Proteins and water
Proteins
All proteins are made from the same monomer – amino acids.
Amino acids
All have a central carbon atom bonded to –
• an amine (–NH2 ) group
• a carboxylic group (–COOH)
• a hydrogen
• a R–group that determines what type of amino acid it is
The peptide bond
• a molecule made up of many amino acids linked together by peptide bonds is a polypeptide
• polypeptides can be broken down to amino acids by breaking the peptide bonds in a hydrolysis reaction
• this happens naturally in the stomach and small intestine during digestion
Structures of proteins
Bonds in the tertiary structure
| PROTEINS | |
| GLOBULAR | FIBROUS |
| spherical/balled shaped • curl up so that their non- polar, hydrophilic R- groups point to the centre of the molecule away from watery surroundings • polar, hydrophilic R- groups are on the outside which makes mixing + dissociating in water easier | proteins that form long strands |
| usually soluble | usually not soluble in water |
| precise shape, have roles in metabolic activities and are specific in nature | have structural roles |
| e.g., enzymes, haemoglobin, myoglobin | e.g., keratin, actin, myosin, collagen |
Haemoglobin: a globular protein
• made of 4 polypeptide chains therefore they have a quaternary structure
• 2 of the haemoglobin chains, ⍺–chains, are made of ⍺– globin
• the other 2 chains, β–chains, are made of β–globin
• each polypeptide chain has a haem group attached (prosthetic group) to it
• haem contains charged particle of iron
• the haem group is also responsible for the colour of haemoglobin
• each polypeptide chain can carry one molecule of oxygen
• therefore, in total, haemoglobin can carry 4 molecules of oxygen or 8 oxygen atoms
Collagen: a fibrous protein
• found in skin, tendons, cartilage, bone, teeth, etc.
• a structural protein
• collagen molecule consists of 3 polypeptide chains, each in a helical shape
• helical polypeptides are wound together creating a triple helix
• strands are held together by hydrogen and some covalent bonds
• every 3rd amino acid in each polypeptide chain is glycine
• each 3 stranded molecule interacts with other collagen molecules running parallel to it
• covalent bonds form between R–groups of amino acids
• these cross–links hold many collagen molecules side by side forming fibrils
• many fibrils lie alongside each other forming strong bundles called fibres
• collagen is flexible but has tremendous tensile strength
• collagen fibres line up according to the forces they withstand