We will look at the main elements found in proteins. Briefly look at how proteins are constructed. Look at the structure of proteins. Overview the major functions of proteins. PROTEIN STRUCTURE

The building blocks of proteins Like carbohydrate and lipid molecules, proteins contain the elements : Oxygen(O), Carbon(C),and Hydrogen(H) In addition they always contain the element Nitrogen(N).

Before we can understand how proteins are constructed, the structure of amino acids needs to be considered.

N C C H H R H O OH AN AMINO ACID Amino group Carboxyl group R represents groups such as CH 3 or C 2 H 5

20 amino acids There are 20 different amino acids Each one has: an amino group a carboxyl group a central carbon an R group (side chain) It is the R group that is different

20 amino acids 8 of the 20 a.a. are considered to be “essential amino acids The essential amino acids cannot be synthesized by the human body and must be obtained in the diet

How are proteins constructed First the Amino acids bond together. They are joined together by what is known as a peptide bond.

Formation of a peptide bond via condensation. H R O H R O N C C + N C C H H H OH OH H Amino acid

A peptide bond between two amino acids. H R O H H O N C C N C C H H R OH H 2 0 [WATER] A condensation reaction

Protein construction When two amino acids join together they form a dipeptide. When many amino acids are joined together a long-chain polypeptide is formed. Organisms join amino acids in different linear sequences to form a variety of polypeptides in to complex molecules, the proteins.

Primary protein structure Peptide bond Amino acid primary structure This is the linear sequence of amino acids

Secondary protein structure Polypeptides become twisted or coiled. These shapes are known as the Secondary Structure. There are two common secondary structures The alpha-helix and the beta-pleated sheet.

Amino acid Hydrogen bonds hold shape together Secondary protein structure Alpha-helix

Secondary Protein structure [The beta pleated sheet] Amino acid Hydrogen bonds Hold shape together

Hydrogen bonds The polypeptides are held in position by hydrogen bonds. In both alpha-helices and beta pleated sheets the C=O of one amino acid bonds to the H-N of an adjacent amino acid. As below: C=O----H-N

Secondary structures Both secondary structures give additional strength to proteins. The alpha-helix helps make fibres like in your nails, e.g. Keratin. The beta pleated-sheet helps make the strength giving protein in silk, fibroin. Many proteins are made from both alpha-helix and beta-pleated sheet.

Fibrous proteins A fibrous protein only achieves a secondary structure. The simple alpha-helix polypeptides do not undergo further folding.

Structure of a fibrous protein Coiled alpha-helix structure

Tertiary protein structure This is when a polypeptide is folded into a precise shape. The polypeptide is held in ‘bends’ and ‘tucks’ in a permanent shape by a range of bonds including: Disulphide bridges [sulphur-sulphur bonds] Hydrogen bonds Ionic bonds.

Tertiary protein structure

Quaternary protein structure Some proteins consist of different polypeptides bonded together to form extremely intricate shapes. A haemoglobin molecule is formed for separate polypeptide chains. It also has a haem group, which contains iron. The inorganic group is known as the prosthetic group. In haemoglobin it aids oxygen transport.

Quaternary protein structure

How useful are proteins? Cell membrane proteins: Transport substances across the membrane for processes such as facilitated diffusion and active transport. Enzymes: Catalyse biochemical reactions, e.g. pepsin breaks down protein in to polypeptides.

Hormones: are passed through the blood and trigger reactions in other parts of the body e.g. insulin regulates blood sugar. Immuno-proteins: e.g. antibodies are made by lymphocytes and act against antigenic sites on microbes. Structural proteins: give strength to organs, e.g. collagen makes tendons tough.

Transport proteins: e.g. haemoglobin transports oxygen in the blood. Contractile proteins: e.g. actin and myosin help muscles shorten during contraction Storage proteins: e.g. aleurone in seeds helps germination, and casein in milk helps supply valuable protein to babies. Buffer proteins: e.g. blood proteins, due to their high charge, help maintain the pH of plasma.

Enzymes Living cells carry out many biochemical reactions. These reactions take place rapidly due to enzymes. All enzymes consist of globular proteins.

Enzymes The tertiary folding of polypeptides are responsible for the special shape of the ‘active’ site. Some enzymes require additional non- protein groups to enable them to work efficiently. e.g the enzyme dehydrogenase needs coenzyme NAD to function.

The lock and key theory Substrate Enzyme + Enzyme-substrate complex

A catabolic reaction [substrate broken down] enzyme-substrate complex enzyme 2 x products

An anabolic reaction [substrates used to build a new molecule] enzyme substrate

Anabolic reaction continued Enzyme substrate complex Single product formed Enzyme ready to Use again

Metabolic reactions Metabolic reactions = anabolic reaction + catabolic reaction. Metabolism is a summary of build up and break down reactions.

Induced fit theory The active site is a cavity of a particular shape. Initially the active site is not the correct shape in which to fit the substrate. As the substrate approaches the active site, the site changes and this results in it being a perfect fit. After the reaction has taken place, and the products have gone, the active site returns to its normal shape.

Induced fit theory Enzyme Sustrate +

Induced fit continued Induced fit Enzyme-substrate complex

products enzyme

Lowering of activation energy Every reaction requires the input of energy. Enzymes reduce the level of activation energy needed as seen in the graph. substrate Reaction without enzyme Reaction with enzyme products Progress of reaction energyenergy

Two minute summary Now you have seen the presentation ! Summarise the most important points of this presentation. What was the ‘muddiest’ point in the presentation? Hand in your paper to the teacher before you leave the classroom.

END OF PRESENTATION by S S Khalsa [science PGCE]