Enzyme Catalysis

Enzymes:

· Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals.

· They are actually protein molecules of high molecular mass and form colloidal solutions in water.

· They are very effective catalysts which catalyse numerous reactions, especially those connected with natural processes.

· Numerous reactions that occur in the bodies of animals and plants to maintain the life process are catalysed by enzymes.

· The enzymes are termed as biochemical catalysts and the phenomenon is known as biochemical catalysis.

· Many enzymes have been obtained in pure crystalline state from living cells.

The following are some of the examples of enzyme-catalysed reactions:

a) Inversion of cane sugar:

The invertase enzyme converts cane sugar into glucose and fructose.

C₁₂H₂₂O₁₁ (aq) + H₂O (l) C₆H₁₂O₆ (aq) + C₆H₁₂O₆ (aq)

b) Conversion of glucose into ethyl alcohol:

The zymase enzyme converts glucose into ethyl alcohol and carbon dioxide.

C₆H₁₂O₆ (aq) 2C₂H₅OH (aq) + 2CO₂ (g)

c) Conversion of starch into maltose:

The diastase enzyme converts starch into maltose.

2(C₆H₁₀O₅)n (aq) +nH₂O (L) nC₁₂H₂₂O₁₁ (aq)

d) Conversion of maltose into glucose:

The maltase enzyme converts maltose into glucose.

C₁₂H₂₂O₁₁ (aq) + H₂O (l) 2C₆H₁₂O₆ (aq)

e) Decomposition of urea into ammonia and carbon dioxide:

The enzyme urease catalyses this decomposition.

NH₂CONH₂ (aq) + H₂O(l) 2NH₃(g) + CO₂ (g)

f) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis.

g) Conversion of milk into curd:

It is an enzymatic reaction brought about by lacto bacilli enzyme present in curd.

Enzyme	Source	Enzymatic reaction
Invertase	Yeast	Sucrose → Glucose and fructose
Zymase	Yeast	Glucose → Ethyl alcohol and carbon dioxide
Diastase	Malt	Starch → Maltose
Maltase	Yeast	Maltose → Glucose
Urease	Soyabean	Urea → Ammonia and carbon dioxide
Pepsin	Stomach	Proteins → Amino acids

Characteristics of enzyme catalysis:

Enzyme catalysis is unique in its efficiency and high degree of specificity. The following characteristics are exhibited by enzyme catalysts:

Most highly efficient:

One molecule of an enzyme may transform one million molecules of the reactant per minute.

Highly specific nature:

Each enzyme is specific for a given reaction, i.e., one catalyst cannot catalyse more than one reaction. For example, the enzyme urease catalyses the hydrolysis of urea only. It does not catalyse hydrolysis of any other amide.

Highly active under optimum temperature:

The rate of an enzyme reaction becomes maximum at a definite temperature called the optimum temperature. On either side of the optimum temperature, the enzyme activity decreases. The optimum temperature range for enzymatic activity is 298-310K. Human body temperature being 310 K is suited for enzyme-catalysed reactions.

Highly active under optimum pH:

The rate of an enzyme-catalysed reaction is maximum at a particular pH called optimum pH which is between pH values 5-7.

Increasing activity in presence of activators and co-enzymes:

The enzymatic activity is increased in the presence of certain substances, known as co-enzymes. It has been observed that when a small non-protein (vitamin) is present along with an enzyme, the catalytic activity is enhanced considerably.

Influence of inhibitors and poisons:

Like ordinary catalysts, enzymes are also inhibited or poisoned by the presence of certain substances. The inhibitors or poisons interact with the active functional groups on the enzyme surface and often reduce or completely destroy the catalytic activity of the enzymes. The use of many drugs is related to their action as enzyme inhibitors in the body.

Mechanism of enzyme catalysis:

The enzyme-catalysed reactions may be considered to proceed in two steps:

Step 1: Binding of enzyme to substrate to form an activated complex.

E + S → ES

Step 2: Decomposition of the activated complex to form product.

ES → E + P

Catalysts in Industry:

Process	Catalyst
1. Haber’s process for the manufacture of ammonia N2 (g) + 3H2 (g) → 2NH3 (g)	Finely divided iron, molybdenum as promoter; conditions: 200 bar pressure and 723-773K temperature. Now-a-days, a mixture of iron oxide, potassium oxide and alumina is used.
2. Ostwald’s process for the manufacture of nitric acid. 4NH3(g)+5O2(g) → 4NO(g)+6H2O(g) 2NO(g) + O2(g) → 2NO2 (g) 4NO2(g)+2H2O(l)+O2(g)→4HNO3(aq)	Platinised asbestos; Temperature 573K.
3. Contact process for the manufacture of sulphuric acid. 2SO2 (g)+O2 (g) → 2SO3 (g) SO3(g)+H2SO4(aq) → H2S2O7(l) H2S2O7(l) + H2O(l) → 2H2SO4(aq)	Platinised asbestos or vanadium pentoxide (V2O5); temperature 673-723K.