Key area: Proteins Overview In this section, study the structure and function of proteins. Learn about how they are formed from amino acids in condensation reactions, and how they are broken down by hydrolysis.

After completing this lesson you should be able to : Proteins are the major structural materials of animal tissue and are also involved in the maintenance and regulation of life processes. Amino acids, the building blocks from which proteins are formed, are relatively small molecules which all contain an amino group (NH2), and a carboxyl group (COOH). The link which forms between two amino acids can be recognised as a peptide link (CONH) also known as an amide link. Proteins are made of many amino acid molecules linked together by condensation reactions. In these condensation reactions, the amino group on one amino acid and the carboxyl group on a neighbouring amino acid join together, with the elimination of water. Proteins which fulfil different roles in the body are formed by linking differing sequences of amino acids together. The body cannot make all the amino acids required for body proteins and is dependent on dietary protein for supply of certain amino acids known as essential amino acids.

Proteins COPY

Introduction to proteins An animation of three proteins which demonstrate common structural elements despite their very different functions.

Globular proteins Proteins which operate within cells need to be soluble. The polypeptide chains are coiled together in spherical shapes. E.g. Haemoglobin and many hormones. e.g. Insulin, was the first protein structure to be worked out. Enzymes are globular proteins. Protein Structures Some proteins are composed of a single polypeptide chain, but many consist of two or more polypeptide chains. Proteins are classified according to their shape into fibrous and globular proteins. Fibrous proteins These have their polypeptide chains interwoven. The polypeptide chains are held together by hydrogen bonding, between the N-H and the C=O groups. This gives these proteins their properties of toughness, insolubility, and resistance to change in pH and temperature. So they are found in skin,tissue, (collagens), hair, nails (keratins). COPY

Silk is a typical example of a fibrous protein. This view shows the protein chains contain 2 different amino acids. This view shows the individual atoms in the protein chains. Silk Protein Structures COPY

Albumin, in egg white, is a globular protein.. backbone viewatom view Albumin Protein Structures COPY

Enzymes are globular proteins. The structure of amylase is shown below. Protein Structures Starch molecule in the enzyme’s active site. COPY

b) Amino Acids Learning intention Learn about the characteristic chemistry of amino acids, the building blocks of proteins.

Amino acids All proteins contain the elements C,O,H, N. They are condensation polymers, made by amino acids linking together. The body cannot make every type of amino acids that it needs. So our diet must contain essential amino acids. (about 10 of them). We synthesise the others. When R is Hydrogen, the amino acid is glycine (Gly) (aminoethanoic acid) When R is CH 3, the amino acid is alanine (Ala) (2-aminopropanoic acid) Amino Acids NH 2 CHCOOH R Most proteins contain 20+ different amino acids HN H CCOH R O H COPY

Amino acids

Essential Amino acids An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo (from scratch) by the organism, but must be supplied in its diet. "Nonessential" means that our bodies produce an amino acid, even if we do not get it from the food we eat. Conditional amino acids are usually not essential, except in times of illness and stress. COPY

This animation illustrates the process of protein formation by the condensation of the carboxylic acid and amine groups of amino acids. It also looks at the reverse process of protein hydrolysis s/chemistry/animations/chemicalequations.asp

c) Amide links Learning intention Learn how proteins are formed by condensation reactions of amino acids to produce amide (peptide) links.

Protein Polymers Proteins are condensation polymers, made by amino acids linking together. An amine group of one molecule links to the carboxyl group of another molecule to form an amide link or peptide bond. COPY

Protein Polymers Tripeptide, ala-gly-ala Polypeptide chain can have amino acids N – C - C CH 3 H + + alanine glycine alanine H H O OH H N – C - C H H O OH H N – C - C H H O OH H O N – C – C - CH 3 N – C – C - H N – C - C CH 3 O HH H H H + 2H 2 O O OH H H amide (peptide) link COPY