P ROTEIN Protein is the largest known molecule and all proteins are made of Carbon, Hydrogen, Oxygen and Nitrogen and most proteins also contain Sulphur and Phosphorus (Proteins are the only nutrients that contain the element nitrogen. Nitrogen is needed for growth) These elements are arranged into units called amino acids which form the building blocks of protein Protein molecules are large. They consist of long chains of amino acids (typically 500 amino acids) chemically combined by peptide links (or bonds) Each amino acid molecule contains at least one amino group (-NH2) and at least one acidic group (-COOH). Amino acids show both acidic and basic properties and are said to be amphoteric. Alkaline Part Acidic Part

A peptide link is formed when the amino group reacts with the acidic group of an adjacent amino acid. A molecule of water is eliminated during the formation of the peptide link (condensation polymerization). Two amino acids joined together form a dipeptide. Longer chains are called polypeptides. Protein molecules can consist of a single polypeptide chain or a number of polypeptide chains joined by cross linkages There are several orders of complexity in the structure of proteins: primary structure (the sequence of amino acids in the protein chain) and secondary structure (where the amino acids are further linked by various bonds to give it a definite shape, often in the form of a spiral). The most important group involved in cross-linking is the SH group, which forms a disulphide bridge. Other links are formed between the amino acids which contribute to the coiling of the chain. Cross links can also be made by the formation of hydrogen bonds. Cross linking determines the secondary structure (i.e. the shape) of the protein. Proteins can be classified into two groups according to the shape of the molecules: Globular (as in ovalbumin, haemoglobin, myoglobin and caseinogen), are rounder in shape. Fibrous (as in gluten, collagen and elastin), are either straight or coiled.

Basic plan of an amino acid Side group changes Depending on the Amino acid All amino acids have same basic structure – side group differs Amino acids join to form many different proteins R

Amino acids are of no use to us unless they are joined to make protein: Amino acid Amino acid + amino acid = dipeptide Amino acid + amino acid + amino acid + tripeptide Many amino acids joined = polypeptide Another name for protein = polypeptide One of the smallest proteins is insulin which is made up of 51 amino acids

When 2 small molecules (amino acids in this case) join to make a larger molecule, it is through condensation and a unit of water is lost in the process. This reaction forms a peptide bond When proteins are digested, the reverse happens, this process is called hydrolysis

There are over 80 amino acids but only around 20 are found in food protein. Of the 20 different amino acids found in proteins from plants and animals, 8 have to be provided by the diet. These are called essential amino acids, which cannot be made by the human body. Non essential amino acids can be made by the body, there are 11 on essential amino acids.

Vincent’s Lovely Mother Took Him To London In a Pram 8 essential for adults and 10 for children An easy way to remember the essential amino acids Valine Lysine Methionine Trytophan Histidine (children) Threonine Leucine Isoleucine Arginine (children) Phenylalnine

T HE B IOLOGICAL V ALUE OF P ROTEIN Biological Value of protein is used as a measure of protein quality. If a protein contains the essential amino acids in the amounts needed it is said to have a high biological value (HBV). If it is low in one or more of the essential amino acids it is said to be low biological value (LBV) High Biological Value foods (HBV): Protein from animal sources provides good amounts of all the essential amino acids needed by the body and have a higher biological value or ‘quality’ than protein from plant sources. Sources: Meat, Poultry, Fish, Eggs, Milk, Cheese and other Dairy foods. Low Biological Value foods (LBV) are: Bread, Cereals, Nuts, Pulses (beans, lentils), Soya, Mycoprotein Protein complementation Different foods contain different amounts and combinations of amino acids. Vegans and vegetarians can get all the amino acids they need by combining different protein types at the same meal. This is known as protein complementation. Examples: bread and cheese, macaroni cheese, rice and peas, beans on toast, hummus and pitta bread, bean chilli served with rice, rice pudding