Chemicals of life

 

Biochemistry is the branch of biology that deals with the study of the chemicals of life and their reactions. Chemicals of life include carbohydrates, proteins and lipids.

Carbohydrates

Are compounds of carbon, hydrogen and oxygen in the ratio of 1:2:1. They have a general formula (CH2O) n where n represents the number of carbon atoms. Carbohydrates are grouped into three categories:

Mono-saccharides

·         These are the simplest carbohydrates.

·         They include glucose, fructose and galactose.

·         Their general formula is C6H12O6.

Properties

·         They have a sweet taste

·         They readily dissolve in water

·         They are crystallizable

·         They are reducing sugars; monosaccharide reduce blue copper (II) sulphate in Benedict’s solution to red brown copper (I) oxide when heated.

Functions

They are the chief respiratory substrate. They are broken down to release energy in the body.

They are condensed to form complex important carbohydrates.

 

Disaccharides

These are complex sugars formed by linking two monosaccharide units through condensation.

They have a general formula C12H22O11.The bond that holds two monosaccharide units is called glycosidic bond.

Examples of disaccharides include: Maltose- common in germinating seeds; Sucrose-fruits and sugarcane. Sucrose is the form in which carbohydrates are transported in plants; Lactose-found in milk.

 

 

Properties of Disaccharides

·         They have a sweet taste

·         They are crystallizable

·         They are water soluble

·         They are non-reducing sugars except maltose is sugar reducing and is known as a complex reducing sugar.

·         They can be broken down into their constituent monosaccharide units through hydrolysis. Hydrolysis is the process through which complex molecules are broken down in the presence of water molecules. In living systems, hydrolysis is carried out by enzymes. However, in the laboratory, hydrolysis can be carried out by boiling the disaccharide in dilute acid such as hydrochloric acid.

Functions

They are hydrolyzed into mono-saccharides and respire do not yield energy

They are the form in which carbohydrates are transported in plants due to their soluble and inert nature.

Polysaccharides

These are formed through linking of numerous monosaccharide units through condensation. Their general formula is (C6H10O5)n where n is a very large number.

Properties of polysaccharides

·         They are non-sweet

·         They do not dissolve in water

·         They are non-crystalline

·         They are non-reducing sugars

Examples of polysaccharides:

·         Starch- Made by linking numerous glucose molecules. It is a form in which carbohydrates are stored in plants.

·         Glycogen- Is a storage carbohydrate in liver and muscles of animals. It is broken down to glucose in animals when blood glucose falls.

·         Cellulose-This is a structural polysaccharide in plants. It is a component of the cell wall

·         Chitin-A structural carbohydrate found in cell wall of fungi and arthropod exoskeletons

Functions of polysaccharides

·         They are storage carbohydrates; their insolubility and inertness makes them ideal for storing carbohydrates.

·         They are structural carbohydrates e.g. cellulose forms the plant cell walls

·         They can be hydrolyzed into monosaccharide and be broken down to release energy

 

Lipids

These are compounds of carbon, hydrogen and oxygen. However, they contain lesser oxygen but higher hydrogen compared to carbohydrates. Building units for lipids are fatty acids and glycerol. To synthesize a molecule of lipid, three fatty acids and a glycerol molecule are linked through a condensation reaction

There is one type of glycerol but numerous fatty acids. There are different types of fatty acids. The property of a lipid therefore depends on the type of fatty acids that link up with the glycerol.

There are complex lipids such as phospholipids, steroids, waxes and cholesterol. These also form through condensation.

Properties of lipids

·         Fats easily change to oil when heated while oils easily solidify when cooled.

·         They are insoluble in water but readily dissolve in organic solvents such as chloroform to form emulsions

·         They are inert hence can be stored in tissues of organisms.

Functions

·         They are a source of energy when oxidized. They yield more energy compared to carbohydrates when oxidized per unit weight. However, they are less preferred as source of energy because they require a lot of oxygen to oxidize. In addition, they are insoluble hence not easy to transport to respiratory sites.

