Proteins
After water and possibly fat, proteins are the most abundant material in the human body. They are made up of molecules called amino acids. Amino acids consist of atoms of carbon, hydrogen, oxygen like Carbohydrates and Fats. Unlike carbohydrates and fats, amino acids also contain nitrogen and sometimes sulfur. Each protein has its own unique sequence of amino acids which determines its shape and function. Most common proteins contain more than 100 amino acids. Let us explore some of the many roles of proteins.
Each gram of Protein that is “burned” by the human body produces potentially 4 Calories of what can be called “metabolizable energy”. When we account for the energy used to digest, absorb and metabolize protein, we actually get 3.2 Calories per gram of “net metabolizable energy,” that can be either converted to physical energy or used to add body weight. Unlike proteins the net metabolizable energies for carbohydrates (4 Cal/g) and fats (9 Cal/g) are practically the same as their metabolizable energies. Needless to say the body prefers to metabolize Carbohydrates or fats over Proteins since there is no wastage of energy and metabolizing protein often means loss of essential body mass.
One of the important role of proteins is to give structural support to cells and to structures around cells. The structural arrangement produces a recognizable appearance. Structure is also important for strength and protection. Collagen is the main structural protein in the extracellular matrix found in the body’s various connective tissues. It gives the structure to bones, skin and connective tissue (tendons and ligaments). As the main component of connective tissue, it is the most abundant protein in humans, making up about a third of the whole-body protein content.
Another structural protein called Keratin is found in the upper layer of skin and well as in hair and nails. It is fibrous structural protein that is coiled into a helix as illustrated in the figure to the right.
The structural protein Elastin gives elasticity to the skin because of its ability to stretch and relax.
Histones are proteins that provide structural support for chromosomes. Without histones, the unwound DNA in chromosomes would be very long. The difference between unwound and wound up DNA in one human cell is 1.8m versus 90mm. The compacted molecule is 40,000 times shorter than an unpacked molecule.
Two specialized structural proteins, Actin and Myosin are found in muscles and allow for movement through muscle contraction and relaxation. How muscle actually work is really complex and cool but is beyond the scope of this lesson.
Others proteins carry or transport substances from one place in the body to another. For example, Hemoglobin is a transport protein in red blood cells that picks up oxygen as it circulates through lung tissue and then carries it to the body’s cells. It also takes up carbon dioxide from the body’s cells and releases into the lung tissue from where it is exhaled.
Other important transport proteins include:
- Transferrin: carries iron in the blood.
- Myoglobin: carries oxygen in the muscles.
- Ferritin: stores iron.
- Cytochromes: carry electrons in the respiratory chain in the cells.
- Albumin carries certain ions, hormones, vitamins, bilirubin, drugs and toxins via the blood and also also helps to retain water in the blood and thus blood in the blood vessels.
Some cell membrane proteins have both a structural and transport role. They regulate two-way flow (influx and outflux) of nutrients and waste products into and out of cells.
Proteins are involved in many regulatory (control processes) roles in the human body. Some of these include:
- Enzymes: enable the synthesis (making) or breakdown of most substances in the body by making the hundreds of chemical processes that occur in (cellular metabolism) and around cells faster and more efficient at normal body temperatures. Some digestive enzymes help us to break down ingested food and harvest the energy from them.
- Coagulation proteins Thrombin and Fibrin facilitate blood clotting by causing platelets to clump together. Without them whenever get a cut or bruise we would never stop bleeding. Fibrin also enables wound healing.
- Hormones are chemical substances that act like messenger molecules in the body. They are produced and released from one part of the body, travel through blood to other parts of the body where they have different effects. The hormones insulin, oxytocin, somatotropin and thyroxine are proteins.
- Other proteins are released into the blood whenever we are ill in order to control inflammation. Some examples are C-reactive protein, prostaglandins, and prostacyclins.
- The production of protein is regulated by DNA. We mentioned Histones earlier. Apart from their structural role they also play a role in gene regulation since when the chromosomes are wound up tightly around the histone spools genes cannot be read. Other proteins are involved in the replication and transcription (reading)of genes.
Some proteins are involved in defense against microbes or cancerous cells. They are called Antibodies (Immunoglobulins IgA, IgE, IgG, IgM).
Proteins are the most common macromolecule found in every cell of an animal’s body. when protein is deficient many of the processes above will not function appropriately. Animal foods like meat, milk, and fish and eggs are good sources of protein. But, protein is also found in many plant foods, especially beans, nuts, seeds and grains.
