Proteins

• Proteins


  Large molecules comprised of long chains of amino acids
• Proteins


  • Form structural components e.g. muscle
  • Can adopt specific shapes e.g. for enzymes and some hormones
  • Membranes have protein constituents which act as carriers and pores for diffusion, facilitated diffusion and active transport
• Amino acids


  Required to make proteins, animals can make some, others must be ingested- essential amino acids
• Plants can make all amino acids they need, if access to fixed nitrogen, e.g. nitrate
• Amino acids


  • Each contains carbon, hydrogen, oxygen and nitrogen
  • There are over 500 amino acids, but only 20 are proteinogenic (found in proteins)
  • The R group is what makes each amino acid different
  • R groups vary by size, charge and polarity
• Amino acids join together

  1. By covalent bonds called peptide bonds, formed in a condensation reaction, broken by hydrolysis

  2. 2 amino acids joined together is a dipeptide, and a longer chain is a polypeptide
• Protein structure


  • Primary structure: sequence, number and order of amino acids bonded by peptide bonds
  • Secondary structure: chain of amino acids twists into shape called secondary structure, some coil into an ⍺-helix, others fold in a zig zag structure, and that folds to produce a β-pleated sheet
  • Tertiary structure: additional bonds between R groups, when coils and pleats further coil, tertiary structure is formed, which is a very precise shape held by different covalent bonds
  • Quaternary structure: describes how multiple polypeptide chains are arranged to make complete protein molecule and may be held together by same bonds as 3° structure
• Protein bonding


  • 1° held by strong covalent bonds
  • 2° mainly hydrogen bonds as well
  • 3°, 4° contain many others e.g. ionic bonds between amino group and carboxyl of R groups, disulphide links between 2 cystines (with sulphur)
• Fibrous proteins


  • Regular, repetitive sequences of amino acids, and are usually insoluble in water, enabling then to form fibres (which have structural function)
  • Examples: collagen, keratin, elastin
• Fibrous proteins


  • Collagen
  • Keratin
  • Elastin
• Globular proteins


  • Protein rolled up into a spherical shape, hydrophobic R inwards, hydrophilic R outwards so soluble
  • Very specific shapes, good for enzymes, hormones, haemoglobin
  • Examples: haemoglobin, insulin, pepsin
• Globular proteins


  • Haemoglobin
  • Insulin
  • Pepsin