The second and probably most widely recognised role of proteins in living structures is enzymes; they act as organic catalysts and catalyse many biochemical reactions in the body and in the world around us. This involves the tertiary structure of proteins where there are bonds held together by hydrogen and ionicly as well as sulphur bridges. This encourages and helps with the highly specific nature of enzymes and their ability to take energy from one reaction to assist their own. Enzymes are found in nearly every living organism, for example even in fungi such as Rhizopus where enzymes are secreted on to the substrate, brown bread, to break it down so it can be absorbed and used as food for the Rhizopus to grow. Some examples of enzymes are :
- Lactase to rid milk of lactose
- Lipase to break down fats into fatty acids and glycerol
- Pepsin breaks down protein into polypeptides
Another use of protein is in the immune system of the human body, these are called immuno-proteins. They are a blood protein whose activities affect or play a role in the immune system. Some soluble proteins bind with antigen to induce or affect antigen specific functions in immuno-regulation and/or hypersensitivity; these then attack microbes to rid the body of infections etc, another way in which protein helps protect the body is fibrinogen. When the body experiences a cut in the skin then the platelets form a “plug” while the fibrinogen in the blood plasma, which on contact with the air, hardens to form fibrin which seals the cut and prevents anything entering the body through the cut.
The next use of proteins in living organisms is as hormones, although many of the hormones have protein structure not all hormones are proteins, the hormone protein is generally quite small compared to most. Hormones are passed through the blood and trigger reactions in other parts of the body i.e. regulating homeostasis. Insulin is one such hormone and is a globular protein, like other proteins its shape is important as it binds to specific receptors on target cells to regulate the amount of sugar in blood. Glucagon is the growth hormone which is also a globular protein and encourages growth in humans.
Proteins can also act as transporters and are commonly found as globular proteins when in this role. Haemoglobin is an example of a transport protein; it carries oxygen from the lungs to the tissue whilst myoglobin performs a similar function by taking oxygen from the haemoglobin and storing it or carrying it around until the muscles need it. Haemoglobin is well adapted to its role, as all protein molecules are, because it can easily bond with oxygen under certain conditions but also release oxygen under other conditions. Haemoglobin can carry four oxygen molecules and changes its shape slightly when oxygen binds to it so that more oxygen molecules can bind with it. Another transport protein is transferrin which carries iron, so as you can see protein has a very important role in the human body to transport many vital molecules.
Storage is yet another role which protein plays, it is a prominent part of seed germination where it is stored in the seed and used as nitrogen sources for the developing embryo during germination. It can also be found in the storage organs of plants such as roots and shoot tubers i.e. potatoes; in these types of situation the normal properties of the protein are:
- The proteins have no enzymatic activities
- They occur normally in an aggregated state within a membrane-surrounded vesicle (protein bodies, aleuron grains).
- They are often built from a number of different polypeptide chains
These plant proteins are also very important for human nutrition, as we need the amino acids to replace the ones we can’t make ourselves. This is also very important in pregnant women as casein is a valuable supply of protein to the baby, which needs it to develop muscles and bones. It is also vital for humans to store iron in the form of ferritin as it can be very toxic if we do not have the ability to store and release iron in a controlled fashion, the ferratins are protein bound as a ferric oxide-phosphate mineral.
Structural proteins are of a fibrous nature, and the most familiar of the fibrous proteins are the keratins. They form the protective coating of all land vertebrates: skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks and feathers. If we look at our hair it grows at about half an inch per month which means we need to grow 36 amino acids a second so we need a big intake of protein each day. Collagen is another fibrous structural protein found in humans, with its insoluble fibres it has very high tensile strength and is found in bone, tendons, cartilage and skin when it needs extra support.
Contractile proteins are proteins which allow contraction in muscles and therefore movement. Actin and myosin form sarcomeres, which are the basic units of contraction, actin is a thin filament contractile protein and myosin is a thick filament. Myosin filaments slide along and pull the actin filaments toward the centre of a sarcomere during contraction and so movement happens.
Buffer proteins help maintain balance in the human body. Blood proteins have a charge which helps maintain the pH of plasma, in this way it is a very important protein because the blood transports lots of vital substances all over the body so without the right pH we could not function properly. From reading this essay you can see the importance of proteins in the world around us and how we would be lost without them, for example we would not have any hair, nails or skin!!!!
