Enzymes II

Question 1

name a, b and c

Answer 1

a = normal

b = competitive

c = noncompetitive

Question 2

describe the difference between reversible and irreversible inhibitors

Answer 2

• Reversible inhibitor drugs bind noncovalently
• Irreversible inhibitor drugs bind covalenty to the enzyme
  
  • Covalent modification of the active site
  • Covalent binding to an SH-group of the enzyme
  • Covalent interference with a metal cofactor (zinc, molybdenum)

Question 3

describe the function of DFP

Answer 3

• DFP = di-isopropyl fluorophosphate
• Irreversible inhibitor of acetylcholinesterase
  
  • Acetylcholinesterase normally cleaves the neurotransmitter acetylcholine in the postsynaptic neuromuscular junction

Question 4

name normal conditions vs DFP conditions

Answer 4

• At the active site: serine normally forms a covalent bond with the acetyl group during catalysis
• DFP blocks this reaction and acetylcholine accumulates
  
  • Continuous stimulation of muscle contraction and overstimulation of the autonomic nervous system
    
    • blurred vision, bronchoconstriction, seizures, respiratory arrest and death

Question 5

describe function of aspirin

Answer 5

• Irreversible enzyme inhibitor
• NSAIDs are mostly reversible inhibitors, but aspirin is not
• Aspirin irreversibly inhibits the enzyme COX which is needed for prostaglandins and thromboxane synthesis
• Aspirin is acetyl-salicylic acid and it acetylates a serine in the channel of the active site of COX

Question 6

describe the process of aspirin

Answer 6

• Aspirin is acetyl-salicylic acid and it acetylates a serine in the channel of the active site of COX
• Aspirin is used for pain treatment and acts as an anti-blood clotting factor
• The action of aspiring is an irreversible COX inhibition in cells
• In most cells the acetylated enzyme is degraded and a new enzyme is synthesized

Question 7

how is COX involved in blood clotting?

Answer 7

• The enzyme is needed in:
  
  • 1. Platelets for eventual release of thromboxane
  • Endothelial cells for eventual release of prostacylcin
• Activated platelets start thromboxane synthesis using COX and thromboxane synthase and release thromboxane
  
  • Thromboxane stimulates blood clotting
• Endothelial cells start prostacyclin synthesis using COX and prostacyclin synthase and release prostacyclin
  
  • Prostacyclin prevents blood clotting

Question 8

how does aspirin affect COX

Answer 8

• Aspirin irreversible inhibits COX in both platelets and endothelial cells
• 1. endothelial cells degrade the modified enzyme and synthesize new COX that is fully active
     * Leads to increased prostacyclin
• 2. Platelets are not able to synthesize new COX and less thromboxane is finally released

Question 9

define suicide inhibitor drugs

Answer 9

• Some drugs have the structure of competitive inhibitors but have also the special characteristics of having their structure modified by the enzyme where the drugs now become an irreversible inhibitor ending the reaction

Question 10

describe the purpose of allopurinol

Answer 10

• suicide inhibitor drug
• Used in patients with gout who are overproducers of uric acid
• Uric acid is formed by xanthine oxidase in the degradation of purine nucleotides

Question 12

describe the function of allopurinol

Answer 12

• allopurinol used to irreversibly inhibit xanthine oxidase
• The enzyme uses allopurinol and forms the irreversible inhibitor alloxanthine (oxypurinol)
• Alloxanthine binds very tightly to a molybdenum-sulfide complex in the active site which is needed for catalysis

Question 13

name the 4 methods of regulating enzymes

Answer 13

1. Concentration of substrates or products
2. Modulation of enzyme concentration
3. Covalent modification of enzymes
4. Modulation of a metabolic pathway (allosteric enzymes)

Question 14

describe regulation of enzymes via substrate concentration

Answer 14

• At substrate concentrations below Km the effect on velocity is strongly responsive to the substrate availability

Question 15

describe the modulation of enzyme concentration

Answer 15

• Induction of enzyme synthesis leads to the synthesis of additional enzyme and can take hours to days
• Repression of enzyme synthesis interferes with normal transcription of the enzyme
• Degradation of an enzyme can be achieved by specific changes in the protein structure
  
  • Degradation is recognized by the ubiquitin-proteasome system

Question 16

describe covalent modifications of the enzyme

Answer 16

• Often performed by phosphorylation and dephosphorylation
  
  • some enzymes are activated by phosphorylation and others inactivated by phosphorylation
• By proteolytic activation (irreversible process)

Question 17

describe the process of proteolytic cleavage

Answer 17

• Irreversible covalent modification
• It is very safe and used for the activation of:
  
  • blood clotting enzymes
  • digestive enzymes that could damage if activated prematurely

Question 18

name examples of proteolytic activation

Answer 18

• Trypsinogen is synthesized in the pancreas and released into the pancreatic juice
• Trypsin is a powerful protease and is active only in the lumen of the small intestine
• Enteropeptidase in the duodenum recognizes trypsinogen and cuts it to trypsin

Question 19

describe the regulatino of metabolic pathways using allosteric enzymes

Answer 19

• Allosteric enzymes are often the regulated enzymes of pathways
• They have an allosteric binding site for specific molecules (effectors) which are different sites than the substrates, coenzymes and products use
• Binding leads to a reversible conformational shift and can reduce or increase enzyme activity
  
  • sigmoid curve results from cooperative substrate binding