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Solutions for Amino Acids, Peptides, and Proteins
N
N
H
H
H
N
N
H
H2N CH
(CH2)3
COOH
HN C NH2
NH
H+
H2O
C
H
N H
N
H
N H
N
H
(CH2)3
NH2
CH COOHH2N
N
H
N
N
N
H
H
H
H2N C
O
NH2
urea
+
In the protonated imidazole, the two Ns are similar in structure, and both NH groups are acidic.
(b)
(c)
resonance-
stabilized
We usually think of protonation-deprotonation reactions occurring in solution where protons can move with 
solvent molecules. In an enzyme active site, there is no "solvent", so there must be another mechanism for 
movement of protons. Often, conformational changes in the protein will move atoms closer or farther. 
Histidine serves the function of moving a proton toward or away from a particular site by using its different 
nitrogens in concert as a proton acceptor and a proton donor.
49 The high isoelectric point suggests a strongly basic side chain as in lysine. The N—CH bond in the 2
side chain of arginine is likely to have remained intact during the metabolism. Can you propose a likely 
mechanism for this reaction? Think of this as a nucleophilic acyl substitution on C=NH instead of C=O.
arginine
ornithine
– H+
48 continued
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