B1.2 Proteins

amino acid

dipeptide formation

• amino acid + amino acid → dipeptide + water

• condensation reaction

dietary requirements for amino acids

• there are 20 amino acids

• 9 essential amino acids, eaten from diet

• 11 non-essential amino acids can be made in liver

• vegans must make sure they are consuming essential amino acids

R group

• is different for each type of amino acid

• 20 types

• R group with more carboxyl groups than amino groups is an acidic amino acid

• R groups with more amino groups than carboxyl groups is a basic amino acid

non-polar R groups

• from hydrophobic bonds (interactions) with each other or other hydrophobic molecules

• found on the inside of most proteins but on outside of integral proteins in contact with fatty acid

• at the active site of enzymes if the substate is non-polar

polar R group (hydrophilic)

• form hydrogen bonds

• found on the outside of proteins in contact with water

• are weak and broken by high temperatures

charged R groups (hydrophilic)

• form ionic bonds (between positively and negatively charged R groups)

• amine and carboxyl groups in R groups can become charged by binding/disassociation of H⁺ ions to participate in ionic bonding

• bonds broken by changes in pH (change the charge on the R group)

R groups containing sulphur

• cysteins form disulphide bridges which are covalent and therefore strong

• each S is part of an R group on separate amino acids

• broken by high temperature

polypeptides

• amino acids joined together at ribosome to make polypeptide

• any number of amino acids can be added to chain

• 20ⁿ possibilities

• all proteins/polypeptides produced by an organism are known as the proteome

examples of polypeptides

• insulin hormone - is made of two polypeptides, one of 21 amino acids, the other 30

• titin - found in the muscle, has 34350 amino acids

• lipase has 449

denaturation

a three dimensional change in protein as a result of bonds being broken by pH or temperature

primary structure

• determines three dimensional shape

• type and order of amino acids

• amino acid sequence is coded for by genes

• mistake in order can lead to change in the molecule’s shape

secondary structure

• structure held in place by hydrogen bonds

• alpha helix and beta pleated sheet

tertiary structure

• the way in which the polypeptide is further twisted and folded to make a £D shape

• held in place by bonds between R groups of the amino acids

• include disulphide bridges, ionic bonds, hydrophobic interactions and hydrogen bonds

soluble proteins

• protein is folded so that polar R groups face outwards and non polar R groups face inwards

• hydrophobic amino acids are clustered in the core of globular proteins

quaternary structure

3D arrangement of two or more polypeptide chains joined together to form one functional protein

conjugated proteins

also have one or more non protein part (prosthetic group) of the molecule

globular proteins

• usually have a metabolic function - all enzymes are globular

• irregular, specific primary structure, folded into a very specific 3D shape

• soluble

• unstable (exposure to pH/temp changes results in denaturation)

haemoglobin molecule

• quaternary - made from 4 polypeptides

• conjugated protein - contains prosthetic group: contains 4 haem groups

• oxygen bind to haem group, x4 increases its ability to carry oxygen

• haemoglobin can change shape to aid the release/binding of oxygen

insulin

• globular

• quaternary - made from two polypeptides

• non-conjugated protein (doesn’t contain prosthetic group)

• binds to liver and muscle cells and causes them to take up glucose and convert it to insoluble glycogen

• small sizes allow it o dissolve and be carried by blood plasma

fibrous proteins

• structural proteins

• repetitive sequences in primary structure

• insoluble

• stable

collagen

• found in skin

• strong, flexible and insoluble

• quaternary - three polypeptides wound round each other

• repeating amino acid sequence P-G-X

• G = glycine - contains small R groups which faces inwards allowing chains to be tightly wound each other

• P - proline, - stops alpha helix from forming to keep structure straight (no coils) and maintain stability

• X varies, allows for different forms of collage in different tissues

collagen structure in relation to function

• long and thin → allows it to provide structural support in skin and connective tissue

• strength → chains closely bonded to provide structural support