Chpt 10 b Flashcards
Reversible Covalent Modification
Used to turn proteins on or off
Phosphorylation
- dephosphorylation
- acetylation
Phosphorylation
-reversible attachment of phosphate group (usually gamma phosphate) to hydroxyl of R group of S, T, Y
Catalyzed by Protein Kinase
- Enzyme that transfer a phosphate from from ATP to R group hydroxyl on S,T, Y of Protein
- Serine/threonine protein kinases
- Tyroseine kinases
Phosphorylation works well to regulate activity because:
- Phosphate’s two negative charges disrupt electrostatic interaction
- Phosphate bonds are directional
- Large free energy
- Rapid rate (less than sec)
- amplified effect
- Use of ATP ties phosphorylation to energy charge of cell
Dephosphorylation
-Removal of phosphate from phosphorylated protein
Catalyzed by Protein Phosphatase
- enzymes that removes a phosphate from phosphorylated protein
- Serine/threonine protein phosphatase
- tyrosine protein phosphatase
Phosphorylation/Dephosphorylation
At physiological conditions
- irreversible
- rate of uncatalyzed reaction is negligible
Kinases/Phosphatases maybe:
1) Dedicated Kinases/Phosphatases
- phosphylates/dephosphorylates specific target
2) multifunctional Kinases/Phosphatases
- phosphylates/dephosphorylates multiple targets
Acetylation
attachment of an acetate group to the R group of Lysine
-Histones are acetylated and deaceltylated
Cyclic AMP (cAMP)
Phosphate connects 3’ carbon to 5’ carbon
- second messenger
- Example: amplification of fight or flight signal from hormone epinephrine (adrenaline)
Protein Kinase A (PKA)
Heterotetramer of two subunits (R2C2)
1) Catalytic subunit:
- phosphorylates large proteins (protein kinase activity) when freed by R subunit
2) Regulatory Subunit
- each R subunit contains two binding site for cAMP
Numerous Isozymes
PKA participates n “fight or flight” response
- Hormone epinephrine (adrenaline) binds membrane receptor
- stimulates synthesis of cAMP (second messenger) by adenylate cyclase
- Two cAMP bind to regulatory subunit of PKA
- catalytic subunit is activated and functions as kinase
Pseudosubstrate binds to active site of the C subunit
X-ray Crystal structure of PKA
Inhibitor and ATP-Mg2+ bind to active site of PKA catalytic subunit
Two lobes:
- smaller lobe binds ATP-Mg2+
- Large lobe binds protein and contains catalytic residues
Binding of substrate causes a conformational change in PKA resulting in the two lobes moving closer to each other
Proteolytic activation
-def
an enzyme is activated by cleaving peptide bond by proteolysis
Zymogens
Zymogen or proenzyme
- inactive precursor of enzyme
- often activated by proteolysis
use secretory pathway:
1) rER-proteins (zymogens) synthesize
2) Transport vesicles (COP II)
3) Golgi- Clathrin coated
4) Stored vesicles (granules)
- stored until nerve impulse or hormonal signal is received
5) Secreted
Processes using zymogens
Digestive enzymes Blood Clotting Protein hormones -preproinsulin-proinsulin-insulin Collagen Development Apoptosis
Synthesis and activation of Chymotrypsin
Chymotrypsin synthesized as 245 amino acid zymogens called chymotrypsinogen in acinar cells of pancreas
- stored in membrane bound vesicles (zymogen granules) until needed
- released into duct of duodenum
Proteolysis of peptide bond between amino acid 15 and 16 causes conformational change
- ile16 turns inward and forms ionic bond to Asp
- Met moves to surface
- Residues 187 to 193 become extended, which begins creating of hydrophobic substrate binding cleft
- oxyanion hole is incomplete
Hydrolysis of single peptide bond causes discrete conformational change
Enteropeptidase
activates trypsin by hydrolyzing lys-6 ole-7 peptide in trypsinogen
Trypsin
- activates more trypsin and other zymogens
- hydrolyzes proteins on c terming side following long, positively charge R groups (R, K)
Elastase
Cleaves peptide on C terminus of small chains (A,S)
