Proteins: types and functions

August 4, 2018

Introduction

They are an integral part of protoplasm and hence are present in every living cell.
They account for 14% part of living and 75% part of dead and dried animal body.
These are the major components of the body and more important for anabolism than for catabolism.
These are compounds of C, H and O but in addition, these essentially contain about 16% nitrogen and may also contain traces of sulphur, phosphorus, iodine and iron, etc.
Protein is polymer of very large molecular mass, composed of one or more polypeptide chains and whose monomers are amino acids, joined together by peptide bonds.
In addition, some have covalent ‘sulphur bonds’ formed by oxidation of between two cysteine radicals, in the polypeptide.
Biological polypeptides are often several hundred amino acids long. So few of the possible polypeptides actually occur in organisms.
Linking by peptide, amino acid molecules form dipeptide, tripeptide, oligo-peptides and polypeptide.
The latter than link with each other forming first the peptones, then proteoses.
Various proteins of biological system can be classified into three categories.
Similarly, based on physico- chemical properties, proteins may be classified into three types. They are:

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Types of proteins

A) Simple proteins

These proteins are made up of amino acids only as they give amino acids or their derivatives after hydrolysis.
e.g., globulin, albumin, collagen, etc.

i) Globular proteins

The polypeptide chains are folded into compact globular or spherical shapes in the molecules of these proteins.
Hence, the length to breadth ratio of molecules is usually 1:3 or 4 (never more than 1:10).
That is why, these proteins are non-contractile and soluble in aqueous systems forming colloidal solutions and easily diffusible.
Examples of globular proteins are: all enzymes, many hormones, the antibodies, albumin, globulin, globin of haemoglobin, myoglobin of muscles, histones of nucleoproteins, glutelins, prolamines, e.tc.

ii) Fibrous proteins

The length to breadth ratio is always more than 1:10 in the molecules of these proteins.
Hence, these are insoluble structural proteins that make the body architecture.
For examples: collagen, elastin, reticulin of connective tissues, tendons, ligaments; keratin of skin, horns, nails, feathers; fibroin of silk; the actin and myosin of muscles; fibrinogen of blood plasma, etc.
Collagen is the most abundant of the body protein.

B) Complex or conjugated proteins

These are compounds of simple proteins conjugated with prosthetic groups. They are of various types:
Phospoproteins: compounds of simple proteins and phosphoric acid. Casein of milk and vitellen of egg yolk are examples.
Nucleoproteins: These form chromatin of chromosomes in nuclei of cells.
Glycoproteins or mucoproteins and proteoglycans: mucus found in connective tissues, cartilage, saliva, etc. Glycoproteins are the largest and most abundant group of conjugated protein.
Chromoproteins: Haemoglobin and haemocyanin of blood and cytochromes of mitochondria are examples.
Lipoproteins: HDL, LDL, IDL, VLDL, ULDL are the examples of plasma lipoprotein particles.

C) Derived proteins

Proteoses and peptones are common examples of this group of protein.
These are smaller polypeptide chains formed as temporary by- products during protein digestion.
Proteins cannot diffuse through the cell membrane. Only amino acid can diffuse. Of the twenty amino acids used by human, only ten are obtained from food.
Other 10 are synthesized inside the body themselves.
Those obtained from food are called ‘essential amino acids’. Food whose proteins yield all essential amino acid on digestion is called complete food.
Food proteins having all amino acids require for synthesis of all structural proteins are referred to as adequate proteins.
Food proteins whose amino acid monomers can be used only for deamination and energy production re called inadequate proteins.

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Functions of proteins

Enzymes like trypsin and pepsin help in catalysis of biochemical reaction.
Hemoglobin of RBC helps in transporting oxygen from lungs to various tissues through blood stream and also provide red colour to RBC.
Myoglobin of muscles stores oxygen until it is needed for energy production.
Myosin and actin are involved in muscle movement.
Collagen and keratin provide structural and protective function in hairs, teeth, nails, etc.
Insulin has an important role in maintaining the sugar level in the blood and deficiency of insulin leads to diabetes.
Antibodies recognize and destroy foreign bodies which are also protein.
Casein and Ferritin stores protein in milk and iron in spleen and liver respectively.

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I am Shailesh Koirala