Proteins- Definition, Classification, Structure, Functions

Proteins are polymers of numbers of amino acids joined together by peptide bonds. Proteins are found in all living organisms. They have high nutritional values and are involved in the various chemical processes that occur in living organisms. There are 20 different amino acids are present in natural proteins. Proteins with similar functions have similar amino acid sequences and compositions.


Proteins are species-specific in the sense that the proteins of one organism differ from the proteins of other organisms. They are also organ-specific in the context that the protein present in the brain differs from the protein present in the liver within a single organism.

Sources of proteins  

  • Cheese
  • Seafood
  • Eggs
  • Beans
  • Soy
  • Lean beef
  • Legumes etc.

Classification of protein

On the basis of structure

On the basis of structure, proteins are classified into two types.

1. Fibrous protein: Fibrous proteins are formed by the linear condensation of neutral amino acids, where protein chains are held together by an intermolecular hydrogen bond. Fibrous proteins are the main structural material of tissues. They are insoluble in other solvents but are soluble in strong acids and bases. Examples: Keratin in skin and hair, the tendon in muscle.

2. Globular protein: They are highly branched and the cross-linked condensation product of amino acids. They are compact spherical molecules in which the peptide chain is stabilized by intramolecular hydrogen bonds. Globular proteins are soluble in water acids and bases. They are crucial for the regulation and maintenance of life processes. Examples: enzymes, hormones, albumin, etc.

On the basis of hydrolysis product

Simple proteins:

Simple proteins are those proteins that give only alpha amino acids during hydrolysis. They are:

a. Albumins: They are soluble in water, acid, and alkalies. These are coagulated by heat. Examples: Seram albumin, egg albumin

b. Globulins: They are insoluble in water but are soluble in inorganic acids and alkalies. They are coagulated on heating and can be precipitated with ammonium sulfate. Examples: vegetable globulin, and tissue globulin.

c. Prolamines:  They are insoluble in water and salt solution but are soluble in dilute acid and alkali. Examples: zein (from maize), hordein (from barley).

d. Glutelins: They are insoluble in water but are soluble in dilute acid and alkali. Example: glutenin (from wheat).

e. Albuminoids: They are insoluble in water and salt solution but are soluble in concentrated acid and alkali. Examples: Keratin (from hair and skin), fibroin (silk).

They are of two types. They are:

 i. Collagens

 ii. Elastins

f. Histones: Histones are water-soluble proteins and are rich in histidine and arginine. Examples: hemoglobin, proteins of nucleic acid.

g. Protamines: They are soluble in water, dilute acids, and ammonia. Protamines are generally found in nucleic acids and are rich in arginine.

Conjugated protein:

Conjugated protein consists of the protein group, which is bounded by another nonprotein group. On hydrolysis, conjugated proteins give alpha amino acids along with the non -proteinous group. The nonproteinous group present in conjugated proteins is known as the prosthetic group, which plays an important role in the biological functions of these proteins.

The conjugated proteins can be further classified into different groups as follows:

a. Glycoprotein: it consists of carbohydrate or its derivative as a prosthetic group. Examples: Collagens, mucins, immunoglobins, antibodies

b. Phosphoproteins: The prosthetic group is phosphoric acid. Examples: Casein, DNA

c. Lipoproteins: Phospholipid is present as the prosthetic group. Examples: adhesins, toxins, structural proteins

d. Nucleoproteins: Nucleic acids is present as a prosthetic group. Examples: ribosomes, nucleosomes

e. Chromoproteins: They consist of colored prosthetic groups. The color of the prosthetic group is due to the presence of metal ions. For example, in hemoglobin, the haemin is present as the prosthetic group while the globin is the protein.

Structure of proteins

Proteins are large macromolecules composed of the chemically building blocks known as amino acids, which are connected by the peptide linkage. The number of amino acids present, their sequence, the shape of the peptide chains, their number and arrangement, and the force with which the peptide chains are arranged determine the structure of proteins.

Structure of protein
Structure of protein

Proteins have four levels of structural organization as follows:

Primary structure

The primary structure of the proteins depends on the number and the nature of amino acid residue sequences. It gives information about the sequence of amino acid residues only but nothing about the conformation i.e. shape of the molecules. The primary structure determines the higher level of structural organization.

Secondary structure

The conformation or shape acquired by the polypeptide chain present in protein, due to secondary bonding is known as the secondary structure. The secondary structure tells about the shape of the polypeptide chain due to the presence of hydrogen bonding and disulfide linkage (-S-S) between different polypeptide chains. The common types of secondary structures are α-helix and β-plated sheets.

