Protein accounts for about three-fourths of the dry matter in human tissues other than fat and bone. It is a major structural component of hair, skin, nails, connective tissues, and body organs. It is required for practically every essential function in the body. Proteins are made from the following elements; carbon, hydrogen, oxygen, nitrogen and often sulphur and phosphorus. Proteins cannot be stored except in eggs and seeds and they form the body’s main structural elements and are found in every cell and tissue. The human body uses proteins for growth and to build and repair bones, muscles, tissue, skin, internal organs and blood. Hormones, antibodies and the enzymes that regulate the body’s chemical reactions are all made of protein. Without the right proteins, blood won’t clot properly and cuts won’t heal and if carbohydrates and fat can’t meet your energy needs, proteins can be broken down and used as a source of emergency energy. So this makes protein very crucial for your health.
Each protein is a large complex molecule; these molecules are made up of a string of amino acids. There are 20 different amino acids that occur naturally to form proteins and they all have the same basic structure. The 20 amino acids the body needs can be linked in thousands of different ways to form thousands of different proteins, each with a unique function in the body. Both the amino acids manufactured in the liver and those derived from the breakdown of the proteins we eat are absorbed into the blood stream and taken up by the cells and tissues to build new proteins as needed.
The functions of different Proteins
Proteins have many functions in the human body and these include transport, catalysis, protection, storage, sensitivity, structure and co-ordination.
They serve as enzymatic catalysts that speed up biochemical reactions while remaining unchanged in the process. Without these biological catalysts, chemical reactions would occur so slowly that life as we know it could not exist. With them, chemical reactions can occur at rates as much as 10 billion times faster than would be possible without enzymes. Enzymes are critical to digestion and metabolism, they are required to release nutrients from foods so they can be absorbed and utilized by the body. If enzymes are not present in sufficient quantities, complete digestion cannot take place. Enzymes also keep the body’s metabolic “machinery” running smoothly. In turn, vitamins and minerals are essential for proper enzyme functioning.
Proteins are also used as transport molecules, such protein is haemoglobin which transports oxygen in red blood cells all around the body. The main haemoglobin in adult humans is Haemoglobin A and it contains two alpha and two beta subunits. Haemoglobin also transports carbon dioxide away from the tissues to the lungs where it is exhaled.
Haemoglobins are a chain of polypeptide which is held in position by three types of bonds; disulphide bond, ionic bond and hydrogen bond. Haemoglobins are formed when four Globin molecules link together.
Regulatory proteins such as insulin which are hormones regulate blood sugar levels. These kinds of proteins are used by people who have diabetes. Vasopressin is another kind of hormone of the posterior pituitary gland which controls blood pressure and controls the amount of urine secreted by the kidneys.
Proteins cannot be stored except in seeds and eggs, Ferritin is a storage protein which holds iron in egg yolk, spleen and liver. The protein albumin acts as water storage in egg white. Another storage protein is myoglobin which stores oxygen in the muscles
Fibrinogen and Prothrombin are protective proteins essential for the clotting of blood. Another protective protein is antibodies which are part of the immune system; they reduce the harmful effects of foreign proteins or are known as antigens. The production of antibodies is one of the main ways in which the white cells in the body respond to viruses and bacteria. Antibodies recognise these harmful micro organisms and attach to particular molecules on their surfaces, this then triggers off mechanisms that lead to their destruction and the infection is controlled.
These proteins can be found in poisonous animals such as a cobra which has venom that is toxic and can kill by blocking nerve functions. It is the venom which is the protein so this tells you that there are good proteins as well as bad.
Myosin is a protein responsible for muscle contraction; it combines with actin, another muscle protein, forming actomyosin in the muscle tissue. The different filaments of actomyosin shorten, causing the contracting action of the muscle. This is known as locomotion
The structure of a protein
The most basic level of protein structure, called the primary structure, is the linear sequence of amino acids. Different sequences of the acids along a chain, however, affect the structure of a protein molecule in different ways. Forces such as hydrogen bonds, disulphide bridges, attractions between positive and negative charges cause a protein molecule to coil or fold into a secondary structure, examples of which are the so-called alpha helix and the beta pleated sheet. When forces cause the molecule to become even more compact, as in globular proteins, a tertiary protein structure is formed. When a protein is made up of more than one polypeptide chain for example haemoglobin it is said to have a quaternary structure.
Collagen, which makes up bone, skin, tendons, and cartilage, is the most abundant protein found in humans. The molecule usually contains three very long polypeptide chains, each with about 1,000 amino acids that twist into a regularly repeating triple helix and give tendons and skin their great tensile strength. When long collagen fibrils are denatured by boiling, their chains are shortened to form gelatine.
Keratin, this protein gives mechanical support to the body. It makes up the outermost layer of human skin, hair, and nails, and the scales, hooves, and feathers of animals. It twists into a regularly repeating coil called an alpha helix. Serving to protect the body against the environment, keratin is completely insoluble in water. Its many contains disulphide bonds which make it an extremely stable protein.