Proteins Flashcards
Define protein primary structure
Sequence of amino acids
Define protein secondary structure
Folding patterns of peptides
Alpha helix, beta sheet, beta bends
Define protein tertiary structure
3D assembly of a protein
Define protein quaternary structure and some possible functions facilitated by this organization
complex of multiple polypeptides
Allows for….
Transmission of info between subunits
Mix-and-match functional units
Passage of substrates directly from one enzyme to another
Define isoforms/isozymes in regards to protein function
Proteins with distinct primary structure that perform the same general function
Describe the two general modes of proteolytic cleavage following removal of a signal peptide
- Removal of short peptides from the N- and C- termini
- Cleavage of polypeptide into multiple, active segments
Transient, reversible post-translational modification of peptides is…
Addition of a chemical group
eg. phosphorylation
Contrast N-linked and O-linked glycosylation
N-linked glycosylation
- Modification occurs in the lumen of RER
- Sugar is transferred to peptide from a ER membrane lipid
O-linked glycosylation
- Modification occurs in the lumen of the Golgi
- Sugar is synthesized directly onto the peptide
Describe cotranslational transport into the RER
- Preproprotein has signal sequence that is recognized by a signal recognition particle while the protein is being synthesized by the ribosome
- Protein-Ribosome complex migrates to the translocator
- Translation continues while new protein feeds into RER lumen
- Signal peptides of most proteins are immediately cleaved following entrance into RER by signal peptidase
- Chaperone proteins bind to translated protein to allow for proper folding and provide protection from high conc. of proteins in RER
- Folded protein is modified as needed and shipped to the Golgi if required
How are cytoplasmic proteins trafficked following translation?
Cytosolic proteins have a sorting signal that is specific for a receptor at the target location (eg. nucleus, mitochondria, peroxisome, etc.)
What is a nuclear localization signal? Describe the mechanism of translocation into the nucleus by a protein with a NLS
One or more short sequences of cationic amino acids (lysine, arginine) on surface of protein
- The NLS is recognized and bound by an importin protein
- Protein/importin complex moves through a nuclear pore complex (NPC)
- Complex is bound by Ran-GTP, which releases protein from importin
- Importin/Ran-GTP complex moves back into cytoplasm
- Cytoplasmic GTPase converts Ran-GTP to Ran-GDP, releasing importin
- Ran-GDP is recycled back into nucleus
Describe the ubiquitin-proteosome system
- Ubiquitin is enzymatically bound to a target protein via formation of a peptide bond (usually between Ubiq. glycine and target protein lysine)
- Marked protein is recognized and bound by proteasome
- Marked protein is migrated to core of proteasome and denatured by proteolytic active sites
Describe the makeup and structure of collagen
Mostly glycine/proline sequence repeats
Forms a compact triple helix structure
What are the steps of collagen biosynthesis?
- mRNA is translated into preprocollagen
- Signal sequence is cleaved
- proline and lysine are hydroxilated by hydroxylases
- hydroxylysine is glycosylated with a glucose/galactose monomor (note: hydroxyproline is not glycosylated)
- glycosylated and hydroxylated peptides twist, forming a triple helix
- Outside of the cell, membrane-bound collagen peptidases remove loose ends of procollagen to form tropocollagen
- Tropocollagen molecules are covalently bound via lysyl oxidases to form collagen fibrils
How can disease arise from improper collagen synthesis?
Mutations or environmental effects that affect triple helix formation, removal of C- and N- terminus peptides, etc.
eg. lack of vitamin C cofactor for hydroxylation, results in weak cross-linking and a looser triple helix
eg. Loss-of-function peptidases that do not remove N- and C-terminal peptides, leave unwound portion in collagen fibril
