The first function of proteins is that they can be used for movement animal bodies. Myosin and Actin are filamentous protein molecules contained in muscles. Movement is achieved in the presence of calcium ions because myosin and actin will slide past each other and form a “cross-bridge” structure which contracts the muscle. Muscles are vital in animals as they work with bones to allow movement.
Proteins are also used in other structural ways, such as in the composition of skin, hair and cartilage. Collagen is one protein that is important in skin, hair and in connective tissues in general and does things such as giving skin its elasticity. Keratin is also used by animals as a structural protein. Body parts such as hair, claws, horns, bird’s feathers and reptile scales are all made of keratin so it is a vital protein in the animal kingdom.
Proteins have other uses apart from structural ones as well. Firstly they can be used as enzymes and hormones. An enzyme is a special that is used to produce or help chemical reactions in cells. Most chemical reactions that take place inside cells happen too slowly to be biologically useful so a class of protein molecules called an enzyme is used to speed them up. An example of an enzyme that is made of a globular protein would be amylase, which can be found in the endosperm of starchy seeds and in the human mouth and duodenum.
Hormones are messengers made of protein and produced by specialized s called the . The hormones produced are secreted directly into the or other body fluids and sometimes into adjacent tissues. Hormones are used to regulate metabolic activity of some other organs or tissues of the body. An example of a protein hormone would be insulin which is made in the Islets of Langerhans in the pancreas. It is a relatively small protein that comprises 2 polypeptide chains linked with 3 . Chain A consists of 21 and chain B of 30 amino acids. When it is present in the blood it decreases bloody sugar levels. If insulin was not present the brain would go into hyper-glycemic shock, thus causing brain damage. This makes the protein hormone insulin essential.
Proteins are also used in the blood. The main blood plasma proteins are albumins, globulins, and fibrinogen. Albumin is necessary in maintaining in cells and also acts as a transport protein for various substances. The globulins are usually antibodies and fibrinogen is involved in the clotting process. The antibodies (globulins) are important as they fight off infection in the blood. Another technique used to prevent infection is by reducing bacterial entry to the body by clotting. This is the job of platelets and fibrinogen, another protein. The blood clots to form a scab that protects the wound.
Proteins can also be used around the blood as a means of transportation. For example, red blood cells themselves contain haemoglobin, the red pigment which is used to transport oxygen and carbon (IV) oxide around the blood. The red pigment itself is in fact classed as a conjugated chromoprotein as it contains haem which is a protein with a metallic (iron) prosthetic group. Proteins are also used in active transport. Active transport is a mechanism whereby molecules or ions can move against their concentration or electrochemical gradients; however this requires energy in the form of ATP. Active transport works by absorbing the ion to be transported and then being activated by an ATP molecule. The activation forces the protein to change shape and force the ion out on the other side of the cell. An ADP molecule is released as a by-product but this can be changed back into ATP and used again. An example of active transport would be the sodium-potassium pump whereby sodium is pumped out of a cell against its concentration gradient and potassium is forced into the cell against its concentration gradient. The sodium-potassium pump is used in various places around the human body, including the intestine. Proteins also enable facilitative diffusion where dissolved substances such as salts and glucose can pass through channel proteins in the cell membrane into the cell.
Homeostasis also requires proteins for one aspect of bodily control. The pH of blood has to be kept at pH7.35-7.45 otherwise chemical reactions slow down and the body does not work at its optimum. The pH balance in blood is done using a biological buffer – the amino acid monomer. Once a protein is broken down into its constituent amino acids, the amino acids can become Zwitterions whereby they can release H+ or OH- ions to allow a solution to change pH. If the zwitterion is in a solution with a higher pH that it’s isoelectric point, it releases H+ ions from the amine group side so the solution becomes more acidic. If the solution is lower than its isoelectric point it will release OH- ions to make the solution more basic.
The final use of proteins is as a storage medium. There are many ways in which this is done in nature, but one of these is the aleurone layer in seeds. This layer contains a lot of protein which is digested by the seed during germination as a source of energy for the growing plant. The aleurone layer is the brown part on cooked rice which gives it its structure.
In conclusion, there are many different types of proteins that have different purposes. Proteins can be used in muscle movement (myosin, actin), substance movement around the blood (haemoglobin), active and passive transport (channel proteins) enzymes (amylase), hormones (insulin), structure (collagen and keratin), protection (antibodies and fibrinogen), homeostasis (amino acids) and storage (aleurone layer).