410 exam 3 Flashcards
(40 cards)
transition state analogues
- chemically and structurally similar to the transition state
- bind more strongly than substrate or competitive inhibitors
- the tighter an enzyme binds to t.s., the higher the rate of catalyzed reaction
- the more t.s. –> less side effects
3 types of transition state analogues
- statins (cholesterol-lowering drugs that inhibit HMG-CoA reductace ex: lipitor)
- protease inhibitors (AIDS drugs, HIV-1 inhibitors ex: squinavir)
- viral neuraminidase inhibitor- treats influenza by inhibiting the neuramidase rxn (essential for viral respiration ex: tamiflu)
specific vs. general acid-base catalysis
specific- H+ or -OH diffusing in from solutions
general- proton transferred in transition state
* acid: proton transfer lowers free energy of t.s.
* base: proton abstraction lowers free energy of t.s.
RNase A Acid-Base Catalysis
- digestive enzyme secreted by pancreas
- hydrolyzes RNA to its component nucleotides
- 2 residues: His 12 (base) and His 119 (acid)
Metalloenzymes
- tightly bound metal cofactors
- Fe 2+, Fe 3+, Cu2+, Zn 2+, Mn 2+, Co2+
- bind to substrates for orientation, mediate redox rxns by changing oxidation state, and stabilize/shield negative charges
Metal activated enzymes
- only loosely bind the metal ions
- Na+, K+, Mg 2+, Ca 2+
- structural role
where do Trypsin, Chymotrypsin, and Elastase cleave?
Trypsin: Lys, Arg
Chymotrypsin: Tyr, Phe, Trp
Elastase: Ala, Gly
catalytic triad
active sites composed of His, Asp, Ser
what stabilizes transition state ?
- oxyanion hole (amide groups)
- Ser 195 and Gly 193 provide primary stabilization of tetrahedral oxyanion
oxyanion hole
- carbonyl oxygen moves deeper into active site due to conformational changes
- preferential binding of t.s. (tetrahedral intermediate) –> enhanced rates
- lowers t.s. free energy for formation of tetrahedral intermediate
aspartic proteases
- acid-base mechanism
- has 2 active site aspartic acid residues (catalytic dyad)
- active at acidic pHs
- a-b: extraction of 2 protons leads to Nu attack
- c-d: Asp 32 extracts proton; Asp 215 donates proton
HIV-1 protease
- aspartic protease
- cleave yields active products
protease inhibitors
- transition state analogs (enzyme inhibitors)
- older meds targeted reverse transcriptase
- newer meds target HIV protease and mimic a T.S.
what are the 2 ways to regulate enzyme activity
1) availability of enzyme
2) control of enzyme activity
zymogens
- inactive precursor of a proteolytic enzyme
- aka proenzyme
- made and activated in different places
digestive enzymes
made as proenzymes so dont destroy tissues
enteropeptidase
cleaves digestive system enzymes at specific peptide bonds (autocatalytic process)
proteolytic cleavage
produces the active enzyme
protein hormones
can also be synthesized in inactive form and later activated
4 ways to regulate enzyme activity
1) synthesis/degradation
2) proteolysis
3) allosteric regulation
4) covalent modification (global signal)
allosteric effectors
- binds to “other site” (allosteric) and regulate their catalytic activity (S-> P)
- dont need to have structural similarity to substrates or products
- effectors shift equilibrium to R state
- effectors shift equilibrium to T state
allosteric enzymes
- give multiple subunits -> multiple binding sites
- hill coefficient x=x 1
- have 2 sites: active (for substrate) and allosteric
example of feedback inhibition pathway
Aspartate Transcarbamoylase (ATCase)
ATCase
- hexameric (6 subunits; dimer of trimers)
- each monomer has a catalytic enzyme positively affected by ATP and negatively affected by CTP
- substrate binding sites change after allosteric binds
- sigmodial shape (T–>R)
