Topics Covered: (UNFINISHED)
- basic codons
- protein processing
- post translational modifications
- Gene regulation
Stop: UGA, UAG, UAA
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Post Translational Modifications
The addition of a glycosyl group to a protein. In the ER this is N-linked glycosylation occurring on the nitrogen of asparagine or arginine. In the Golgi this is O-linked glycosylation occurring on the oxygen of serine, threonine, or tyrosine. This addition assists in proper folding, attachment, and stability. Un-glycosylated proteins may be readily degraded.
The addition of a phosphate group to a oxygen or nitrogen. Usually serine, threonine, and tyrosine or histidine. This often results in turning a protein on or off, thus regulating the protein’s activity.
The addition of a methyl group to positively charged residues, lysine or arginine. Methylation is generally considered an inactivating modification.
Acetylation is both a co and post-translational modification that can occur to proteins. A acetyl group (CH3O) is added to the N-terminus or Lysine residues of the protein. This is generally considered an activating modification.
SUMO proteins (Small Ubiquitin-like Modifier) covalently attach and release from proteins to modify their functions. These modifications aid in nuclear-cytosolic transport, transcriptional regulation, apoptosis, protein stability, stress response, and cell cycle progression. SUMO is mature when the last 4 amino acids from the C terminus have been cleaved. Attachment to proteins imilar to ubiquitin via E1, E2, E3 proteins (activating, conjugating, and ligases).
Ubiquitin acts as a marker for protein degradation if in chain form. Individual molecules acts as signals for cell location, activity, and protein-protein interactions. Addition of ubiquitin to a lysine in the protein of interest is performed by E1, E2, and E3 proteins.
Transcription is regulated at the nucleosome level by covalent modification of the histone. These include the acetylation and mono-, di-, and tri-methylation of lysine residues. These are all reversible modifications that occur largely on the N-terminal histone tails but can occur on the core protein as well. There are specific enzymes for each modification which are recruited by gene regulatory proteins that bind to specific DNA sequences:
- HATs (histone acetyl transferases)
- HDACs (histone deacetylase complexes)
- histone methyl transferases
- histone demethylases
The effects of these is acetylation of lysines loosens chromatin studctures by canceling the positive charge and thus reducing tail affinity of adjacent nucelosomes. The precise structure and proteins depends on the gene encoded in the DNA segment. The sequence of modifications creates a Histone Code.