Biology Reference
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Amino acid sequence analyses suggest that Aer2 consists of the
N-terminal poly-HAMP (the first to third HAMP), PAS, di-HAMP, and
MCP domains. Aer2 shows the unusual domain arrangement compared
to most well-studied chemotaxis signal transducer proteins, which contain
a single HAMP domain immediately following a membrane-spanning seg-
ment. The N-terminal poly HAMP domain of Aer2 adopts a four-helix
bundle ( Fig. 7.9 ). Deleting the second HAMP and/or the third HAMP
results in a kinase-off phenotype, whereas deleting the fourth HAMP
and/or the fifth HAMP results in a kinase-on phenotype that is unresponsive
to changes in O 2 concentration ( Watts et al., 2011 ).
The isolated PAS domain (residues 173-289) shows typical UV-vis spec-
tra of a 5-coordinate and high-spin haem with the Soret band at 395 and
433 nm in the ferric and ferrous states, respectively ( Watts et al., 2011 ).
On the other hand, the full-length Aer2 show the Soret bands at 394
(sh)/412 nm and 425/434(sh) nm in the ferric and ferrous states, respec-
tively, indicating the existence of a 6-coordinate haem as a predominant spe-
cies ( Sawai, Sugimoto, et al., 2012 ). These results suggest that a truncation of
the HAMP domains affects a conformation of the haem pocket, which cau-
ses changes in the coordination structure of the haem.
Sawai, Sugimoto, et al. (2012) report the crystal structure of a truncated
Aer2, PH-Aer2 consisting of the PAS and di-HAMP domains (the residues
178-384) in cyano-met (Fe 3 þ -CN) form, at 2.4 ˚ resolution ( Fig. 7.10 ).
Though the HAMP domain in PH-Aer2 is disordered for the most part (res-
idues 308-384), the PAS domain is well ordered. The residues 184-285
constitutes a core of the PAS domain that is composed of two a -helices
( a 2 and a 3), one 3 10 helix ( Z 1), and a single antiparallel, five-stranded
b -sheet ( b 1- b 5) ( Sawai, Sugimoto, et al., 2012 ). The a 1 helix is a part of
the linker between the N-terminal poly-HAMP and PAS domains. Though
the overall structure of the PAS domain in Aer2 is similar to that in FixL and
Ec Dos, it shows differences in haem pocket region and haem ligation. In
FixL and Ec Dos, the proximal His is located in the F a helix, but it is not
the case in Aer2. His234 in the 3 10 helix provides the proximal ligand of
the haem in Aer2 ( Sawai, Sugimoto, et al., 2012 ).
A b-type haem is accommodated in a hydrophobic pocket composed of
non-polar side chains. The 7-propionate forms a salt bridge with His251,
and the 6-propionate has hydrogen bonds with a water molecule, the
main-chain N atom of Lys235, and NE2 atom of Gln240 ( Sawai,
Sugimoto, et al., 2012 ). The refined model indicates that the Fe d C d N
bond angle is 159 and the Fe d C distance is 2.0 ˚
( Sawai, Sugimoto,
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