Protein - Technical Details

Proteins are large organic compounds essential to life. They are made up of complex combinations of amino acids and are the most common macro-molecules found in cells.
Proteins are obtained from animal and vegetable matter, but they can also be made synthetically. Many commercial products are at least partly made up of proteins. Proteins are in such diverse substances as milk and mushroom poisons. The molecular building blocks of all proteins are amino acids and they consist of atoms of carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur.
Amino acids are linked by peptide bonds to form what is called a polypeptide chain. A peptide bond is a link between the amino, NH2, group of one amino acid and the carboxyl, COOH, group of another. Most proteins found in living organisms are composed of 20 different kinds of amino acids. Thus, a large number of polypeptide chain combinations are possible.

The precise number of protein types in the human body is unknown. However, scientists have isolated and described several thousands of them. Some are involved in the processes of growth, movement, reproduction, repair, digestion, and ageing. Many proteins are enzymes; compounds that accelerate chemical reactions in and around cells. By following the instructions coded in genes, cells use their molecular machinery to build the proteins required by the organism. In order to do this, they need a supply of amino acids. Plants and animals differ in their ability to manufacture their necessary amino acids. Some plants are capable of producing all of their life-essential amino acids. Human adult cells, however, can produce only 11 of their 20 necessary amino acids. The remaining nine called essential amino acids are obtained by eating foods that already contain them.

Some proteins, called transport proteins, carry substances from one place in the body to another. Haemoglobin, for example, is a transport protein in red blood cells that picks up oxygen as it circulates through lung tissue and then carries it to the body's cells. Other transport proteins are located in cell membranes. These proteins shuttle nutrients and waste products from one side of the membrane to the other. Many proteins make up the supportive elements that provide biological structures with strength and protection. Regulatory proteins help to prevent the depletion of nutrients in tissues or the harmful accumulation of the products of respiration.

Structure

Each type of protein has its own unique sequence of amino acids. This sequence is known as its primary structure and determines the shape and function of the protein. The chemical properties of the amino acids determine how the polypeptide chain bends, twists, and folds. If both ends of an outstretched polypeptide chain were held and then released, the chain would spontaneously fold into the structure crucial to the protein's function.
Secondary structure refers to the arrangement of amino acids in space and is a function of the angles formed by the peptide bonds. Helical and zigzag structures are examples of secondary structures.
The tertiary structure refers to how the various helical, zigzag, and less ordered regions of a polypeptide chain fold back upon themselves to form a compact, spherical, or globular shape. Tertiary structure is determined largely by the side chains of the amino acids. Side chains that carry opposite electrical charges attract one another and form ionic bonds; those with like electrical charges repel one another. Permanent hair waving is achieved by altering the tertiary structure of a protein in hair. Quaternary structure refers to the way in which several different polypeptide chains assemble into larger protein complexes.

Types

There are two common groups of proteins: fibrous and globular. In fibrous proteins, polypeptide chains are arranged into long strands or sheets. The fibrous proteins collagen and elastin are essential to connective tissues, including tendons, cartilage, bone, and the deeper skin layers. Leather is almost pure collagen.
Other fibrous proteins, such as tubulin, are the building blocks of microtubules, tiny hollow tubes inside cells. Microtubules play a role in cell movement, in material transport within nerve cells, and in the maintenance of cell shape. Fibrin, derived from fibrinogen, is a fibrous protein that binds platelets together to form blood clots. Actin and myosin are fibrous proteins that play a major part in skeletal muscle contraction.
Globular proteins consist of amino acid chains that are tightly folded into a spherical or globular shape. Unlike most fibrous proteins, they have many electrically charged groups of atoms exposed to cytoplasm and bodily fluids. This feature makes many globular proteins highly soluble.
Immunoglobulins, or antibodies, make up perhaps the largest category of globular proteins. Most enzymes are globular proteins. They catalyse the hundreds of reactions that together constitute cellular metabolism. Through these enzymatic reactions, cells are able to generate, conserve, and transform chemical energy for other processes, such as nutrient metabolism and the production of large molecules from smaller ones.
Some globular proteins are hormones, or chemical messengers made in endocrine glands. Hormones are circulated to target tissues, where they stimulate biochemical or physiological responses. The hormones insulin and glucagon, for example, maintain safe glucose levels in the blood.

Amino Acids

All living organisms, from plants to animals, have in common the fact that they synthesise protein by using the same 20 amino acids, even though more than 100 amino acids occur in nature. If proteins are the bricks from which the body is made, then amino acids are the grains of sand and clay that make up those bricks. Chemists define an amino acid as any group of organic molecules that consist of a basic amino group (-NH2), an acidic carboxyl group (-COOH), and a specific organic side chain that is unique to each amino acid. Arginine, glycine, cystine, histidine, and tryptophan are a few examples of amino acids.
The human body is unable to synthesise the nine so-called essential amino acids. Therefore humans must make sure they get these amino acids by eating a proper diet, with a good balance of grains and legumes (peas and beans). Deprivation of all amino acids leads to an overall loss of protein from the body, resulting in malnutrition, weight loss, anaemia (low blood iron), and muscle deterioration. Specific disorders are associated with deficiencies of particular amino acids. Inherited disorders of amino-acid metabolism are generally defects in either transport or breakdown of amino acids.
The remaining 11 so-called non-essential amino acids are synthesised in the human body by a reaction called transamination. In this process, the organic chemical deoxyribonucleic acid (DNA) normally involved in hereditary processes directs the placement of amino acids in a specific sequence to form a molecule of protein. Amino acids are joined by special bonds to yield such proteins as keratin (the principal component of hair), egg albumin (present in egg whites), casein (a major milk protein), enzymes, hormones, and other molecules of special biological activity. Most of the common proteins contain more than 100 amino acids. For example, haemoglobin, the oxygen-carrying component of blood, is made of 287 amino acids arranged into four chains.

Further Reading : -

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