Tertiary and Quaternary Proteins

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Features of Tertiary structure

  • Interior is non-polar (hydrophobic)
  • Exterior is mainly polar (hydrophilic)
  • Amino acids are in close proximity to allow bonds to form (electrostatic, van der Waals, hydrogen and disulphide).
  • Adds strength to extracellular proteins
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Domains are either:

  • alpha - 4-helix (Rop protein in plasmids); globin fold
  • beta - antiparallel; (enzymes, transport porteins, antibodies); immunoglobin fold, b-barrel
  • alpha/beta - the most common, found in all types of proteins Rossman fold; a/b barrel

There are many variants of these three general categories...

e.g. the GLOBIN fold, 8 helicies (A-H)

an a-domain: single domain protein

Sperm whale myoglibin (heamoglobin similar with 4 subunits)

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Quaternary proteins

  • Quaternary is the term given to polypeptides that exist as multisubunits.
  • Haemoglobin is an example of a quaternary protein
  • Each B globin chain consists of 8 with a Haem molecule
  • Conservative for the hydrophobic interior as it contains non-polar groups
  • The tetramer is composed of 2 identical dimers, a1b1 and a2b2.
  • The function of haemoglobin is to bind to oxygen reversibly; the tetramer undergoes co-operative binding.
  • The binding of one oxygen molecule assists the binding to the next molecule etc.
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Functions of Proteins

  • CATALYSIS (enzymes) - e.g. lysozyme, pepsin, superoxide, dismutase, amylase, hexokinase, glucose oxidase.
  • STRUCTURAL - e.g. keratins, fibrion, collagen, elastin, fibrin, titin.
  • MEMBRANE COMPONENTS (integral/peripheral proteins) - e.g. receptors, ion channels, spectrin, glycophorin, G-proteins, adenyl cyclase.
  • CONTRACTION/MOVEMENT/FORCE TRANSDUCTION - myosins (acting on actin), dyneins and kinesins (acting on microtubules)
  • TRANSPORT - e.g. storage haemoglobin, cytochromes, transferrin, albumin, myoglobin, retinol binding protein.
  • TRANSLOCATION - e.g. Na+/K+ pump, glucose transporter, aquaporin membranes.
  • RECEPTORS - e.g. adrenoreceptors, insulin-R, glucaon-R, growth factor-Rs, odoreceptors.
  • REGULATORS (cell function) - e.g. calmodulin, cyclins.
  • REGULATORS (DNA function) - repressor proteins, transcription factors, replication factors.
  • RIBOSOMES - ribosomal proteins, initiation factors, elongation factors, release factors.
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Functions of Proteins 2

  • INTERCELLULAR COMMUNICATION - e.g. insulin, glucagon, growth hormone, grwoth factors (PDGF, EGF, NGF, erythropoietin)
  • ANTIBODIES - e.g. immunoglobins.
  • PROTECTION/PACKAGING - e.g. virus coat proteins, histones
  • LUBRICATION - e.g. mucin
  • OSMORALITY - e.g. serum proteins (albumins, globulins)
  • PH BUFFER - e.g. serum proteins (albumins, globulins)
  • AMINO ACID STORAGE - e.g. casein, ovalbumin
  • CYROPROTECTION - e.g. serum proteins (of marine fish, antifreeze proteins)
  • LUMINESCENCE - e.g. luciferase (fireflies;similar in some jellyfish)
  • SWEETNESS - e.g. monellin, thaumatin
  • TOXINS - e.g. tetanus toxin, pertussis toxin, bungarotoxin (snake venom), melittin (bee venom), ricin.
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Enzyme Classification

  • OXIDOREDUCTASES - oxidation-reduction reactions, e.g. dehydrogenases, oxidases, peroxidases.
  • KINASES/TRANSFERASES - transfer groups from one molecule to another, e.g. amino, carboxyl, carbonyl, methyl, phosphoryl.
  • HYDROLASES - cleave a bond by adding water, e.g. esterases, phosphotases, peptidases.
  • LYSASES - either form or add to (and thus remove) a double bond, e.g. decarboxylases, hydratases, deaminases, synthases.
  • ISOMERASES - intramolecular rearrangements, e.g. epimerases (asymmetric carbon atoms) and mutases (moving functional groups)
  • LIGASES - form a covalent bond between two substrates, energy supplied by ATP and often called synthetases.
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