Biology Reference
In-Depth Information
Fig. 7.5 A portion of an enzyme (histidine decarboxylase) viewed as the network of atoms of
amino acid residues linked together through both
covalent
and
non-covalent
bonds.
D ¼
aspartic
acid;
E ¼
glutamic acid;
I ¼
isoleucine;
R ¼
arginine;
S ¼
serine.
Solid lines
and the
ribbon
indicate
covalent
bonds, and
dotted lines
and
empty space
indicate
non-covalent
interactions
(Downloaded from the web site of Jon D. Robertus, University of Texas,
http://research.cm.
As an example of an enzyme viewed as an atomic network, let us examine
histidine decarboxylase (HDC), a portion of which is shown in Fig.
7.5
.This
enzyme catalyzes the removal of carbon dioxide from amino acid histidine:
HDC
Hisitidine
!
Histamine
þ
CO
2
(7.15)
HDC exists as a trimer (i.e., three identical units combined to form a functional
unit). Each unit consists of a linear chain of 662 amino acids (molecular weight
¼
74,017 Daltons). Each of the three active sites is located at the interface between
two HDC molecules. Each HDC molecule can exist in two conformational states
denoted as
T
and
R
. Low pH and high histidine concentration favor the R confor-
mation which has a high affinity for histidine, while high pH favors the T confor-
mation which has a low affinity for histidine:
H
þ
$
HDC(
T
Þþ
HDC(
R
Þ
(7.16)
One active site of HDC in the
R
form is shown in Fig.
7.5
. The active site is partially
formed by helix B from a neighboringmolecule. At acidic pH, two protons are trapped