Proteins: Definition, Classification and Function

Proteins are a kind of substance that has presence of amino acids, carbon, nitrogen, hydrogen, oxygen, compounds. In some cases proteins also do contain sulphur. They are mainly found in food, for example, meat, beans, eggs, etc. Proteins are the building blocks of life and form 60% of dry cells in the body and form the fundamental basis of structure and functions of life. Chief sources of protein are eggs, pulses, fish, meat, milk and other milk products which in turn help in growth and maintenance of the body. “Proteios” is a Greek word meaning something which is of prime importance and the term ‘protein’ is derived from it.

Key Terms: Protein, compounds, unit of life, cells, amino acids, peptide bond, amino acids, carbon, nitrogen, hydrogen, oxygen, sulphur

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Building Blocks

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Amino acids are the basic blocks which form protein molecules formed by linking the carboxyl group (-COOH) of one amino acid with the amino group (– NH₂) of another amino acid releasing a molecule of water (H₂O). The resultant bond formed is called peptide bond and can belong to the α, β, γ, δ category. Hydrolysis of proteins leads to generation of α-amino acids. Amino acids are usually colourless, crystalline solids. These are water-soluble, high melting solids and act like salts rather than simple amines or carboxylic acids.Depending upon the relative position of amino group with respect to carboxyl group dipeptide, tripeptide, and polypeptide linkages are formed. When two amino acids are linked together a dipeptide linkage is formed. While three, four, five and six amino acid linkages lead to generation of tripeptides, tetra peptide, penta peptide & hexa peptide respectively. On the other hand, more than ten amino acid linkages produce polypeptides with more than 100 amino acid residues. They have molecular mass greater than 10,000, or less than 100 amino acid remains. They also have a precise shape and are referred to as proteins. Amino acids may be of three different kinds: acidic, basic or neutral. The nature of the amino acid depends upon the respective number of amino and carboxyl groups present in their molecule. When amino and carboxyl groups share equal numbers the amino acid is treated as neutral, while greater number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups generate acidic properties. Essential amino acids are those which cannot be synthesized in the body and must be obtained through diet.

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Classification of Proteins

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Proteins can be classified into various categories on the basis of the following parameters:

• Shape
• Constitution
• Nature of molecules

Based on shape, proteins can be further classified into

• Fibrous protein (Scleroprotein)

These proteins are insoluble in water and are resistant to proteolytic enzymes and are coiled and exist in threadlike structures to form fibres. e.g. collagen, actin, and myosin, keratin in hair, claws, feathers, etc These proteins are mainly available in animals.

• Globular proteins

These are water soluble proteins, made up of polypeptides that are coiled about themselves to form oval or spherical molecules e.g. albumin, insulin, and hormones like oxytocin, etc.

Based on constitution, proteins can be further classified into

• Simple proteins

These types of proteins have amino acids as their only constituent e.g. albumins, globulins, prolamins, etc.

• Conjugated proteins

These kinds of proteins are complex in nature and are fused with the characteristics of a non–amino acid substance known as the prosthetic group. These can be nucleoproteins, mucoproteins, glycoproteins, chromoproteins, lipoproteins, metalloprotein, phosphoprotein depending upon the percentage of combination of proteins and nucleic acid/ proteins and Carbohydrates/ proteins and coloured pigments/ proteins and lipids/ proteins and metal ions/ proteins and phosphate group.

• Derived proteins

Hydrolysis of proteins by acids, alkalies or enzymes, lead to generation of derived proteins.

Based on nature of molecules, proteins can be further classified into

• Acidic proteins

These proteins exist as anions and are rich in acidic amino acids. e.g. blood groups.

• Basic proteins

These proteins exist as cations and are rich in basic amino acids e.g. lysine, arginine etc.

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Structure of Proteins

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Proteins are polymers of α-amino acids connected to each other by peptide bond or peptide linkage. This linkage is an amide formed between the –COOH group and –NH2 group. The reaction between the amino group of one molecule with the carboxyl group of the other leads to the generation of peptide bonds –CO–NH- with the release of water molecules. Peptides can be dipeptide, tripeptide, and polypeptide depending upon the relative position of the amino group with respect to the carboxyl group. Polypeptides that have fewer amino acids are known as proteins. They have a well structured conformation of the protein such as insulin which contains 51 amino acids. Amino acids are substituted methane whose four valencies are occupied by hydrogen, amino group, carboxyl group and the remaining one by a variable R- group. The R group controls the variety of amino acids, out of which 20 are found in a polypeptide chain. These properties control the ultimate structure of protein.

Structure of protein can be classified into primary, secondary, tertiary and quaternary based on ascending order of their complexity.

• Secondary structure of protein: Secondary structure of proteins refers to the shape where a long polypeptide chain exists.
• Tertiary structure of protein: Tertiary structure of proteins refers to further folding of the secondary structure. Hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction stabilize the structure of this protein.
• Quaternary structure of protein: Quaternary structure deals with spatial arrangement of two or more polypeptide chains relative to one another.

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Functions of Protein

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Protein plays a crucial role in proper functioning of body. Few of the prominent functions of protein are:

Enzymes –  Enzymes regulate different biochemical reactions occurring in the body. For instance digestive enzymes which are composed of protein help in digestion.

Movement –  Contraction and expansion of muscles for making movement is controlled by myosin, a protein found in muscles.

Structure and Support – Proteins are essential components of various structures in the cell and tissues Growth of hair, nails and horns in animals is attributed to keratin, a structural protein found in animals.

Protective – Antibodies play a vital role in protecting our body against antigens and pathogens. Proteins are the main constituent of these antibodies.

Hormonal regulation – Hormones which are composed of proteins play vital role in regulating muscle mass, sex hormones, growth and development.

Cellular communication –  Cells communicate with one another via receptors present on the surface of cells. These receptors are made of proteins.

Act as a messenger –  These proteins function as the chemical messengers, which help in the communication between the cells, tissues, and organs.

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