LEARNING OBJECTIVES
In this article, author has explained the introduction, structures, properties, classification and functions of proteins.
Proteins are the most abundant organic molecules found in living organisms. Proteins form about 50% of the cellular dry weight. Proteins contain carbon, hydrogen, oxygen, and nitrogen. These C, H, O, and N are arranged to form amino acids. Amino acids are linked to form long polypeptides called proteins. Proteins are polymers of amino acids.
Structure of protein
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Depending upon the folding of the polypeptide chain, proteins have four types of structures.
Primary structure of proteins
The linear arrangement of amino acids forms the primary structure of proteins. A large number of functions are related to the primary structure of the proteins. The exact arrangement of amino acids is necessary for the proper functioning of proteins. Any change in the linear sequence of amino acids alters the whole protein.
Amino acids are connected by a peptide bond to form the primary structure of proteins. Each gene encodes a specific protein. The sequence of amino acids is determined by the DNA of the gene which encodes protein.
Secondary structure of proteins
The structure formed by the folding of the polypeptide chain and interaction between the atoms of the polypeptide chain is called the secondary structure of proteins. The most common secondary structures are α-helix and β-sheet. These structures are formed by the hydrogen bonding between the carboxyl group of one amino acid and the amine group of the next amino group.
Tertiary structure of proteins
The three-dimensional structure of a polypeptide is called the tertiary structure of proteins. The tertiary structure is formed by the interaction between R groups of amino acids. This interaction can be hydrogen bonding, London-dispersion forces, ionic bonding, and dipole-dipole interaction. The tertiary structure has a disulfide bond which is much stronger than other bonds.
Quaternary structure of proteins
Many proteins are made of single polypeptides but some proteins are made of multiple polypeptide chains. The interaction between these polypeptide chains gives rise to the quaternary structure of the protein.
Hemoglobin is an example of the quaternary structure of the protein. Hemoglobin is made of four polypeptide chains. Two of them are alpha chains and two are beta chains. Another example is the DNA polymerase enzyme. DNA polymerase is composed of 10 polypeptide chains.
Properties of proteins
- Proteins are colorless and tasteless.
- They are crystalline.
- The shape of proteins varies from simple to spherical to long fibrils.
- Proteins have a large molecular weight.
- Because of the large size of proteins, the diffusion rate of proteins is slow.
- They scatter light in the solution.
- When proteins are denatured, they lose their functions.
- Proteins are amphoteric; they act as both acid and base.
- They can form a salt with both cations and anions.
- Proteins are levorotatory; the solution of proteins reflects the plane of polarized light to the left.
- The solubility of proteins depends upon the pH. At an isoelectric point, solubility is the lowest. When acidity or alkalinity increases, solubility also increases.
Classification of proteins
Based on shape proteins are divided into two classes;
Fibrous proteins
Fibrous proteins have structural functions to provide support to the cells or the whole organism. Fibrous proteins are insoluble in water as they contain so many hydrophobic amino acids. Fibrous proteins form supramolecular structures.
Here are some examples of fibrous proteins;
Fibroin; is produced by spiders and insects.
Collagen is the main component of connective tissues. It is also found in many other organs such as tendons and the organic matrix of the bones. Collagen is also present in the cornea of the eyes.
α-keratin is found in the nails, hair, horns, hooves, beaks, and a large part of the outer layer of the skin. The disulfide bonds present in these structures make them stiff and flexible.
Elastin is present in the skin and blood vessels. It is the most elastic fibrous protein.
Globular proteins
Globular proteins are more complex than fibrous proteins. They are more compact and less spherical. Motif, domains, tertiary and quaternary structures are found in these proteins. These proteins are soluble in water.
Hemoglobin, myoglobin, antibodies, enzymes and membrane transporters, receptors are examples of globular proteins.
Functions of proteins
- Proteins are building blocks of the body. They are found in tissues, cells, the outer membrane of the cells, etc. Proteins are present in the hair, nails, skin, and muscles. Proteins strengthen these structures. Protein allows the movement of the body by the contraction of muscles. They are also involved in the movement of food through the digestive system. They are needed for growth, development, healing, etc.
- Proteins are part of our immune system. Antibodies are made of proteins. Antibodies protect our body from antigens and thus prevent diseases and infections.
- Hormones are composed of proteins. Hormones perform so many functions in the body. They are involved in growth, development, reproduction, etc.
- Enzymes are proteins. The enzyme catalyzes all reactions in our body. They regulate many biochemical reactions occurring in our bodies.
- Many proteins work as transporters. They transport organic and inorganic substances inside or outside the membrane, in the extracellular environment, bloodstream, and from one place to another. For example, hemoglobin carries oxygen from one part of the body to another.
- Storage proteins store different types of substances. For example, ferritin stores iron.
- Proteins are involved in nerve transmission.