Samuelson BCH lec 8/15 Flashcards
Protein Biochemistry
Translation, Processing, Trafficking, Function, Degradation
There are over ~25,000 genes in the human genome that contain protein coding information for
~100,000 protein sequences
Proteins have highly specialized
functions
Signal Sequence of N-Terminal Amino Acids
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SRP: Signal Recognition Particle
a universally conserved ribonucleoprotein that recognizes the ER signal peptide
Protein Glycosylation
adding carbohydrates to proteins
Mitochondrial Localization Signal
- Amino or N terminal amino acid sequence
- Binds to chaperone
- Protein/chaperone inserted into mitochondria
through a transport complex - Signal sequence is cleaved in mitochondria
Nuclear Localization Signal/Sequence (NLS)
- Binds to importin subunit
- Protein/importin passed through nuclear
pore. (energy dependent) - NLS NOT removed
Proteolytic Processing of Preproteins - Insulin
Pteproinsulin, proteases cleave, proinsulin, keep cleaving, insulin.
C chain is packaged in the secretory vessicle and is secreted along with active insulin
Protein Phosphorylation is the most abundant
protein
Protein Post-Translational Modification (PTM) Phosphorylation
Kinase enzymes add phosphate (PO4) groups. Over 500 exist
Reversible: Phosphatase enzymes remove phosphate groups. Over 100 exist
A dynamic mechanism that regulates multiple
types of protein functions
PO4 results in conformational change and has effects on protein interactions
Reasons for protein degradation
Metabolic needs
removal of abnormal proteins, unnecessary proteins, and regulatory proteins
Protein stability: proteins are degraded at different rates
One dependent factor is the N-terminal amino acid
PEST Proteins are
rapidly degraded
Pro (P), Glu (E), Ser (S), Thr (T)
These proteins often contain phosphorylation sites that target them for ubiquitination
Ubiquitination = post-translational modifications
Protein Degradation - Lysosomes
Lysosomes degrade many proteins
* ~50 hydrolytic enzymes
* Multiple proteases called cathepsins
- Internal pH ~5 (lysosomal enzymes are mostly
inactive at cytosolic pH)
Post-translational Modification - Poly-Ubiquitination
a PTM of a linked chain of
Ubiquitin that marks a protein for degradation
Ubiquitin
76-residue monomeric protein
Ubiquitous and Abundant
Highly conserved eukaryotic
protein
Polyubiquitination marks proteins for degradation
Poly-Ubiquitination
a repeated 3 step process that leads to protein degradation
Poly-Ubiquitination - E3
E3 transfers activated Ub from E2 to a Lys (K) on the protein to be marked for degradation (Substrate) or the previous Ub
Proteasome - Polyubiquitinated proteins are
unfolded and proteolytically degraded in a 26S proteasome
Multiprotein complex
ATP dependent
20S proteasome: a hollow
cylindrical core
19S cap
