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Proteins - structure and functions

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Proteins Proteins play a number of vital roles in all organisms. It is an essential part of the diet and helps the body repair muscles, grow tissue, control metabolism as well as many more things. In fact, half of the non-water mass of the body is protein. Unlike carbohydrates or lipids, proteins all contain nitrogen as well as carbon, hydrogen and oxygen. Sulphur and phosphorus are also sometimes present. The Structure of Proteins Proteins are built up from amino acids. There are over 100 amino acids and they all have an NH2 (amino) and a COOH (carboxyl acid) group, as shown below. This is why they are called amino acids. Both groups have opposite charges - there is a positive charge at one end of the molecule and a negative charge at the other end. The 'R' group represents a side chain which in this molecule of glycine (the simplest amino acid) is a hydrogen atom: On its own the body can manufacture 14 of the 22 amino acids it needs. The other 8 must be obtained from diets. Amino acids bond to form proteins. ...read more.


The polypeptide may be folded and twisted at places. These cross links may occur because of the hydrogen bonding as well as sulphur bridges. The sulphur bridges are the strongest and mostly determine the shape. The quaternary structure is only present when a protein consists of 2 or more polypeptides. It refers to the way the polypeptides are arranged in the protein. The Functions of Proteins The functions of proteins are are related to their shapes. Proteins fall into 2 groups- globular and fibrous. In globular proteins, the polypeptide chains are tightly folded to form a curved, spherical shape. One of the most important types of proteins are enzymes, biological catalysts which control the rate of reactions (their purpose is to speed them up but they can slow reactions down as well!). The functioning of an enzyme is related to its shape more than anything. All types of proteins have unique shapes and enzymes take advantage of that. The active site of any enzyme has a unique shape in which the specific substrate molecule will fit in. The shape of the site makes sure that only the intended molecules combine with the enzyme (lock and key analogy). ...read more.


The fixed angle of the carbon-nitrogen peptidyl-proline or peptidyl-hydroxyproline bond allows each chain to fold into a helix with a shape such that three polypeptide chains can twist together to form the three-stranded helix. Collagen is the main insoluble fibrous protein in connective tissue (connective tissues are mainly involved in the structure and support of living organisms) in animals and is the most abundant protein in mammals. About one quarter of all proteins in the body is collagen. It represents such a large proportion of the body proteins because it has great strength but is at the same time very flexible. It is therefore used to create tendons and ligaments. It is also responsible for the elasticity of the skin, but decreases with aging, causing wrinkles to be formed. The collagen molecules bundle together to form a long fibril structure. Collagen is found in many places throughout the body and there are more than 28 types of collagen. Over 90% of collagen, however, is either type 1, 2 or 3. Type 1 is used in the skin, tendons and bones. Type 2 is used as the main component of cartilage whereas type 3 collagen is used as the main component in reticular fibres (they form the supporting mesh for soft organs such as the liver, spleen and tonsils). ...read more.

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