α-helix: The α-helix is the rigid, rod-like structure formed by twisting the polypeptide chain into helical conformation. This type of structure is possible when the alkyl group of amino acids is large. The hydrogen bond between -NH and -CO group gives stability to the protein structure.

β-plated sheets: β-plated sheets are formed when two or more polypeptide chain segments are line up side by side in a zig-zag manner. β-plated sheet structure is possible in proteins consisting of amino acids with small R groups.

Tertiary structure

Tertiary structure in proteins arises due to the folding and superimposition of various secondary structural elements. It also refers to the three-dimensional structure of a protein obtained as a result of the interaction between the side chains in their primary structure. Different types of covalent and non-covalent bonding are involved in the stabilization of the tertiary structure of the protein. They are:

1. hydrophobic interaction

2. Electrostatic interaction

3. Hydrogen bonds

4. Vander walls force of interaction

Quaternary Structure

The quaternary structure of the protein consists of any protein in which the native molecules are made up of several different subunits. These subunits also have their own independent three-dimensional conformation. The polypeptide chain is held together by non-covalent interactions like hydrophobic interaction, electrostatic interaction, and hydrogen bonds. They also consist of the covalent cross-link.

Denaturation protein

The primary structure of a protein determines the amino acid sequence, while the secondary and tertiary structures determine the conformation of protein structure. Denaturation is the process of the breakdown of different covalent and non-covalent bonds that are involved in the protein structure. The denaturation process does not involve the breaking of the peptide linkage. Depending upon the degree of denaturation, proteins lose their biological activities.

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Denaturing agents

a. Strong acids and bases: Strong acids and bases cause the protonation and deprotonation of the amino acids’ side group, altering the hydrogen bonding and salt bridging patterns.

b. organic solvent:  Organic solvents like ethanol disturb the hydrogen bonding pattern by forming intermolecular hydrogen bonds.

c. Reducing agents: Reducing agents reduce the disulfide bonds to the sulfhydryl group and break the intra and inter-disulfide linkage.

d. Heavy metals: Heavy metals disturbs the protein structure by disturbing the salt bridge.

e. Heat: Vibration created by the heat causes the disruptions of the weak interactions like Van der Waals interaction hydrogen bonds so causes the denaturation of the proteins.

Solubilities of protein

The solubilities of protein in an aqueous solution depend upon several factors like pH, ionic strength, nature of the protein, and temperature.

Factors affecting the solubilities of proteins

Effect of pH: The protein molecules consist of both the positive and negative charged groups at their surface. T the isoelectric point (pI), the positive and negative charge on the surface of protein molecule candle to each other, and the protein carries no net charge. This reduces the repulsive interaction so the attractive interaction becomes predominant causing the precipitation of the proteins. This is also called isoelectric precipitation.

Effect of ionic strength: The solubility of protein at low ionic strength increases with an increase in salt concentration; this process is called salting in. This is due to the binding of salt ions to protein ionizable groups. But when a large amount of salt is added, it causes protein precipitation. This is called salting out. When salt is added in the excess amount, the water is removed around the protein exposing the hydrophobic groups. The one hydrophobic group interacts with each other resulting in the aggregation and precipitation of the protein.

Effect of solvent: Organic solvents like ethanol decrease the dielectric constant of the aqueous solution, which allows the aggregation of protein molecules by the electrostatic force of attraction.

Temperature:  The solubility of protein increases with an increase in temperature up to 40-50oC.  But at high temperatures, denaturation of protein occurs.

Functions of proteins

Proteins are the building block of many muscles and play an important role in any biological process. The different proteins have different specific functions. Some important functions of proteins are as follows:

i. Enzymes: Enzymes present in the cells are the proteins that catalyze many biological reactions.

ii. Hormones: Hormones present in the body are involved in chemical regulation.

iii Protein as food reservoir: Some protein acts as the food reservoir. Such as the albumin in eggs and Casein in milk.

iv. Structural protein: Many proteins acts as the main structural material of animal tissues. Examples: collagen in  the tendon, keratin in hair and skin.

v. Repressors: Some proteins act as repressors and are involved in the regulation of genes. example: lac repressor.

vi. Antibody proteins: Antibody proteins provides the natural defense to the body against the entry of antigen inside the body system.

Effects of protein deficiency

Low protein consumption has various effects on our health. some of them are as follows:

  • Protein deficiency in children causes malnutrition, such as kwashiorkor and marasmus.
  • It affects proper growth and development
  • Weak muscle tone
  • Skin lesions
  • cause hormonal imbalance
  • Edema



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