Unit 7: Enzymes — Long Questions

Definition
Metabolism is the sum of all chemical reactions that occur within an organism to sustain life.

Types
There are two sub-sets of metabolism i.e., catabolism and anabolism

1. Catabolism
Definition
It involves the breakdown of complex molecules into simpler ones, releasing energy in the process.

Examples
i. Cellular respiration i.e. oxidation of food (glucose) into CO2 and H2O to get energy.
ii. Lipolysis i.e., break-down of lipids (fats) into fatty acids and glycerol, which can be used for energy production.

2. Anabolism
Definition
It involves building up complex molecules from simpler ones. This process consumes energy.

Examples
i. Photosynthesis i.e. conversion of carbon dioxide and water into glucose and oxygen using sunlight.
ii. Protein synthesis i.e., formation of proteins from amino acids, which are vital for cell structure and function.

Introduction
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process.

Nature of Enzymes
i. They are primarily proteins and are highly specific to their substrates (the molecules that undergo enzyme- controlled reactions).
ii. Some RNA molecules also act as enzymes. Such RNA is called ribozyme. Ribozymes are primarily found in ribosome. They are also found in specific viruses and bacteria.
iii. Most enzymes can speed up reactions millions of times faster than uncatalyzed reactions.

Characteristics of Enzymes

i. Chemical Nature of Enzymes: Enzymes are predominantly proteins. Typically, they contain 100 to 1,000 amino acids.

ii. Globular Structure (Active sites): Enzymes possess a three dimensional globular structure. This structure allows them to form active sites that can bind specifically to substrates.

iii. Specificity of Enzymes: Enzymes are highly specific to the reaction they catalyse they are also very specific for the nature of substrate.

Example Enzyme amylase specifically catalyses the breakdown of starch into simple sugars.

iv. Intracellular and Extracellular Enzymes: Enzymes can be classified based on the location where they function.

a) Intracellular Enzymes operate within cells e.g., enzymes of cellular respiration.

b) Extracellular Enzymes are secreted outside the cells to catalyse reactions e.g., enzymes secreted by the cells of stomach walls into stomach cavity for the digestion food.

v. Cofactors of Enzymes
Definition
Many enzymes require additional non-protein molecules to be fully active. Such non-protein molecules are called cofactors.

Main Groups
There are two main groups of cofactors. i.e., inorganic cofactors and organic cofactors.

a) Inorganic cofactors include metal ions like iron and magnesium ions.

b) The organic cofactors are of two types.

• Prosthetic Groups tightly bind with the enzymes. Examples are certain vitamins (e.g., biotin) and the haem group.

• Coenzymes loosely bind to the enzyme and may be released during the reaction. Examples include many vitamins and nucleotides (NAD and NADP).

vi. Enzyme Actions in Complex Metabolic Reactions: Enzymes often function in pathways. Multiple enzymes work in a sequence to carry out a series of reactions. Each enzyme in the pathway catalyses a specific step. After speeding up the reaction the product is passed on the next enzyme for further reaction.

vii. Use of Enzymes in Different Industries: Enzymes have extensive applications in various industries. For example

• Food Industry: Enzymes that break starch into simple sugars are used in production of white bread, buns and rolls. Enzymes are also used for the production of cheese.

• Paper Industry: Enzymes degrade starch to lower its viscosity that aid in making paper.

• Biological Detergent: Protease enzymes are used for removal of protein stains from clothes. Amylase enzymes are used in dish washing to remove resistant starch residues.

• Fermentation Industry: Enzymes degrade starch and proteins to produce simple sugar, amino acids and peptides that are used by yeast for fermentation.

Introduction
An enzyme has one or more pockets or clefts on its surface called active site. The active sites are directly involved in catalysis. Two models have been proposed to explain the mechanism of enzyme action.

1. Lock and Key Model of Enzyme Action
Introduction
The model was proposed by a German chemist Emil Fischer in 1894.

Explanation
According to it, the active site of an enzyme has fixed structure. The substrate molecule fits precisely into it to form an enzyme-substrate complex. The enzyme catalyzes the reaction and substrate is transformed into product/s. Then, the product is released from the enzyme.

2. Induced Fit Model
Introduction
This model was proposed by an American biologist Daniel Koshland in 1958.

Explanation
According to this model, the active site of an enzyme is not rigid. When substrate interacts with the enzyme, its active site is reshaped to perform its function.

Enzymes are sensitive to their environment. The activity of an enzyme is affected by the following factors.

1. Temperature / Describe how temperature extremes can inhibit enzyme activity and lead to enzyme denaturation?

Optimum Temperature
Each enzyme works at maximum rate at a specific temperature called optimum temperature.

Optimum Temperature for Human Enzymes
The optimum temperature for most of the human enzymes is 37°C.

Denaturation of Enzymes
When temperature rises to a certain limit, the heat adds in the movement of molecules. So, the rate of enzyme action increases. But when temperature is raised well above the optimum temperature, heat breaks the bonds in enzyme molecule. In this way the globular structure of enzyme is lost. This is called denaturation of enzyme. It results in a rapid decrease in the rate of enzymes action.

2. pH / How does pH affect enzyme activity?

Optimum pH
Enzymes are sensitive to hydrogen ion concentration (pH) of the fluid in which they work. They show maximum activity at a specific pH called their Optimum pH.

Affect of Change in pH
Change in pH can affect the ionization of the amino acids at the active site of enzyme. It slows down enzyme activity or blocks it completely. Different enzymes have different optimum pH values. For example, Pepsin (working in stomach) works in acidic medium (pH 1.5 to 2.0) while trypsin (working in small intestine) works in alkaline medium(pH 7 to 8)

3. Substrate Concentration
An increase in substrate concentration increases the rate of reaction.

Saturation of Active Sites
At high substrate concentration, all active sites of the enzymes are occupied.
In this condition, any more substrate molecules do not find free active sites. This state is called saturation of active sites and reaction rate does not increase.

Introduction
Certain substances, called enzyme inhibitors, bind to enzyme and decrease its activity. This phenomenon is known as enzyme inhibition.

Types of Enzyme Inhibition

1. Competitive Inhibition
Introduction
Some inhibitors resemble the enzyme's substrate. They compete with the substrate to attach to the active site of enzyme. When the inhibitor is attached to the active site, it blocks it and does not allow the substrate to attach.

Examples
Examples of competitive inhibitors are antibiotics. The antibiotic molecules compete with the substrates of bacterial enzymes. They attach to bacterial enzymes and inhibit them.

2. Non- Competitive Inhibition
Introduction
Some enzyme inhibitors do not have similarity to the substrate. They do not attach to the active site of enzyme. Rather, they attach to some other location of enzyme. This attachment changes the overall shape of enzyme and also the shape of active site. So, this changed active site does not fit substrate and enzyme is inhibited.

Examples
Heavy metals like mercury and certain drugs used in cancer therapy.