Chapter 9/6 Catalytic Strategies
sources of catalytic power and specificity of enzymes
classes
serine proteases, carbonic anhydrases, restriction endonucleases, and myosins
each requires addition of water to substrate
challenges
serine proteases: promote reaction that is really slow at neutral pH
carbonic anhydrase: achieve high absolute rate
endonuclease: attain high degree of specificity
myosins: use free energ associated with hydrolysis of ATP to drive other processes
A few basic catalytic principles are used by many enzymes
binding energy
free energy released in formation of a large number of weak interactions between the enzyme and substrate
used by enzymes
establishes substrate specificity
increases catalytic efficiency
promotes structural changes in both enzyme and substrate that facilitate catalysis (induced fit)
9.1 Proteases Facilitate a Fundamentally Difficult Reaction
proteases cleave by hydrolysis
Chymotrypsin possesses a highly reactive serine residue
cleaves on carboxyl-terminal side of hydrophobic aa’s
covalent catalysis
Chymotrypsin action proceeds in 2 steps linked by a covalently bound intermediate
chromogenic substrate
for chymotrypsin, N-acetyl-L-phenylalanine p-nitrophenyl ester
many proteases hydrolyze amides and esters
one of the products is yellow
starts with a burst, first phase is way faster
Serine is part of a catalytic triad that also includes histidine and aspartate
catalytic triad
histidine acts as general base catalyst
step one: substrate binding
step two: attack
O from serine attacks carbonyl carbon of target peptide bond
charge stabilized byNH groups from protein in oxyanion hole
step three: tetrahedral intermediate collapses to generate acyl-enzyme
facilitated by transfer of proton
step four: amino component leaves