzymes perform poorly at low temperatures:

pH

5673

C '5 ti '"<l> E >­N C W

4. pH (acidity/alkalinity) Like all proteins, enzymes are denatured by extremes of pH (very acid or alkaline). Within these extremes, most enzymes are still influenced by pH. Each enzyme has a preferred pH range for optimum activity.

(a) State the optimum pH for each of the enzymes:

Pepsin: _ Trypsin: ~ Urease: ___

(b) Pepsin acts on proteins in the stomach. Explain how its optimum pH is suited to its working environment: