Chapter Seven The Behavior of Proteins: Enzymes, Mechanisms, and Control. Allosteric Enzymes. Allosteric: Greek allo + steric , other shape Allosteric enzyme : an oligomer whose biological activity is affected by other substances binding to it
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• Rate of ATCase catalysis vs substrate concentration
• Sigmoidal shape describes allosteric behavior
• ATCase catalysis in presence of CTP, ATP
Note: for an allosteric enzyme, the substrate concentration at one-half Vmax is called the K0.5
Any unbound R is removed KR<KT
Ratio of dissociation constants is called c
• Effect of binding activators and inhibitors
Sequential model for cooperative binding of substrate to an allosteric enzyme
• R form is favored by __________________ activator
• Allosteric inhibition also occurs by the ____________ mechanism
• Unique feature of Sequential Model of behavior:
_____________ __________________ - Induced conformational changes that make the enzyme less likely to bind more molecules of the same type.
• Sequential Model:
Source of PO43- is ______________
• When ATP is hydrolyzed, energy released that drives other energetically unfavorable reactions to take place
• PO43- is donated to residue in protein by protein ____________
Important questions to ask about enzyme mode of action:
• Which amino acid residues on an enzyme are in the active site and catalyze the reaction?
• What is the spatial relationship of the essential amino acids residues in the active site?
• What is the mechanism by which the essential amino acid residues catalyze the reaction?
• As a model, we consider chymotrypsin, an enzyme of the digestive system that catalyzes the selective hydrolysis of peptide bonds in which the carboxyl group is contributed by Phe or Tyr
N-tosylamido-L-phenylethyl chloromethyl ketone
The active site of chymotrypsin shows proximity of 2 reactive aa
• Serine oxygen is nucleophile
• Attacks carbonyl group of peptide bond
General acid-base catalysis: depends on __________
• Nucleophilic substitution catalysts - ___________ _____________ atom attacks ______________ atom.
Zn2+ of _____________ is complexed with:
Activates the carbonyl group for nucleophilicacyl substitution
Enzymes can be _____________________(Specificity where optical activity may play a role)
Binding sites on enzymes must be ______________
*(Biochemical Connections, p. 196)
Nicotinamide adenine dinucleotide (NAD+) is used in many biological redoxrxns
1) nicotinamide ring
2) Adenine ring
3) 2 sugar-phosphate groups
• The B6 vitamins are coenzymes involved in _______________ ____________ ___________ from one molecule to another.
• Important in ______________ _____________ biosynthesis
Figure 7.21 p. 197