There are many different kinds of proteins are present in any given cell. Proteins are necessary for every activity in an organism, from metabolism and growth to development and reproduction. Different types of proteins serve different purposes.
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 RNA, which then synthesize proteins at ribosomes. The sequence of the of amino acids in a polypeptide chain is also called its primary structure. Secondary and tertiary structuring produce a three-dimensional form, resulting in a finished protein. If there are more than one polypeptide chains, a quaternary structuring holds them together to give a macromolecule. The sequence of amino-acids and the three-dimensional form of the proteins determine its function.
Each cell has many proteins that determine its structure and function. Each protein can have more than one role, or just one single role.
- Storage of material
- DNA regulation
- Responding to stimuli
Structural proteins work at various levels, from cell to tissue to the whole body. In the cell, there are different kinds of structural proteins, which are responsible for maintaining its form. Eukaryotic cells are bigger than prokaryotic cells, and need more of them.
Structural proteins are present and very important to any tissue that is hard, such as bones, horns, hair, skin, etc. The structural proteins collagen and elastin are found in tendons and ligaments, and provide support to the whole body. Another structural protein, collagen, 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.
- Transport of material
These proteins bind to electrons or atoms and transport them throughout a cell or body. Within the cell, cytochrome help in electron transport during cell mechanisms.
Transport proteins also provide for metabolic respiration by transporting nutrients to all parts of the body. Waste produced by cells is also transported away by proteins. For example, hemoglobin is a protein with a quaternary structure that is found in red blood cells.. It carries oxygen from the lungs to every cell in the body.
- Storage proteins
Storage proteins store atoms or molecules at the cellular or tissue level. For example, ferritin binds to iron and stores it. Most the iron in the body is bond to ferritin. This protein is found in the liver, spleen, bone marrow, and the blood. Some proteins also store amino-acids, like the ovalbumin in egg whites and the casein in milk.
- DNA regulation
Some proteins, like histone and cohesin, regulate the structure of chromosomes, and therefore gene expression. Various complexes of proteins are necessary for the repair of DNA. For example (MutSα) and (MutSβ) in eukaryotes have functions like recognition of strand mismatch, which is the first step in DNA repair.
- Responding to stimuli
Proteins are capable of picking up signals from outside the cell. They then help the cell to react to these stimuli. As part of this job, some proteins also regulate movement of certain molecules in and out of a cell by acting as channel proteins.
One of the major roles of proteins is to act as catalysts as enzymes. They lower the activation energy needed for biochemical reactions to levels found in the cells, and thereby catalyze these reactions. Enzymes act by providing active receptor sites for substrate/substrates to bind to, and help in cleaving molecules or synthesizing bigger molecules. Anabolic enzymes build more complex molecules from smaller ones, and catabolic enzymes breakdown large molecules to smaller ones. Without enzymes, cells would not be able to function at all.
At the level of organs, enzymes are needed for the digestion of food and are found in the saliva and digestive tract. Catabolic enzymes, like pepsin and lipase, breakdown proteins and lipids respectively.
As hormones, proteins act as messengers between different tissues or organs of a body to co-ordinate body functions. They act as receptors and detect concentrations of chemicals, then they send signals to attract chemicals if concentrations are low. Hormones are secreted by endocrine cells, and are involved in all functions of the organism like metabolism, growth, and development.
Examples of hormones are insulin, oxytocin, and somatotropin. Insulin co-ordinates the amount of glucose cells absorb. This action by insulin regulates the level of sugar in blood.
Antibodies are proteins that protect the body against foreign substances and prevent infections. They are produced by the immune system to immobilize pathogens like bacteria, viruses, and fungi 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. Antibodies, for example immunoglobulins, are specific to each foreign particle.
- Contractile proteins
These proteins allow the body to move, and are found in muscles. For example, actin and myosin help in muscle contraction.
Proteins are chains of amino acids, and are built at ribosomes by mRNA from the genetic information decoded from DNA. There are many different types of proteins present all throughout the body, and each type serves one or more unique function.
Structural proteins provide structure and aid in cell division. Transport and storage proteins transport and store materials, and contractile proteins allow for movement in the body. Hormones are messenger proteins. Antibodies are specialized proteins that fight off certain infections, and enzymes lower the required activation energy, enabling chemical reactions to occur. Some proteins regulate and repair DNA, while others yet recognize and initiate cellular responses to stimuli surrounding the cell. Proteins are a vital component of cellular and body functions, and without them, life would not exist.