Chemistry Reference
In-Depth Information
known asmaltoporin, fromEscherichia coli [95] and the sucrose-speci
c channel ScrY
of Salmonella typhimurium [96].
High-resolution crystal structures of several of these speci
c porins are available.
Although Lamb and Scry display only moderate sequence homology, their structures
can be superimposed [97, 98]. They are composed of trimeric proteins when each
monomer consists of 18 membrane spanning
-strands forming a barrel with a
central water
filled poor, short turns on the periplasmic side and large irregular loops
on the outside of the cell. Based on structures of LamB complexed with a sugar, three
adjacent subsites of binding were observed at the constriction zone located in the
middle of the pore (Figure 7.3B), and a speci
b
c translocation pathway has been
postulated [99]. The hydrophobic face of the sugar rings is in van der Waals contact
with a slide of aromatic residues, while hydrogen bonds are formed between the
sugars hydroxyl groups and polar residues.
Maltoporin has been extensively studied using the black lipid membrane
technique [100]. Single maltoporin trimers were reconstituted in lipid bilayers
and the temporary binding events of maltodextrin molecules inside the
channel were observed as transient closures of the individual monomers to ion
current [101
-
104]. Analysis of the occupancy probability for the four particular
states (fully open, single blocked, double blocked, fully blocked) shows that all
three channels of the maltoporin protein transport sugars independently [101].
Figure 7.3A shows current recordings from single channels in solutions of
maltodextrins of various lengths, containing from three (maltotriose) to seven
(maltoheptaose) glucose moieties [102]. It can be seen that shorter carbohydrates
block the current over a shorter period. However, they all totally obstruct one
channel as indicated by the amplitude of the current transitions of one-third of the
total current.
Power spectra of the sugar-induced current
fluctuations can be described by a
Lorentzian function (Figure 7.3C), which is characteristic of a two-state Markov
process. The association and dissociation rate constants governing the open
-
close
sequence experienced by one channel can be determined using Equation 7.8 by
means of a linear plot of the reciprocal relaxation times versus sugar concentrations
(see e.g. [105]). For maltohexose, values of k
ass
and k
diss
are about 10
7
M
1
s
1
and
1000 s
1
, respectively, at a 150-mV applied voltage in the symmetrical addition of
the sugar. The asymmetry of binding of sugar molecules entering the channel
from either the extracellular or the periplasmic side has been investigated in
detail [88, 102, 103].
According to basic principles of thermodynamics, an electric
field strength E,is
a variable of state just like temperature and external pressure. Therefore, equilibrium
constants depend on E according to the appropriate vant Hoff equation that
involves the molar dipole moment of the reaction (an analogous relationship can
be applied to elementary rate constants, which involves the reaction dipole moment
in the transition state). Actually the effect is negligible in ordinary bulk reactions
since the values of E are usually much too small. However, this
field readily becomes
very large during reactions of oriented molecules in biological membranes [106],
