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 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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- 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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