Protein

Roles of Protein in the Body

The human body contains an estimated 10,000 to 50,000 different protein.

Proteins carry out a wide range of functions within the body. Proteins are important for body structure, protection, transport and communication.

The body uses proteins as building blocks and are an integral component of the skeleton, supporting tissues, connective tissues and skin. Collagen is a protein that is laid down to make bone or teeth. It is also the material ligaments and tendons are made from.

As old skin falls off, new cells made largely of protein, grow underneath.

Enzymes are proteins which facilitate chemical reactions in the body. You would have heard of digestive enzymes. Digestion is only one of many processes which enzyme facilitate. Enzymes not only break down substances, they also build substances and transform one substance to another. Without enzymes chemical processes within the body would become too inefficient to maintain life. When an enzyme facilitates a chemical reaction it remains unchanged.

For an example of the enzymatic process one only needs to look at what happens when you eat food. When a person eats protein, the protein is broken down by digestive enzymes (which are themselves protein) into amino acids. The amino acids enter the body cells, where they are linked together into long chains by synthases (protein enzymes which build proteins). The chains twist and fold to form proteins.

Hormones are messenger molecules. Some, but not all, hormones are proteins. Various glands in the body release hormones in response to changes in the internal environment. The blood carries the hormones to their target tissue, where they elicit the appropriate response to restore normal conditions.

An example of a protein hormone is insulin. When we have eaten and blood glucose levels rise, the pancreas releases insulin into the blood stream which stimulates the cells transport system to pump glucose into the cells. After acting on the message, the cells destroy the insulin. Then as the blood glucose falls, the pancreas reduces its insulin output.

Proteins help maintain the body's fluid and electrolyte balance. The body's fluids are contained in blood vessels, cells, and surrounding the cells. Fluids can flow freely between these compartments, but cells cannot directly move fluids. Cells can, however, manufacture proteins which are too big to pass across cell membranes, but can attract water. Therefore by making and retaining these proteins, cells can retain fluid. Cells can also transport proteins into the blood and intercellular spaces to maintain fluid volume there.

An example of a protein which maintains fluid balance is the membrane-bound transport protein which maintains the balance of sodium and potassium. The balance of these two electrolytes is important for neural transmissions and muscle contractions. Imbalance can cause irregular heartbeats, muscular weakness, kidney failure, and if severe, death.

Proteins help maintain the balance between acid and bases within body fluids. During everyday body processes acids and bases are formed. These are carried to the kidneys and lungs for excretion. It is important that this occurs without upsetting the body's acid-base balance. Proteins can act as buffers. If the fluid is more acid this means that there are more hydrogen atoms. In this case, proteins which have a negative charge on their surface can attract the hydrogen ions which have a positive charge and so maintain acid-base balance.

There are not only transport protein attached to cell membranes, but also proteins which move about the body's fluids, carrying nutrients and other molecules. A well known example of this is the protein haemoglobin which carries oxygen from the lungs to the cells.

Proteins also defend the body against disease. When a virus enters the body it replicates itself within our cells. As a virus replicates itself it yields many virus particles. If replication is not stopped the virus will overwhelm the body with disease. To defend itself, when the body detects an invader it produces antibodies which are giant protein molecules designed specifically to combat the invader. Each antibody is designed to destroy one invader. Once the body has manufactured antibodies against a particular antigen (e.g.virus), it remembers how to make them. Therefore, the next time the body encounters the same invader it will produce antibodies even more quickly. This is known as immunity.

Proteins may also contribute in a small way to overall daily energy expenditure. For example, endurance exercise protein may contribute 2-10% of the energy costs. During starvation protein will be used to fuel the body.

The oxidation of 1g of protein produces 17KJ of energy.

• Growth and maintenance - proteins form integral components of most body structures such as bones, organs, muscles, tendons, membranes and skin. They support the growth and repair of body tissues
• Enzymes - protein facilitate chemical reactions within the body
• Hormones - Some hormones are made of protein. They help regulate a variety of different processes in the body
• Antibodies - These are large proteins produced by the body in response to the invasion of foreign molecules. They inactivate these foreign invaders and so protect the body against diseases
• Fluid and Electrolyte balance - Protein help maintain the fluid volume and the composition of body fluids
• Acid-base balance - proteins can act as buffers and so maintain the acid-base balance of body fluids
• Transportation - Proteins transport substances such as lipids, vitamins, minerals, and oxygen, around the body
• Energy - protein provides some fuel for the body's energy requirements. 1 gram of protein can provide 17KJ of energy.