GCE A-level Biology Archives

Proteins are large molecules made of long chains of amino acids Amino acids All proteins have the same basic structure. They consist of an Amino Group (NH2), Carboxyl group (COOH), and a Carbon in the middle which bonds with a Hydrogen atom and an 'R' group, which is specific to individual amino acids Peptide bonds Condensation reaction: where two amino acids are joined by a peptide bond to form dipeptide and water molecule Hydrolysis reaction: Dipeptides are split into two amino acids by breaking the peptide bond using a molecule of water Organization of proteins Primary structure The sequence of amino acids in a polypeptide or protein molecule The function of a protein is determined by its primary structure. Secondary structure its formation of a 3D structure when a chain of amino acids coils, or folds. The most common secondary structure of proteins is the alpha helix (α-helix) which forms when the chain coils. Beta-pleated sheet (twisted, pleated sheet) There are hydrogen bonds which hold the coils in place, giving the shape great stability. Tertiary structure An α-helix can wrap itself into a 3-dimensional complex shape. Polypeptides that do this form a globular protein. Their shape is maintained by bonding between the R groups of individual amino acids. Tertiary structure is held by: 1. Hydrogen Bonds Hydrogen bonds can form between oppositely charged ions in the groups which are polar, such as this hydroxyl group. Hydrogen bonds, as always, are very weak and easily broken; temperatures above approximately 40oC will cause a loss in their tertiary structure as the bonds become broken – this is called denaturation 2. Disulphide Bonds When two cysteine amino acids are found near each other, they can form a very strong covalent bond called a disulphide bridge (or disulphide bond). They lose their hydrogen ions in an oxidation reaction, and the two sulphurs can then form the bond. This bond can be broken by reversing the reaction (i.e. in a reduction reaction) 3. Ionic Bonds When amino acids bond with each other, the COOH longer exists, as the OH is lost, but some amino acids have an extra acid group as their R group, The amino acid on the left is therefore an acidic amino acid and it donates H+ ions in solution, leaving an oxygen δ- and the nitrogen of the other amino acid accepts the H+ ion. An ionic bond then forms between the O- and the H+ ions 3. Hydrophobic interactions Non-polar R groups are hydrophobic (i.e. hate water). Two amino acids close to each other with hydrophobic R groups will bond together, clustering to exclude water. The bond formed is a hydrophobic bond which is a very strong type of bond and hard to break. Quaternary structure the state when it polymerizes i.e. when more than one globular protein joins together. They bond together in exactly the same ways as the R groups join in the tertiary structure development: through ionic bonding, hydrogen bonding, hydrophobic bonding and forming disulphide bridges Hemoglobin molecule is made of four polypeptide chains. Each chain is wrapped around a group of atoms, called a haem group which holds an iron Fe2+ ion in the centre, Each iron ion is able to bond with two oxygen atoms (one oxygen molecule), so the haemoglobin molecule as a whole can carry up to eight oxygen atoms (or four molecules of oxygen) The molecule consists of two α-chains and two β-chains The relation between the structure and function in hemoglobin Structure Function Explanation Tertiary structure Soluble The hydrophilic R-groups are on the outside of the membrane. they form hydrogen bonds with water molecules Haem group in peptide chain Ability to combine with O2 Haem group contains a Fe2+ ion that can bind with O2. Hb molecule combine with 4 O2 molecules (8 O atoms) Shape of Hb Sick up and release of O2 Pick up O2 when its conc is high and release it when its conc is low. After 1 O2 molecule has combined with haem group, the whole molecule changes its shape sp that it is easier for O2 to combine with other three haem groups Collagen Collagen is a fibrous protein found in skin, bones, cartilage, tendons, teeth etc.. Collagen consists of three polypeptide chains, each in the shape of a helix. The three helices wind around each other to form a rope. Almost every third amino acid in each chain is glycine. The small size of glycine allows the three strands to lie close together and form a tight coil. The strands are held together by hydrogen bonds. R groups of individual collagen molecules form bonds with other collagen molecules These cross-links form fibrils. Many microfibrils bond together to form larger macrofibrils. These associate together to form much bigger bundles called fibres. Collagen, a fibrous protein, has a tremendous amount of tensile strength, i.e. can withstand a high pulling pressure The relation between the structure and function in hemoglobin Structure Function Explanation Molecule size Insoluble Molecules are long and too large to dissolve in water Three stranded molecule High tensile strength 3 polypeptide chains wind around each other held together by h-bonds and can withstand high pulling forces without breaking and stretch slightly when pulled Glycine Compactness Every third amino acid in each polypeptide is glycine. The small size of glycine allows the three strands to lie close together and form a tight coil. lysine Formation of fibres lysine molecules form covalent bonds between lysine R groups of different collagen molecules to form fibres Difference between globular proteins and Fibrous proteins GLOBULAR PROTEIN FIBROUS PROTEINS It has polypeptide chains with irregular sequence of amino acids It has polypeptide chains with regular repetitive sequences of amino acids Its shape is a compact globule of polypeptides It has long chins running parallel It is chemically less stable and its activity if affected by factors such as its concentration, pH and temperature It is chemically stable and relatively unaffected by temperature, concentration and pH Each molecule of the same type of globular proteins has a specific sequence Each molecule of the same type of fibrous proteins may vary in length with slightly different sequences of amino acids It is water soluble It is insoluble in water It is involved in various body systems such as the digestive system, the endocrine system and the immune system Its roles is mainly in helping to maintain structure and providing support Practice questions Explain how DNA structure determines the specific shape of enzymes. DNA codes for , protein / polypeptide ; transcription and translation (or described) ; enzyme is globular (protein) ; 3 bases 1 amino acid ; sequence of , bases / triplets , determines , sequence of amino acids / primary structure ; coiling / helix / pleated sheet / particular secondary structure ; determines projecting side groups ; folding / bonding , for tertiary structure ; 3-D structure is tertiary structure ; e.g. ref. active site related to shape 2 or more genes produce quaternary structure

Category: GCE A-level Biology

Proteins

Enzymes

Water

Lipids

Carbohydrates

Cell Membrane and Transport

Structure of Chromosomes

Nucleic Acid & Protein Synthesis

Chloroplasts