Many kinds of proteins are present in any given cell. Proteins are necessary for every activity in an organism- metabolism, growth, development, and reproduction. Proteins have different functions like structural, transport, storage, DNA regulation and repair, responding to stimuli and contractile. They also act as enzymes, hormones, and antibodies.
Proteins and their structure
Proteins are chains of twenty different amino-acids. The sequence in which the amino-acids are arranged is decided by the genetic code in the DNA, which is transcribed and passed on through the RNAs which then synthesize proteins at ribosomes. The sequence of the of amino acids in a polypeptide chain is also called its primary structure (1, 2). Secondary and then tertiary structuring produce a three-dimensional form to give a protein. If there are more than one polypeptide chains, a quaternary structuring holds them together to give a macromolecule (2).
The sequence of amino-acids and the three-dimensional form of the proteins determine its function (3).
Each cell has many proteins, that determine its structure and function (3, 4). Each protein can have more than one role (5). Proteins have many roles such as
- Storage of material
- DNA regulation
- Responding to stimuli
- Contractile (1, 3, 5).
Structural proteins work at various levels from cell to tissue and the whole body (1, 5). In the cell there are different kinds of structural proteins. Those responsible for maintaining its form.(1, 4). Eukaryote cells that are bigger than prokaryotic cells in particular need them (4). Then there are microtubules that form the mitotic spindle during cell-division, and actin filaments that help in cell movement (4).
Tissues which are hard contain structural proteins, as in the case of bones, horns, hair, or skin etc (2). Structural proteins present in collagen and elastin in tendons and ligaments provide support to the whole body (5). Collagen is important as it connects different tissues and organs in the vertebrate body like bones, muscles, blood vessels and skin, and accounts for 25% of the proteins in a body (6).
Image credit (https://askabiologist.asu.edu/venom/what-are-proteins)
- Transport of material
These proteins bind to electrons, or atoms and transport them within a cell or in the whole body (1, 3). Within the cell, cytochrome help in electron transport during cell mechanisms (3).
These proteins also transport atoms, and nutrients to all parts of a body (2, 5, 6). Waste produced by cells is also transported away by proteins (3). For example, hemoglobin is a protein with a quaternary structure that is found in red blood cells of the vertebrates. It carries oxygen from the lungs to every cell in the body (5, 6).
- Storage proteins
Storage proteins store atoms or molecules at the cellular or tissue level. For example ferritin which as its name indicates binds to iron and stores it. Most the iron in a body is bond to ferritin and this protein is found in liver, spleen, bone marrow, and the blood (7). Some proteins also store amino-acids, like ovalbumin in egg whites, and casein in milk (3).
- DNA regulation
Some proteins are associated with DNA and they regulate the structure of chromosome, and therefore also gene expression, example histones and cohesin (1). Various complexes of proteins are necessary for repair of DNA. For example (MutS?) and (MutS?) in eukaryotes have functions like recognition of strand mismatch, the first step in DNA repair (8).
- Responding to stimuli
Proteins are capable of picking up signals from outside the cell, and then help the cell to react to these stimuli (4). As part of this job, some proteins also regulate movement of certain molecules in and out of a cell, by acting as 'channel proteins' (6).
One of the major role of proteins is as enzymes. They lower the threshold energy needed for biochemical reactions, to levels found in the cells, and thereby catalyse these reactions. Enzymes act by providing active receptor sites for substrate/substrates to bind and help in cleaving molecules or synthesizing bigger molecules (6, 9). Anabolic enzymes build more complex molecules from smaller ones. And catabolic enzymes breakdown large molecules to smaller ones (3). Without enzymes the cells or the organisms would not be able to function or exist (3).
At the level of organs, enzymes are needed for digestion of food, and are found in the saliva or the digestive tract (3). Catabolic enzymes like pepsin and lipase breakdown proteins and lipids respectively in the digestive tract (3).
As hormones, protein act as messenger between different tissues or organs of a body to co-ordinate body functioning (1, 5). They act as receptors and detect concentrations of chemicals, based on which they send signals to attract chemicals if concentrations are low (3). Hormones are secreted by endocrine cells, and are involved in all functions of the organism like metabolism, growth and development (3).
Examples of hormones are insulin, oxytocin, and somatotropin (2). Insulin co-ordinates the amount of glucose cells absorb. This action by insulin regulates the level of sugar in blood (5).
Antibodies are proteins which protect the body against foreign substances and prevent infections (1, 5). They are produced by the immune system of the body to immobilize pathogens like bacteria, viruses, fungus by binding to them; these pathogens are then destroyed by the immune system. Sometimes they are involved against harmful cells of the same organism like cancer cells (2, 10). Antibodies, for example immunoglobulins are specific to each foreign particle, and there are five broad categories of these antibodies in humans (10)
- Contractile proteins
These proteins allow the body to move, and are found in muscles. For example actin and myosin in humans, help in muscle contraction (2, 5).