Exam 3: Lecture 20

Question 0

Reversible Inhibition

Answer 0

• 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

Question 1

Enzyme Inhibition

Answer 1

•Molecules that interfere with analysis by slowing or halting enzymatic reactions

Question 2

Irreversible Inhibition

Answer 2

• 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

Question 3

Competitive Inhibitors

Answer 3

• compete with substrates fro the active site of the enzyme

* These increase the effective Km for an enzyme

Question 4

Uncompetitive Inhibition

Answer 4

• 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

Question 5

Noncompetitive Inhibiors

Answer 5

• Can bind with both the enzyme and enzyme-substrate comples

* Affects only the Vmax

Question 6

Plot on L-B of kinetics shows ______ has no effect on Vmax but increases Km

Answer 6

•competitive inhibitor

Question 7

Higher concentrations of substrate are required to maintain a particular velocity in ______ inhibition.

Answer 7

•Competitive

Question 8

Reaction pathway shows that _______ inhibitor binds only to the ES complex.

Answer 8

• uncompetitive

* Vmax cannot be attained, Km lowers as more inhibitor is added

Question 9

What inhibitor reduces Vmax and Km equivalently and doesn’t affect the slope?

Answer 9

•uncompetitive

Question 10

Pathway shows that ______ inhibitor binds both to fee enzyme and ES. Vmax cannot be attained and Km is unchanged.

Answer 10

•Noncompetitive

Question 11

Noncompetitive inhibitor L-B plot, Km is _______, while Vmax is ______.

Answer 11

•unaltered, decreased

Question 12

Allosteric Inhibition

Answer 12

• 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

Question 13

Allosteric activation

Answer 13

• 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

Question 14

Aspartate Transcarbamoylase (ATCase)

Answer 14

• 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)

Question 15

Which state is ATCase less active?

Answer 15

• T state (tense)

* Favored by CTP binding

Question 16

Which state is ATCase more active?

Answer 16

• R state (relaxed)

* Favored by substrate binding

Question 17

What is a potent copetitive inhibitor used to examine the active site?

Answer 17

•PALA, an analog of the reaction intermediate

Question 18

Protein phosphorylation

Answer 18

• Facilitated by kinases

* Dephosphorylation ob phophatases

Question 19

Protein Kinase A

Answer 19

• activated by multiple signals including epinephrine
• 4 molecules of cAMP bind to regulatory subunits of PKA
• dissociates form inhibited holoenzyme, potentiating signal

Question 20

Protein phosphatase 2B (calcineurin or calmodulin dependent protein phosphatase)

Answer 20

• Consists of catalytic subunit (calcineurin A) and a regulatory, Ca-binding subunit (calcinerin B)
• Increases in Ca can activate, turning off AC

Question 21

Serine proteases

Answer 21

• 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

Question 22

–––––––becomes active protease following peptide cleavage.

Answer 22

•Zymogen

Question 23

_________Cleaves and activates pancreatic and stomach zymogens.

Answer 23

•Enteropeptidase

Question 24

_________ is the zymogen secreted by the stomach.

Answer 24

• pepsinogen, pepsin active site

* ***all others secreted by the pancreas