Proteins: types and functions

  • 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, xipeptide, 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:

 

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.

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.

Proteins: types and functions

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