Proteins have many different biological functions in living systems. Arguably they are primarily what makes us what we are. Our dry mass is significantly protein in nature. So it is at least important if we know alittle about them. Here is a beginners look into proteins.
Classification of proteins
The most varied and most highly specialized proteins are those with catalytic activity-the enzymes. Virtually all the chemical reactions of organic biomolecules in cells are catalyzed by enzymes. M any thousands of different enzymes, each capable of catalyzing a different kind of chemical reaction, have been discovered in different organism. Hexokinase catalyses the transfer of a phosphate group from ATP to glucose – the first step in glucose metabolism. Another example is cytochrome C which transfers electrons toward molecular oxygen during respiration. Also DNA –polymerase and amino-acid-activating enzymes, participate in the biosynthesis of cell components. Each type of enzyme molecule contains an active site to which its specific substrate is bound during the catalytic cycle. Many enzymes contain a single polypeptide chain, others contain two or more.
Transport proteins in blood plasma bind and carry specific molecules or ions from one organ to another. Hemoglobin or erythrocytes binds oxygen as the blood passes through the lungs, carries it to the peripheral tissues and there releases it to participate in the energy yielding oxidation of nutrients. The blood plasma contains lipoproteins which transports lipids between the intestine, liver and adipose (fatty) tissue. Other kinds of transport proteins are present in the plasma membrane and intracellular membranes of all organisms; these are adapted to bind glucose, amino acids or other substances and transport them across the membrane.
Nutrient and storage proteins
Another class of proteins have the function of storing amino acids and using them as building blocks for the growing embryo for eg the seeds of many plants store nutrient proteins required for the growth of the germinating seedling. Particularly well studied examples are the seed proteins of wheat, corn and rice. Ovalbumin, the major protein of egg white and casein, the major protein of milk are other examples of nutrient proteins.
Contractile or Motile Proteins
Some proteins endow cells and organisms with the ability to contract, to change shape or to move about. Actin and myosin function in the contractile system of skeletal muscle and also in many nonmuscle cells. Tubulin is the protein from which microtubules are built. Microtubules act in concert with the protein dynein in flagella and cilia to propel cells
Many proteins serve as supporting filaments, Cables or sheets to give biological structures strength or protection. The major component of tendons and cartilage is the fibrous protein collagen which has very high tensile strength. Ligaments contain elastin, a structural protein capable of stretching in two dimensions. Hair, fingernails and feathers consist largely of the tough insoluble protein keratin. The major component of silk fibers and spider webs is fibroin. Spiders and silkworms secrete a thick solution of the protein fibroin, which quickly solidifies into an insoluble thread of exceptional tensile strength used to form webs or cocoons. The wing hinges of some insects are made of resilin, which has nearly perfect elastic properties
Many proteins defend organisms against invasion by other species or protect them from injury. The immunoglobulin or antibodies, specialized proteins made by the lymphocytes of vertebrates can recognize and precipitate or neutralize invading bacteria, viruses or foreign proteins from another species. Fibrinogen and thrombin are blood clotting proteins that prevent loss of blood when the vascular system is injured. Some proteins are extremely toxic to higher animals in very small amounts. These are called toxins. They include ricin of the castor bean, Snake venoms, bacteria toxins hence have defensive functions. Some of these including fibrinogen, thrombin and some venoms are also enzymes.
Some proteins help regulate cellular or physiological activity. Among them are many hormones. Examples include insulin, which regulates sugar metabolism, other regulatory proteins bind to DNA and regulate the biosynthesis of enzymes and RNA molecules involved in cell division in both prokaryotes and eukaryotes. Hormones are chemical substances produced in one part of plant or animal’s body by ductless (endocrine) glands and produce its effect on distant parts of the body. Much of the integration and regulation of physiological processes in the body is accomplished through these hormones. Examples include; Somatotropin (growth hormone) – a hormone of anterior pituitary gland. Insulin – secreted by certain specialized cells of the pancreas. It regulates glucose metabolism and its deficiency in man causes a disease called diabetes mellitus. Auxins produced at the apical meristem of plants, Adrenaline- fight or flee (it increases heartbeat due to increased productionof ATP), ADH produced by the pituitary glands and acts on kidneytubules to make it permeable to water.