Lecture 22 Flashcards
Where can polypeptide chain cleavage occur along a polypeptide?
1) N-terminal end, most often including Methionine (Met) as well
2) C-terminal end
3) Internal sequence
What are function of polypeptide chain cleavage?
1) Maturation - generation of active products from inactive precursors
2) Inactivation or partial inactivation - regulation of activity
Who was the first person to sequence a protein and what protein was it?
Fred Sanger; insulin
Describe the process of insulin maturation
1) pre-prohormone undergoes signal sequence cleavage (signal peptidase) in ER to form proinsulin
2) In ER, C peptide holds A and B peptides adjacent to each other to allow disulfide bridges to form between A and B peptides
3) C peptide is cleaved off to form insulin (A and B peptides only)
What type of cleavages occur to activate insulin?
1) Tryspin-like (cleaves lysine and arginine - basic amino acids)
2) Carboxypeptidases (chew off amino acids from C-terminal end)
What is familial hyperproinsulinemia?
It is an autosomal dominant disorder characterized by an excess of proinsulin in the blood. There is defective proinsulin cleavage resulting in equal amounts of insulin and partially-cleaved proinsulin in blood. No diabetes will ensue however in a heterozygote affected by the disorder
What are uses of insulin C-peptide?
1) Because the C-peptide is relatively stable in plasma, one can measure insulin production, which is elevated in insulinoma. This can also be used to distinguish type 1 from type 2 DM
2) Monitor endogenous insulin production in patients receiving injected insulin
3) Insulin overdose. In factitious hypoglycermia (self-inflicted insulin overdose) can use forensic tests
4) *It is thought to have physiological properties preventing some DM complications (kidney & CNS)
Proteolytic cleavage is ________ specific
Tissue specific; You can get different combinations of products based on which tissue the proteolytic cleavage occurs
What is a polyprotein?
Viruses create a polypeptide that is destined to be cleaved into multiple proteins
What are two types of proenzymes?
Zymogens & Activation cascades
What do the synthesis of prothrombin and factors VII, IX & X require?
Formation of gamma carboxyglutamate residues via vitamin K. They are inhibited by coumarins
What activates cell death by apoptosis?
Proteolysis cascade. During apoptosis chromatin condenses, DNA is fragmented, cells shrink, and cells are engulfed by macrophages
What does the poliovirus protease do to induce polio?
Poliovirus protease 2A cleaves a polyprotein for activation and also cleaves some cellular proteins, including translation initiation factor eIF4G. Intact eIF4G is required for initiation on capped cellular mRNAs. Cleaved eIF4G still works, but only for poliovirus IRES-mediated initiation. Therefore, there is reduced cap-dependent scanning
What does the poliovirus RNA do?
1) Initiates by internal ribosome entry mechanism (IRES)
2) unaffected by eIF4G cleavage
What are advantages of precursor proteins?
1) Protection of cell/organism (digestive proteases)
2) Allows folding & modification (insulin)
3) Multiple gene products (proopiomelanocortin, poliovirus)
4) Control (apoptosis, blood clotting, poliovirus/eIF4G)
