Proteins

Here’s a fact that will boggle your mind. There is a retinal protein named after the cartoon character Pikachu! It is called Pikachurin. In fact, proteins are known to have unique names. Proteins are complex biomolecules that are made up of smaller units known as amino acids. Let us learn about them in detail.

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Structure of Proteins

Due to different rearrangement of amino acids, the structure of proteins divides into four types:

• Primary- the covalent linkages of the proteins

(Source: Wikipedia)

• Secondary- the linear peptide chains fold either into an alpha-helical structure(coiled) or a beta-pleated structure(sheets) which contain hydrogen bonds.

(Source: Wikipedia)

• Tertiary- The arrangement and interconnection of proteins into specific loops and bends forms the tertiary structures. This structure contains hydrogen, ionic and disulfide bonds.
• Quarternary- this structure is proteins containing more than one peptide chain.

Proteins are made up of smaller units known as amino acids and the bond linking them is known as a peptide bond. This bond is formed when the carboxyl group (-COOH) of one amino acid bonds with the amino group (– NH₂) of another amino acid releasing a molecule of water (H2O).  A peptide may be dipeptide, tripeptide, and polypeptide.

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Classification of Proteins

Classification of proteins is done on the basis of the following:

• Shape
• Constitution
• Nature of molecules

On the basis of shape

• Fibrous protein(Scleroprotein): We can find these proteins in animals and are insoluble in water. Fibrous proteins 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.
• Globular proteins: These proteins, unlike fibrous proteins are soluble in water. They are made up of polypeptides that are coiled about themselves to form oval or spherical molecules e.g. albumin, insulin, and hormones like oxytocin, etc.

On the basis of Constitution

• Simple proteins: These proteins are made up of amino acids only. e.g. albumins, globulins, prolamins, etc.
• Conjugated proteins: These are complex proteins that are combined with the characteristic of non–amino acid substance called as a prosthetic group. These are of following types:–
  • Nucleoproteins: Combination of protein and nucleic acid
  • Mucoproteins: Combination of proteins and carbohydrates (>4%)
  • Glycoproteins: Combination of proteins and carbohydrates(<4%)
  • Chromoproteins: Combination of proteins and coloured pigments.
  • Lipoproteins: Combination of proteins and lipids.
  • Metalloprotein: Combination of proteins and metal ions.
  • Phosphoprotein: Combination of proteins and phosphate group.
• Derived proteins: When proteins are hydrolyzed by acids, alkalies or enzymes, the degradation products obtained from them are called derived proteins.

On the basis of nature of Molecules

• Acidic proteins: They exist as anion and contain acidic amino acids. e.g. blood groups.
• Basic proteins: They exist as cations and are rich in basic amino acids e.g. lysine, arginine etc.

Functions of Proteins

• Structural functions: Proteins are called as the building blocks of the body. They are an essential component of various structures in the cell and tissues. We also find these proteins in the outer membrane of all cells in the human body. We can also find structural proteins in hair, skin, and muscles. Proteins often act to strengthen these structures. Proteins working together can allow movement within the body, such as contraction of muscles and movement of food through the digestive system etc. They are needed for the growth, development, healing, and repair of tissues.
• Protective: Proteins are the main constituent of antibodies that protect our body against antigens and pathogens thus preventing infections.
• Hormonal regulation: Hormones are majorly composed of proteins. Hormones play a vital role in regulating muscle mass, sex hormones, and growth and development.
• Enzymes: Proteins are called as biological buffers because they, as enzymes, regulate many different biochemical reactions that are occurring in the body.

Solved Example for You

Q: Peptide bond form between two amino acids through

(a)    Addition of water          (b) Loss of water

(c)    Decarboxylation           (d) Deamination

Sol. (b) loss of water.

The formation of a peptide bond is a dehydration reaction where a molecule of water is released. Therefore, the correct answer is the option (b).