Form 1:
Nutrition in Plants and Animals

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Enzymes are organic catalysts that are protein in nature and regulate the rate of metabolic reactions.They speed up or slow down the rate of metabolic reactions but to not get used up in the process.

Types of enzymes

a) Extracellular: Are produced within the cells but used outside the cells e.g. digestive enzymes.

b) Intracellular: Are enzymes produced and used within the cells e.g. respiratory enzymes.

Importance of Enzymes.

•They speed up the rate of chemical reactions that would otherwise be too slow to support life.

•Some enzymes take part in synthesis/building of useful complex substances such as DNA.

•Digestive enzymes breakdown complex food substances into simple foods that can be utilized by the cells.

•Some metabolic enzymes such as catalase play a vital role in detoxification (making poisonous substances less harmful.

Enzyme nomenclature

Two systems of naming enzymes have been adopted.

a). Trivial naming

This is where an enzyme is named by the scientist who discovered it.

In trivial naming all enzyme names end in prefix –in.


Pepsin (Theodor Schwann, German physiologist -1836).

Ptyalin (Anselme Payen, a French chemist- 1833).


b). Use of suffix –ase

Enzymes are assigned names by adding suffix –ase to the food substrate acted by the enzyme or by adding the suffix to the reaction being catalyzed by the enzyme.


Amylose (starch)…………..amylase.


Protein……………………. .protease.







Mechanism of action of Enzymes.

Enzymes are not used up during metabolic reactions. They do have “active sites” through which the substrate molecules bind to the enzymes. The reaction is then catalyzed and the end products released. The enzyme is free to bind with another substrate molecule. The enzymes can be used again and again.

Properties of Enzymes.

They are protein in nature; hence affected by temperature and pH.

They are substrate specific e.g. maltase cannot digest sucrose.

They are efficient in small amounts since they are re-used in the reactions.

They mostly take part in reversible reactions.

They regulate the rate of metabolic activities but are not used up.

Factors affecting enzyme activity.



Substrate Concentration.

Enzyme Concentration.

Enzyme co-factors and co-enzymes; Fe, Mg, Zn, Cu ions.


Enzyme inhibitors.

a) Temperature

At low temperatures, kinetic energy of enzymes and molecules are low. There are few collisions leading to low enzyme activity.

As temperature increases, the kinetic energy of the enzyme and substrate molecules increases leading to increased collisions hence increase in enzyme activity.

Enzyme activity is optimum at (35 -40)°C.

Beyond 40 °C the rate of enzyme activity decreases and eventually stops. This is because enzymes get denatured and their active sites get destroyed.

b) pH

Enzymes work best under different pH conditions.

Some enzymes work best under alkaline conditions e.g amylase. Some also work better under acidic conditions e.g. pepsin. However, most intracellular enzymes work better under neutral conditions.

Altering the pH conditions would affect enzyme activity.

c) Enzyme Specificity

A particular enzyme will only act on a particular substrate or will only catalyze a particular reaction.

For instance, sucrase enzymes can only breakdown sucrose.

d) Substrate Concentration

Assuming all other factors are constant, low substrate concentration, the rate of enzyme activity is low.

Increase in substrate concentration increases the rate of enzyme activity since more active sites of the enzymes will be occupied and there will also be an increase in enzyme substrate collisions leading to increased reaction.

The reaction increases up to a point at which it becomes constant. At this point, all active sites are utilized. The enzymes become the limiting factor of reaction. Increasing enzyme concentration would increase the rate of enzyme activity.

e) Enzyme Concentration

An increase in enzyme concentration increases the rate of enzyme reaction up to a level beyond which the rate of reaction becomes constant.

At low enzyme concentration, rate of enzyme activity is low because there are fewer sites and also fewer enzyme-substrate collisions that would lead to reactions.

Increasing enzyme concentration increases rate of enzyme activity since there will be an increase in number of active sites and enzyme-substrate collisions.

At optimum enzyme concentration, substrate concentration is the limiting factor.Increasing substrate concentration increases the rate of reaction.

f) Enzyme co-factors

These are inorganic substances which activate enzymes.

Without them, most enzymes would not function properly.

Co- factors include mineral ions like iron, magnesium, copper, manganese, zinc as well as vitamins.

They are used again and again since like enzymes, they do not get used up during the reactions.

g) Co-enzymes

These are organic molecules that are required by some enzymes for their efficient functioning. Some enzymes will not function without them.

Most co-enzymes are derivatives of vitamins.


NAD- Nicotine Adenine Dinucleotide.

FAD- Flavine Adenine Dinucleotide.

NADP- Nicotine Adenine Dinucleotide Phosphate.

h) Enzyme inhibitors

These are chemical substances which slow down or eventually stops enzyme activity.

They are of two types:

1. Competitive

2. Non- competitive

Competitive inhibitors

These are chemical substances which are structural analogs of the substrates i.e. they take up the shape of the substrates and compete for the active sites of the enzymes.

They bind with the enzymes and do not disentangle easily (they stay in the enzyme active site for a long time) thereby slowing down the rate of enzyme activity.

The reaction can be increased by increasing the substrate concentration.

Non competitive inhibitors

These are inhibitors that do not resemble the substrate molecules but they combine with the enzyme at any site other the active site and alter the structure of the active site of the enzyme. The normal substrate, therefore, fails to bind to the active site leading to decreased rate of reaction.

Note that these substances do not compete for the active sites of the enzymes.

The enzymes are destroyed permanently hence the effect cannot be reversed.

Examples of non competitive inhibitors

Heavy metals (such as lead, mercury, silver), Cyanide, organo-phosphates such as malathion.

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