Exam 3: Lecture 20 Flashcards
Reversible Inhibition
- Bind to enzymes with non-covalent interactions such as H+ bonds, hydrophobic interactions, and ionic bonds
- do not undergo chemical reactions when bonds to the enzyme
- Easily removed by dilution or dialysis
- Ex. HIV protease inhibitors: mimics the enzymes’ substrates
Enzyme Inhibition
•Molecules that interfere with analysis by slowing or halting enzymatic reactions
Irreversible Inhibition
- Covalently modifies an enzyme, and inhibition can’t therefore be reversed
- Aregenerally specifi for one class of enzyme and don’t inactivate all proteins
- They do not function by destroying protein structure but by specifically altering the active site of their target
- Example: FFR-CK covalently binds to the active site serine in plasmin
Competitive Inhibitors
- compete with substrates fro the active site of the enzyme
* These increase the effective Km for an enzyme
Uncompetitive Inhibition
- Does not affect the formation of the enzym-substrate complex but inhibits the enzyme at a site other than the active (allosteric binding site)
- Binds only to the enzyme-substrate complex. Both Km and Vmax are altered
Noncompetitive Inhibiors
- Can bind with both the enzyme and enzyme-substrate comples
* Affects only the Vmax
Plot on L-B of kinetics shows ______ has no effect on Vmax but increases Km
•competitive inhibitor
Higher concentrations of substrate are required to maintain a particular velocity in ______ inhibition.
•Competitive
Reaction pathway shows that _______ inhibitor binds only to the ES complex.
- uncompetitive
* Vmax cannot be attained, Km lowers as more inhibitor is added
What inhibitor reduces Vmax and Km equivalently and doesn’t affect the slope?
•uncompetitive
Pathway shows that ______ inhibitor binds both to fee enzyme and ES. Vmax cannot be attained and Km is unchanged.
•Noncompetitive
Noncompetitive inhibitor L-B plot, Km is _______, while Vmax is ______.
•unaltered, decreased
Allosteric Inhibition
- Active in the uncomplexed form, which has high affinity for substrate
- Binding stabilizes the enzyme in its low-affinity form, resulting in little or no activity
Allosteric activation
- Is inactive in its uncomplexed form, which has a low affinity for its substrate
- Binding stabilizes the enzyme in its high-affinity form, resulting in enzyme activity
Aspartate Transcarbamoylase (ATCase)
- allosterically inhibited by the end product of its pathway
- Catalyses first step in biosythesis of pyrimidines (cyt, thy, and uracil)
- Trimer, 3 regulatory dimer
- Zinc domain (cofactor, stabilizer)
Which state is ATCase less active?
- T state (tense)
* Favored by CTP binding
Which state is ATCase more active?
- R state (relaxed)
* Favored by substrate binding
What is a potent copetitive inhibitor used to examine the active site?
•PALA, an analog of the reaction intermediate
Protein phosphorylation
- Facilitated by kinases
* Dephosphorylation ob phophatases
Protein Kinase A
- activated by multiple signals including epinephrine
- 4 molecules of cAMP bind to regulatory subunits of PKA
- dissociates form inhibited holoenzyme, potentiating signal
Protein phosphatase 2B (calcineurin or calmodulin dependent protein phosphatase)
- Consists of catalytic subunit (calcineurin A) and a regulatory, Ca-binding subunit (calcinerin B)
- Increases in Ca can activate, turning off AC
Serine proteases
- use active site serine for peptide backbone cleavage, chymotrypsin active site
- phe preferred substrate, binds to hydrophobic pocket
- Active site dictates specificity
- Serine uses active site serine 195
- Cleavage of peptide backbone occurs in the third step, creates acyl-enzyme intermediate, rearrangement to release carboxylic acid component
–––––––becomes active protease following peptide cleavage.
•Zymogen
_________Cleaves and activates pancreatic and stomach zymogens.
•Enteropeptidase
