What is protein?
Proteins are polymers of α-amino acids bonded by peptide linkages. Their molecular load range from 5000 to many millions. They occur in all living cells. Without proteins, life would not be possible.
Points to Remember.
- Proteins are polypeptides.
- Proteins undergo hydrolysis to give a-amino acids.
- Proteins are present in muscle, skin, hair, and other tissue that make up the body’s nonbony structure As enzymes they catalyze biochemical reactions; as hormones, they regulate metabolic processes.
Isolation of proteins.
Proteins invariably occur as mixtures in the animal (or vegetable) source material. It is treated with a solvent and sodium chloride, sodium carbonate, or sodium hydroxide in very low concentrations. From the solution so obtained, proteins are precipitated by a suitable reagent like ethanol, ammonium or magnesium sulfate or phosphotungstic acid. The mixture of proteins is then separated by column chromatography. Careful control of pH and cataphoresis are amongst the modern techniques used for securing individual proteins from mixtures.
Importance of proteins.
Proteins are indispensable components of all living things, including the simplest forms of life, such as bacteria, algae, and other microorganisms. They are required in the diets of all animals, including humans, to synthesize tissues, enzymes certain hormones, and some blood components, In addition, they are used in the maintenance and repair of existing tissues and as the source of power for the body.
Structure of proteins.
Proteins have a complex 3-dimensional structure. It is analyzed step-by-step at four different stages, rather than as a whole.
Primary Structure. The primary structure of a protein relates to the sequence of amino acids. The amino acid sequence in many proteins have been determined and the following observations have been made:
- A protein may contain more than 1 amino acid chain.
- The sequence of amino acids in the protein chains is essentially random.
- Small variations in the sequence of amino acids have pronounced effects on the properties of the protein.
Secondary Structure. The secondary structure of a protein refers to the shape in which long amino acid chains exist. Many proteins exist. Many proteins consist of an amino acid chain coiled into a spiral known as an a-helix. The spiral is held together by hydrogen ties between N-H and C=O groups vertically adjacent to one another in the helix. See Fig. 1.1.
Tertiary Structure. An α-helix may be considered to be a piece of rope that is free to bend, twist, and fold. The tertiary structure of a protein refers to the final 3-dimensional shape that results from twisting, rotating, and foldable of protein helix.
Quaternary Structure. Complex proteins are often formed from two or more amino acid chains rather than a single amino acid chain. Each chain is a complete protein with a characteristic primary, additional, and tertiary structure. The quaternary structure refers to the way in which these amino acid chains of a complicated protein are associated with each other.
Denaturation of proteins.
The physical properties of proteins can often be altered by heating, or by adding salts, dilute acids or other chemicals. Proteins changed in this way are said to be denatured and the process is called denaturation. A simple example is the heating of white of an egg which is coagulated to solid mass (denatured protein). There are two important effects of denaturation :
- Denaturation does not after the primary structure of proteins. It only changes the secondary and tertiary structure of the protein. The highly ordered natural protein structures are converted to more random conformations.
- Denaturation results in loss of biological activity of proteins.