Proteins, Characteristics, Structure and Types

THE PROTEINS

  1. Proteins are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen and hydrogen
  2. Proteins are the most abundant organic compounds in cells. They are over 50% of total dry weight of the cell.
  3. About 170 types of amino acids are present in the cells and tissues. Of these, about 25 types are involved in the formation of proteins. However most of the proteins are made of 20 types of amino acids.
  4. The unique properties of each amino acid are determined by its particular R group.
  5. The linkage between C of carboxyl group of one amino acid and N of amino group of next amino acid is called peptide bond.
  6. Glycylalanine has two amino acids and is called dipeptide.
  1. Each protein has specific properties which are due to: Number of amino acids, Kinds of amino acids, specific sequence of amino acids and the shape of protein molecule.
  2. F. Sanger was the first scientist who determined the sequence of amino acids in a protein
  3. Insulin is composed of 51 amino acids in two chains. One with 21 amino acids and the other with 30 amino acids. Both the chains are linked by di sulphide bonds.
  4. Hemoglobin is composed of four chains, two alpha and two beta chains. Each alpha chain contains 141 amino acids while each beta chain contains 146 amino-acids.
  5. There are over 10,000 proteins in the human body. These are formed by the specific arrangements of 20 types of amino acids.
  6. The sequence of amino acids is determined by the order of nucleotides in the DNA.
  7. In the sickle cell hemoglobin only one amino acids (Glutamic acid) in each beta chain out of the 574 amino acids does not occupy the normal place in the proteins. Actually glutamic acid is replaced by valine. Therefore hemoglobin fails to carry sufficient oxygen. The result is the death of the patient.
  8. Secondary structure of proteins is the coiling of primary polypeptide chains.
  9. The α helix is very uniform geometric structure with 3.6 amino acids in each turn of the helix.
  10. Tertiary protein structure is formed when a polypeptide chain bends and folds upon itself forming a globular shape.
  11. Amino acids join together in long chains to form proteins by means of peptide bonds. This is an example of a condensation reaction.
  12. Quaternary proteins are polymers of several tertiary structures.
  13. All enzymes are proteins (e.g. amylase, lipase, pepsin etc.). Enzymes control the cell metabolism.
  14. Some hormones are proteins (e.g. insulin). Hormones regulate metabolic processes.
  15. Antibodies (Immunoglobulins) are proteins which protect the body from pathogens.
  16. Blood clotting proteins (like fibrinogens) prevent the loss of blood from the body after injury.
  17. Movement of organs and organisms are caused by proteins (e.g. actin and mysosin etc. are involved).
  18. Movement of chromosomes during anaphase of cell division, are caused by proteins (tubulins are involved).
  19. The term protein is for the finished, functional molecule.
  20. Some proteins consist of one polypeptide, others consist of two or more than two. Hemoglobin, for example, contains four polypeptides.

FOUR LEVELS OF PROTEIN STRUCTURE:

All proteins are complex molecules and biochemists look at their structure at four different levels; primary, secondary, tertiary and quaternary.

Primary Structure:

The primary structure of proteins depends upon the number, kind (types) and sequence of amino acids in a protein molecule.

Real proteins usually consist of a lot more than five amino acids. The hormone insulin, for example, a relatively small protein, has 51 amino acids.

The code for the primary structure of any protein is contained in the gene. This code determines the order in which amino acids are assembled. This order then dictates the way they will twist and turn to produce the three-dimensional shape that allows the protein to carry out its specific function.

Secondary Structure:

The first level of three dimensional twisting is described as the secondary structure of the protein.

When combinations of amino acids join together in a chain they fold into particular shapes and patterns (such as spirals).

These shapes form because the amino acids twist around to achieve the most stable arrangement of hydrogen bonds. The main types of secondary structure in proteins are:

  • The a-helix, a spiral, is the most common type of secondary structure. The hydrogen bonds stabilize the α-helix.
  • The β-pleated sheet, a flat structure that consists of two or more amino acid chains running parallel to each other, linked by hydrogen bonds.

The secondary structure of a protein depends on its amino acid sequence; some amino acids produce α-helices. Others usually make β-pleated sheets.

Tertiary Structure:

The tertiary structure of a protein is its overall three-dimensional shape and is produce as a result of the following:

  • The sequence of amino acids that produces α-helices & β-sheets bends at particular places.
  • The hydrophobic nature of many amino acid side chains. Globular proteins are surrounded by water and so the hydrophobic side chains tend to point inwards.

Tertiary structure is maintained by ionic bonds, hydrogen bonds and disulfide (-S-S-) bonds.

Functional proteins, such as enzymes and antibodies, must have an exact shape-and sometimes the ability to change shape — to fulfill their role in the organism.

Many structural proteins depend on their tertiary structure for strength. The large number of disulphide bridges in keratin, of example, makes body structures such as hair and nails very tough.

If a protein consists of only one polypeptide, the tertiary structure is the final shape of the molecule. If, however, the protein has more than one, it has a further, quaternary level of structure.

Quaternary Structure:

Quaternary proteins are polymers of several tertiary structures.

In quaternary structure, the highly complex polypeptide tertiary chains are aggregated and held together by:

(i) Hydrophobic interactions,

(ii) Hydrogen bonds and

(iii) Ionic bonds.

FIBROUS AND GLOBULAR PROTEINS

The final three-dimensional structure of proteins results in two main classes of protein — fibrous and globular.

  • Fibrous proteins contain polypeptides that bind together to form long fibres or sheets. They are physically tough and are insoluble in water.
  • Globular proteins are usually individual molecules with complex tertiary and quaternary structures. They are spherical, or globular in shape, hence the name. Most are soluble in water and they have a biochemical function.

How Stable are Proteins?

As the final shape of globular proteins is maintained by relatively weak molecular interactions such as hydrogen bonds, proteins are very sensitive to temperature increases and other changes in their environment.

As the temperature goes up beyond 40C, molecular vibration increases and bonds that are holding together the tertiary or quaternary structure break, changing the shape of the molecule. This is known as denaturation.

Different proteins are denatured at different temperatures. Some begin to be denatured after about 40-45C or even below, but many are not totally denatured until 60C or even higher.

It is an oversimplification to say ‘organisms die at temperatures over 44C because their proteins become denature’. In practice organisms die because of a metabolic imbalance, caused when enzymes work at different rate.

Proteins can also be denatured by adverse chemical conditions. Chemicals that affect weak bonds change the overall structure. Even a slight change in protein shape mean loss of function. Some proteins are particularly sensitive to changes in pH.

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