PROTEIN

 Protein

Protein is an organic compound made up of a chain of amino acids bonded together by the peptide bond. Simply, it’s a linear chain of amino acid residues. The term protein is derived from the Greek word proteios, meaning holding the first place. Berzelius (Swedish chemist) suggested the name proteins to the group of organic compounds that are of utmost importance to life.

Proteins are large, complex macromolecules made up of amino acids joined together by peptide bonds. Biochemically, proteins are polymers of α-amino acids, typically containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S). They are synthesized based on the genetic code (DNA) via the process of transcription and translation, and they perform a vast range of functions in living organisms. E.g.- albumin, globulin.

Amino acids are organic compounds that combine to form proteins. They are building blocks of protein. They are better defined as organic compounds containing functional groups, such as amine (-NH2) and carboxyl (-COOH), along with a side chain (R group) and hydrogen. E.g., Glycine, Alanine.

As many as 300 amino acids occur in nature. Of these, only 20—known as standard amino acids are repeatedly found in the structure of proteins, isolated from different forms of life—animal, plant, and microbial.

General structure of amino acids

The amino acids are termed as α-amino acids if both the carboxyl and amino groups are attached to the same carbon atom, as depicted below

There are altogether 20 different types of amino acids that can be combined to make a protein. There are two main classes based on their dietary requirement in the diet. They are

Essential amino acids-They cannot be synthesized in the body. They have to be supplied in the diet. E.g., methionine, arginine, threonine, tryptophan, valine, isoleucine, leucine, phenylalanine, histidine, and lysine. Of the ten listed above, two amino acids, namely arginine and histidine, can be partly synthesized by adult humans – hence called Semi-essential amino acids.

Non-essential amino acids- The other ten amino acids can be synthesized in the body, hence called Non-essential amino acids. E.g., glycine, alanine, serine, tyrosine, cysteine, glutamic acid, aspartic acid, asparagine, glutamine, and proline.

Functions of protein

Caloric value:- One gram of protein provides 4 calories of energy

Transport Proteins: Proteins are involved in the transport processes. For example, Hemoglobin in erythrocytes is involved in the transport of O2. Lipoproteins  transport lipids.

Nutrient and Storage Proteins: - Some proteins are used as nutrients, Eg, ovalbumin of egg white and casein of milk are used as nutrients.

Contractile Proteins: - Some proteins, like actin and myosin, are involved in the contraction of skeletal muscles.

Regulatory Proteins:- Many hormones are proteins in nature. E.g., Insulin, which regulates sugar metabolism.

Defense Proteins: Proteins have a protective role in the body. Immunoglobulins and interferons are proteins that protect humans against bacterial and viral infections.

Enzymes:- Most of the enzymes involved in the biochemical reactions in the body are protein in nature.

Regulation of genes:- Proteins are seen in association with DNA molecules, where it control the gene transcription and translation.

Structural proteins:- Structural proteins like collagen and elastin provide structural strength and elasticity to organs and the vascular systems.

Classification of proteins

Based on chemical composition.

Simple protein: The simple proteins are those that are made of amino acid units only joined by a peptide bond. Upon hydrolysis, they yield a mixture of amino acids and nothing else, i.e, it yield only amino acids and no other major organic or inorganic hydrolysis products.

Conjugated Protein: Conjugated proteins are composed of simple proteins combined with a non-proteinous substance. The non-proteinous substance is called a prosthetic group. The prosthetic group may be either a metal or a compound. Their further classification is based on the nature of the prosthetic group present.

E.g. Nucleoprotein (a protein containing Nucleic acids)

Lipoprotein (a protein containing lipids)

Phosphoprotein (a protein containing phosphorus)

Metalloprotein (a protein containing metal ions of Fe2+)

Glycoprotein (a protein containing carbohydrates)

Derived Protein: These are not naturally occurring proteins and are obtained from simple proteins by the action of heat, enzymes, and chemical agents. Examples: peptones, peptides, proteoses, etc.