Enzymes are protein compounds that catalyse a specific reaction. Living cells contain thousands of 
different enzymes that accelerates or decelerates one kind of reaction without itself being changed. In 
some of these reactions, small organic molecules such as; acids, sugars, nucleotides and lipids are 
broken down to provide energy for the cell. In other reactions, small molecules are built into complex 
macromolecules, such as; proteins, DNA, RNA, and polysaccharides, or used to carry signals, or to
control cell movement or gene expression. Sometimes the enzyme proteins have non proteins 
attachment called prosthetic group when bounded by covalent bond, e.g. metals like; Cu, Mg, and Fe, 
and co-enzyme or co-factors when bonded by loose hydrogen bond, e.g. Vitamins. The co-enzyme 
and prosthetic group may become attached to several different protein forming different enzymes. Enzymes exhibit enormous catalytic power, in some cases increasing reaction rates by a factor’ of 
over 1014 (one hundred million). Enzymes dictate the pattern of chemical changes in a cell and 
without them life as we know it would be impossible.
Properties characteristics of Enzymes
a) Small amount of enzymes are needed to convert large amount of substrate into 
products.
b) Same substrate can be utilized by several enzymes. For instance, isomers, 
phosphoglucomutase and glucose – 6 – phosphate dehydrogenase act on glucose to 
produce fructose – 6- phosphate, glucose -1- phosphate and 6- phosphoglucolactone 
respectively.
c) Enzymes work at narrow range of temperature. Optimum temp is 40o
c and could be 
destroyed at 600
c.
d) They work at specific pH. Most function at around neutral pH (5-7). However, pepsin 
in stomach works at pH 2-3 and trypsin in duodenum works at pH 8.5.
e) Catalytic actions of enzymes may be specific. Hence an enzyme which catalyse one 
reaction may not catalyse another. For instance, invertase works on sucrose to produce 
glucose while zymase act on glucose and give carbondioxide and ethanol.
f) Enzymes are not destroyed by the reactions they catalysed and could be used again.
g) Enzymes could be poisoned by chemical compound like; mercury, chloride, silver 
chloride and hydrogen cyanide. This inactivate the enzymes, for example HCN blocks 
the enzymes involve in respiration.
Mechanism of Action (working) of Enzymes.
This can be explained by chemical hypothesis A B. Chemical energy needed could be 
in the form of heat (temperature) to activate passive A by bombarding A’s molecules to activate it 
and later turned into B molecules. The energy that A molecule required to react and be converted 
into molecule B is the activation energy of reaction. Enzymes are believe to catalyse reaction by 
lowering the activation energy e.g. in 
2H2O catalase 2H2O + 2O2 the activation energy in the absence of catalase is 
18,000cal mol. While in the presence of catalase, it is 6,400 cal mol. Lock and Key Hypotheses: 
the enzyme is believed to be the padlock and substrate the key.