Protein Structure and Function

Introduction to Proteins and What is the Primary Function of the Protein

Proteins are large, specialized, and complex molecules which include oxygen, carbon, nitrogen, hydrogen, and sometimes sulfur. Proteins are composed of thousands of smaller units known as amino acids which are attached together to form a long chain of polypeptides (proteins). There are a total of 20 different types of amino acids that combine together to make proteins. These amino acids are identical but have different side chains. The amino acid sequence of proteins determines the unique 3- dimensional structure of each protein and its specific function. The function of the protein in the human body is that it is required for the structure, regulation, and function of the tissues and organs of the body.
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Protein Structure

The structure of a protein is a 3-dimensional arrangement of amino acid residues that link-up to form polypeptide chains. Proteins are polymers whose structure is formed by link-up of several such long chains that are made from amino acid (monomer of protein)  sequences. The position and property of amino acids decide the ultimate structure and function of the protein.
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Amino acids are substituted methane, in which the alpha-carbon valencies are occupied by a carboxyl group (-COOH), amino group (-NH2), hydrogen, and a variable R-group. A variety of amino acids are present depending on the R-group, out of which 20 are used in the making of the polypeptide chain. The structure of a protein is better described by using its types.

Types of Proteins

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Primary Structure 

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The primary protein structure is simply a linear polypeptide chain made up of the sequence of amino acids. Changing even a single amino acid position as there are limited amino acids monomers i.e 20 presents in the human body will result in alteration of the 3-dimensional structure of the protein which further leads to different chains and finally a different protein. This simple sequencing of protein by amino acids is called its primary structure.
For example, Human insulin has two polypeptide chains, A and B.

Secondary Structure

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The secondary structure is the local folded structures formed by interactions (hydrogen bond) between atoms of the polypeptide chain except for atoms of the R-group. This causes the chain to fold or coil and affect the 3-D shape of a protein in two different conformations known as α-helix and β-pleated sheets. Both the structure results due to the hydrogen bonds, which forms between the amino H atom of one amino acid and the carbonyl O of another.

  • α-Helix: The carbonyl group (C=O) in the backbone forms a hydrogen bond with the amino H (N-H) group between every 4th amino acid residue. The bonding pattern forms a helical structure that resembles a curled ribbons within the polypeptide chain.
  • β-Pleated Sheet: This structure is formed by hydrogen bonding between two or more strands of the polypeptide chain which are lined-up next to each other shaping a sheet-like structure. N-H groups form hydrogen bonds with the C=O group, while the R-group either extends above or below the plane of the sheet.

There can be other numerous functional groups that can be linked to each protein like carboxylic acid, alcohols, carboxamides, etc. These functional groups are also responsible for affecting protein folding and its function.

Tertiary Structure

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The final 3-dimensional shape of a polypeptide is called protein tertiary structure. The tertiary structure is mainly due to repulsive and attractive forces of different R-groups of amino acids which make up a protein.
The secondary interactions that are seen in the tertiary structure include ionic bonding, hydrogen bonding, London-dispersion, dipole-dipole interactions.
R-groups that are polar in nature form hydrogen bonds and dipole-dipole interactions. Similarly, R-groups with opposite charges form the ionic bonds.
Non-polar hydrophobic R-groups assemble together within the protein. Disulfide bond also contributes to tertiary structure, by covalent linking between the cysteine chain’s sulfur-containing side. By keeping polypeptide parts to attach firmly to each other, they act as “Molecular safety-pins”.

Quaternary Structure

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Multiple polypeptide chains that are linked together to form certain proteins are called subunits. The orientation and arrangement of subunits which come together with multi-subunits to give the quaternary structure of proteins. Proteins and other macromolecules present in the body interact to form such complex assemblies. These assemblies are required because protein can develop specialized functions in them that stand alone. Proteins are unable to perform transmission of cell signals and carrying out DNA replication. Its example includes:

  • Hemoglobin is used to carry oxygen in the blood. It is a form of two subunits α and β type, a total of four subunits.
  • DNA polymerase is composed of 10 subunits that an enzyme uses to synthesize new DNA strands.

FAQ (Frequently Asked Questions)

  1. Why are Proteins Called Building Blocks of the Body?

Proteins are called building blocks of the body because they are found in abundance throughout the body. They account for 20% total weight of the body and are important for all the functions of the body. They are recruited in all the reactions that are biochemical in nature, taking place inside the cell. Growth and development of the body, making of new cells, repairing damaged cells and tissues all depend upon proteins. Proteins are also present in food like milk, pulses, and egg, etc. It is also present in nails and hairs.

  1. What are the Biological Functions of Proteins?

Functions of Protein in the Human Body are:

  • Support and Structure: Structural proteins such as elastin or collagen provide mechanical support and keratin makes our nails, hairs, etc.
  • Generate Movement: Protein (Myosin) found in muscles enables the muscle contraction and hence makes the movement possible.
  • Act as a Messenger: Proteins also function as chemical messengers, allowing communication between the cells, tissues, and organs via receptors made up of protein that are present on the cell surface.
  • Control Cell Process: Regulatory proteins like enzymes act as a catalyst for controlling cell differentiation and growth.

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