·         They are a source of metabolic water. When oxidized, they yield a lot of metabolic water. This explains why some desert animals such as camels store large quantities of fat in their bodies.

·         Lipids offer protection to internal organs as they are deposited around them to act as shock absorbers.

·         Lipids provide heat insulation when stored underneath the skin as they are poor conductors of heat hence do not conduct heat away from the body. Organisms in cold areas tend to be short and plump as they have fatter fat adipose.

·         Lipids form structural compounds for instance phospholipids in cell membrane.

·         Complex lipids such as waxes in leaves help minimize water loss through transpiration.

·         Some lipids mediate communication between cells

 

 

Proteins

·         These are compounds of carbon, hydrogen and oxygen. In addition, they also contain nitrogen and sometimes phosphorous or sulphur or both.

·         Some proteins molecules contain other elements. In particular, haemoglobin contains iron.

·         Proteins are made up of amino acids. There are about twenty known amino acids. Amino acids are of two kinds:

a)         Essential- These are those amino acids that cannot be synthesized by the body systems hence have to be supplied in the diet.

b)         Non-essential- These are amino acids that can be synthesized by the body mechanisms hence do not need to be supplied in the diet.

·         An amino acid has an amino group, carboxyl group, hydrogen atom and an alkyl, R group. Amino acids differ from each other by the alkyl group.

·         Proteins are of two kinds:

a)         First class proteins- Contain all essential amino acids

b)         Second class proteins- Proteins lack one or more essential amino acids

Protein synthesis

·         Two amino acids combine through a condensation process to form a dipeptide molecule. Several amino acids link up to form a polypeptide chain. Proteins are made up of long chain polypeptides.

·         Properties of a protein depend on the type of amino acids present in its chain and the sequence in which the amino acids link up in the polypeptide chain.

 

Properties of Proteins

•           They dissolve in water to form colloidal suspensions in which the particles remain suspended in water.

•           They are denatured at temperatures beyond 40°C. Strong acids, bases, detergents and organic solvents also denature proteins.

 

•           They are amphoteric- possess both basic and basic properties.

•           This property enables them to combine with other non protein substances to form conjugated proteins such as:

•           Mucus- Protein plus carbohydrate

•           Haemoglobin- Protein plus iron

 

Functions of proteins

a)         They are structural compounds of the body. Cell membrane is protein in nature. Hair, nails and hooves are made up of protein keratin.

b)         Proteins are broken down to release energy during starvation when all carbohydrate and lipid reserves are depleted.

c)         Functional proteins play vital roles in metabolic regulation. Hormones are chemical messengers while enzymes regulate the speed of metabolic reactions.

d)         Proteins such as antibodies provide protection to the body against infections

e)         Some protein molecules are transport molecules. Haemoglobin molecule plays a crucial role in transportation of respiratory gases.

f)         Proteins play a vital role in blood clotting e.g. fibrinogen.

g)         Contractile proteins such as actin and myosin bring about movement.

 

ENZYMES

What are 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.

The 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.

Examples:

·         Pepsin (Theodor Schwann, German physiologist -1836).

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

·         Trypsin.

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.

Substrates

Amylose (starch)         amylase

Lipids                          lipase

Protein                         protease

Carbohydrate               carbohydrase

Lactose                                    lactase

Processes/Reactions Hydrolysis           hydrolase

Reduction                    reductase

Oxidation                     oxidase

 

 

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

·         Temperature

·         pH

·         Substrate Concentration

·         Enzyme Concentration

 

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

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, t 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.

Examples

NAD- Nicotine Adenine Dinucleotide. FAD- Flavine Adenine Dinucleotide

NADP- Nicotine Adenine Dinucleotide Phosphate

h)         Enzyme inhibitors

This is a chemical substance which slows 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, organophosphates such as malathion.