Proteins

• Biological Molecules
• Biological Molecules Elements

  • Carbohydrates - C,H,O
  • Lipids - C,H,O
  • Proteins - C,H,O,N,S
  • Nucleic Acids - C,H,O,N,P
• proteins - made up of one or 4 more large polymers, creating a macromolecule
• Primary Structure

  • basic sequence, number and type of amino acids in polypeptide,determined by the codons sequence in mRNA
  • structure determines the function
  • bonds: peptide bonds
• Secondary Structure -

  • the sequence of amino acids causes parts of a protein to bend into alpha-helix / beta-pleated sheets
  • shape depends on hydrogen bonding

  -> bonds form between O electronegative attached to C=O & H electropositive attached to -NH

  • alpha-helix , all N-H bonds on same side of chain (held by bonds) spiral shape, hydrogen bonds parallel to helical axis
  • beta-pleated sheets, N-H + C=O groups on alternate from one side to another
• Tertiary Structure -

  • 3D structure formed by further folding
  • Disulfide Bridges, strong S-S bonds between molecules of amino acids cysteine
  • Ionic Bonding, relatively strong bonds between charged R groups (pH changes cause these bonds to break)
  • Hydrogen Bonding, numerous and easily broken
• Quaternary Structure -

  • functional proteins may consist of more than one polypeptide
  • precise 3D structure held together by same bonds as tertiary structure
  • may involve an addition of prosthetic groups e.g. metal ions or phosphate groups
• Hydrogen Bonds -

  • formed between hydrogen atoms with slightly positive charge + other atoms with a slightly negative charge
  • in amino acids, they form in hydroxl, carboxyl + amino groups
  • they may form between polar areas of R groups

  -> these in particular are involved in keeping the tertiary + quaternary structure in shape

  • a lot of hydrogen bonds = collective strength
• Ionic Bonding -

  • electrostatic attraction between two oppositely charged ions
  • can form between hydroxyl + amino group that are part of R groups

  -> these ionise into NH3+ and COO+ groups
• Disulfide Links -

  strong covalent bonds

  • R groups of amino acids cysteine contains sulphur
  • disulfide bridges are formed between R groups of the cysteine
• Hydrophilic + Hydrophobic Interactions

  • interaction causes twisting of amino acid chain which changes shape of protein
  • can be a very important influence given that most proteins are to be found surrounded by water inside a living organism
• globular :

  • spherical + compact
  • hyprophobic face inwards,usually water soluble
  • involved in metabolic processes e.g enzymes such as amylase and insulin

  fibrous :

  • regular repetitive sequences of amino acids + usually insoluble
  • enables to form fibres + tend to have structural function
• Haemoglobin

  • made of 4 polypeptide chain
  • 2 alpha-globin + 2 beta-globin
  • the chain are each tertiary structure and together form haemoglobin
  • outside each chain , there's an empty space for haem group

  -> haem group contains an iron ion which o2 binds to, when it does haemoglobin turns from purple red to bright red
• Pepsin

  • single polypeptide of 327 amino acids that fold into a symmetrical tertiary structure
  • has 4 amino acids in basic R groups + 43 amino acids in acidic R groups
  • the reason it's stable in stomach is because it has little effect on enzyme structure
  • held by 2 disulfide links + hydrogen bonds
• Insulin

  • 2 polypeptide chains
  • A chain starts with alpha-helix, B chain ends with beta-pleats
  • both chains fold in to tertiary structure + then joined by disulfide links
  • hydrophilic on outside so it's soluble in water
  • insulin binds to glycoprotein receptors on muscle cells
• Keratin

  • rich in cysteine so lots of disulfide bridges form between its polypeptide + hydrogen bonding
  • keratin is found where a body part needs to be hard + strong
  • found in finger nail, hair etc
  • barrier to infection + waterproof from water pollutants
• Collagen

  • artery walls, a layer of collagen prevent artery from bursting with withstanding high b.p.
  • tendons are made of collagen + connect muscles to bones , allowing to pull on muscles
  • bones are made of collagen, and then reinforced with calcium phosphate (CaPO) which makes them hard
  • cartilage + connective tissues are made of collagen
• Elastin

  • highly coiled structure
  • crossed links with other peptide bonds (quaternary structure)
  • cross-linking + coiling makes the structure of elastin strong + flexible
  • found where living things need to stretch to adapt as part of life processes
  • recoils when deformed
  • found in skin, lungs, bladder + blood vessels