Post-Translational Modification of Proteins

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  • Created by: rosieevie
  • Created on: 10-01-17 19:29

Crick's Central Dogma (2003)

Genes can produce multiple proteins using RNA splicing, addition, foliding, modifications and targeting e.g. K+ channels in squid giant axon 1 gene = 4.5 x1015 distinct channel types

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Protein Synthesis Later Stages

To become a functional protein:

  • Unique 3D folding
  • Modifications e.g. cofactor binding, uses kinases or peptidases
  • Sorted into correct location
  • Assembly correctly with other protein subunits

Required information for processes is within primary amino acid sequence

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Modification of Amino Acids

EX - Glutamic acid to y-carboxy-glutamate (COO- group) by vitamin k-dependent enzyme system

  • e.g. prothrombin (blood clotting) - first 10 glutamates carboxylated so it can bind to calcium moore effciently = bind to cell membrane to coagulate

EX (stable modifications) - conversion of lysine to 5-hydroxylysine in collagen by lysyl hydroxylase

Other examples inc. glycosylation, phosphorylation (reversible)

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Modification During Folding

2 enzymes promote correct folding by catalysing modification reactions:

  • Peptidyl prolyl isomerase - catalyses proline isomerisation (interconversion of prolines cis cyclic structure to trans)
  • Protein disulphide isomerase (PDI) - promotes correct disulphide bond formation (nearest -SH groups bonding is not always desirable)
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Modifications During Transport

Signal sequences direct proteins to correct cellular address. N-terminal peptides - range between 15-60 amino acids and are cleaved by peptidases once transport is complete:

  • Signal peptide - direct to ER
  • Transit peptide - direct to mitochondira/chloroplasts

Sometimes 2 signal sequences to be more specific e.g. chloroplast then thylakoid lume

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Attachment of Fatty Acids and Lipids

Anchor protiens to membranes by attaching fatty acids/lipids:

  • Mytistolylation - protein with glycine at end terminus = adds C14 fatty acid
  • Palmitoylation - protein with internal cysteine = C16 fatty acid
  • Pronylation - protein with cysteine 4 residues on C-terminus = add prenyl lipid
  • GPI anchor helps binding to lumerial or extracellular surfaces of membrane
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Attachment of Fatty Acids and Lipids

Anchor protiens to membranes by attaching fatty acids/lipids:

  • Mytistolylation - protein with glycine at end terminus = adds C14 fatty acid
  • Palmitoylation - protein with internal cysteine = C16 fatty acid
  • Pronylation - protein with cysteine 4 residues on C-terminus = add prenyl lipid
  • GPI anchor helps binding to lumerial or extracellular surfaces of membrane
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Proteolytic Cleavage

  • EX - Insulin Biosynthesis

(http://biochem.co/wp-content/uploads/2008/08/biosynthesis-of-insulin.png)

  • EX - zymogens are enzyme precursors that are activated in a proteolytic cascade 
    • e.g. trypsinogen -> trypsin in duodenum (reduces damage to other cells)
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Binding of Cofactors

Helps to stabalise the protein - allows enzymes to work:

  • Fe-S centres (electron transport)
  • Coordination of metals
  • Binding of tetrapyrroles e.g. haem, chlorophyll
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Assembly of Oligomeric Complexes

Often needed to function:

  • Oligomer of identical subunits
  • Oligomer of two different subunits e.g. ribosomes
  • Large complex of many different subunits
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Reversible Modifications

Regulate protein activity. Not modified constants - dynamic molecules.

  • Glycosylation
  • Oxidation
  • Mehtylation
  • Acetylation
  • Phosphorylation
  • Ubiquitnation
  • Carbonylation
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