Protein Structure

• proteins are macromolecules comprising one or more polypeptide chains
• polypeptides are long chains of amino acids joined by peptide bonds
• proteins have characteristic 3D structure determined by the sequences of amino acids
• 3D structure held together by bonding between the amino acids

amino acids

• have 5 components
• central carbon atom
• a hydrogen atom
• an R group (varies from single H atom to more complex structures)
• an amino group (NH₂) sometimes called amine group
• a carboxyl group (COOH)



• carboxyl group has acidic properties (i.e. donates H⁺ ions)
• amino group has basic properties (i.e. accepts H⁺ ions)
• amino acids are thus amphoteric



• there are about 20 amino acids found in organisms

peptide bonds

• two amino acids join to form a peptide bond
• reaction is a condensation reaction - releases water
• result is a dipeptide
• many amino acids in a chain = polypeptide
• this is a polymerisation reaction - many monomers (essentially similar molecules) forming a polymer cf lipids

primary structure

• sequence of amino acids in polypeptide chain

secondary structure

• regions within a polypeptide adopt stable structures
• 2 types: -helix (a spiral, link a phone cord)
• -pleated sheet (like corrugated iron)



• both structures are held together by hydrogen bonds between peptide bonds
• in helix bond is between peptide groups close together in chain
• in sheet bonds are between peptide bonds much further apart (or on two different chains) resulting in chains running in parallel



• collagen is a fibrous protein specified in the syllabus
• found in bone and tendons
• 3 chains wound round each other
• resulting in triple helix
• can not be stretched
• is flexible



• some proteins e.g. collagen and keratin have only primary and secondary structure
• fibrous proteins are insoluble in water
• are usually structural

tertiary structure

• most proteins have an additional level of 3D structure = tertiary structure
• these are globular proteins
• globular proteins are enzymes, hormones and antibodies
• 3D structure held together by bonding between the R groups
• note these proteins also have regions of helix and pleats
• some R groups are polar and hydrophilic
• some are non-polar and hydrophobic
• proteins arrange accordingly



• bonds between R groups
• hydrogen bonds
• ionic bonds between ionized R groups
• covalent bonds between the sulphur atoms on the R groups of cysteine residues = disulphide bridges. These are the strongest types of bonds so the least likely to be broken during protein denaturation

quaternary structure

• some proteins comprise more than one polypeptide chain
• e.g. haemoglobin = 2 and 2 chains, each associated with an iron-bearing haem group which can bind to oxygen
• chains are held together by range of bonds
• insulin (syllabus specified) comprises 2 chains held together by 2 disulphide bridges.
• is a globular hormone which causes several tissues to respond to a rise in blood glucose bringing it back to normal - i.e. homeostasis
• held together by range of bonds

denaturation

• when a protein loses its 3D shape = denaturation
• caused by breaking of bonds hold tertiary structure together
• caused by high temp - heat energy breaks hydrogen bonds
• change in pH - alters charge on ionic groups - breaks ionic bonds
• presence of ions - bind to ionic groups
• of particular importance in enzymes as changes shape of active site

questions on protein structure

• What is the general term for the monomers which comprise proteins
• Draw a diagram of one of these monomers
• Draw a diagram showing 2 monomers joining
• What type of reaction is this
• Define the primary structure of a protein
• How is the tertiary structure of a protein held together

exam style question on protein structure

Describe, using named examples to illustrate your answer, the structure of proteins (8)