protein

A protein molecule is a long chain of amino acids linked by peptide bonds. The properties of a protein are determined by the order, or sequence, of amino acids in its molecule, and by the three-dimensional structure of the molecular chain. The chain folds and twists, often forming a spiral shape.

The digestion of protein. Protein is broken down into amino acids by the enzymes pepsin (secreted by the stomach) and trypsin and peptidase (in the small intestine).

Large, complex, biologically-important molecules composed of amino acids joined by peptide bonds. The number of amino acids used can be many hundreds. There are 20 different amino acids and they can be joined in any order. Proteins are essential to all living organisms. As enzymes they regulate all aspects of metabolism. Structural proteins such as keratin and collagen make up skin, claws, bones, tendons, and ligaments; muscle proteins produce movement; haemoglobin transports oxygen; and membrane proteins regulate the movement of substances into and out of cells. For humans, protein is an essential part of the diet, and is found in greatest quantity in soy beans and other grain legumes, meat, eggs, and cheese. During digestion protein molecules are broken down into amino acids which are then easily absorbed into the body.

Protein synthesis occurs in cells. The information describing the order in which the different amino acids are joined is found in DNA in the form of a code. The part of the DNA that carries the code for making one protein is called a gene. Each protein described in the code has an effect on the appearance and characteristics of the organism.

During digestion in the body, proteases (any enzymes that break down a protein) are produced by the stomach, the pancreas, and the small intestine. These enzymes catalyse the breakdown of protein into amino acids. Pepsin is an enzyme which is produced by the walls of the stomach. It digests large protein molecules into smaller protein molecules (smaller polypeptides). The conditions in the stomach are very acid and pepsin works at its fastest rate in such conditions.

The amino acids produced by the digestion of proteins are absorbed into the blood in the small intestine. They are transported around the body by the blood for use by cells to make the proteins they need. This protein synthesis needs energy and this energy is provided by respiration.

Plants and protein Plants also make proteins. Unlike animals, plants make proteins from minerals, such as nitrates, that they absorb from the soil, and carbohydrate produced in photosynthesis. So it is plants that make the protein that animals need to have in their diet. There is a shortage of minerals like nitrates (containing the element nitrogen) on Earth. The movement of nitrogen between organisms is therefore important and is referred to as the nitrogen cycle.

Uses of proteins as fibres Proteins are used by organisms in a variety of ways. Some large proteins are insoluble and form fibres. These fibres are strong and are used to form structures in organisms. An example is the protein fibre that forms tendons - these join muscles to bone and are important in joints. Protein fibres can usually bend. Bones are mainly composed of brittle minerals (calcium phosphate) but the presence of protein stops bone from being too brittle.

Enzymes Enzymes are soluble proteins that are involved in and facilitate many processes in organisms, such as respiration, photosynthesis, and digestion.

Haemoglobin Proteins can act to transport materials. For example, haemoglobin is a protein that transports oxygen in the blood.

Proteins and disease Antibodies are modified proteins that can provide immunity (see immunization) to disease. Proteins coat genes to form virus particles that can infect cells.

Protein structure Types of bond other then peptide bonds, such as sulphur-sulphur bonds, hydrogen bonds, and cation bridges between acid sites, are responsible for creating the protein's characteristic three-dimensional structure, which may be fibrous, globular, or pleated. Protein provides 4 kcal of energy per gram.