A theory of inhaled anesthetic action by
disruption of ligand diffusion chreodes
Lemont B. Kier, PhD
Richmond, Virginia
A theory is proposed for the clinical actions and behavior of
volatile anesthetic agents. Evidence supports the nonspecific and ubiquitous effects of these drugs in which many
ligand-receptor systems may be involved. The volatile
anesthetic drugs interfere with the diffusion of ligands to
receptors across the hydrodynamic landscape on the protein surface. The hydropathic states of the amino acid side
chains on the landscape of an active site influence the water
to form evanescent paths, called chreodes as defined by
T
he mechanism of action of inhaled anesthetic
agents has been considered for more than a
century and is summarized in recent
reviews.1,2 The earliest contribution to the
mechanism of these drugs came at a time
before there was any understanding of the chemical
nature of cell membranes or proteins. It was observed
that there was a notable correlation between potency
and the solubility of the drug in olive oil.3 When the
lipoidal nature of cell membranes was recognized, a
formal mechanism was proposed wherein inhaled
anesthetic drugs produced their effect by entering and
altering, in some way, the normal function of these
structures.4 The consequences of this alteration were
assumed to have an impact on the proteins imbedded
in the membrane. Some effects of inhaled anesthetics
on membranes have been noted, but they require concentrations well above clinical levels.5,6 Recently, the
observation that stereoisomers of anesthetics such as
isoflurane exhibit a modest but real difference in
potency casts doubt on the cell membrane lipid bilayer
being the target.7,8 An alternative mechanism invokes
an induced lateral pressure on the phospholipid-water
boundary created by the presence of anesthetic molecules in the membrane.9,10 The presence of anesthetic
molecules may disrupt the interface between the
bilayer molecules and the imbedded proteins, altering
their function.
Another focus of attention has been on proteins as
targets of inhaled anesthetic drugs. One observation
that stimulated this consideration was the correlation
between anesthetic potency and the inhibition of the
enzyme luciferase.11 The concentrations needed for
clinical effectiveness, however, do not support the
concept of a receptor on a protein as the target of an
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AANA Journal/December 2003/Vol. 71, No. 6
Waddington and offered as an appropriate term by Kier.
These chreodes are altered by the presence of volatile anesthetic drugs, leading to a reduction in the diffusion of many
ligands to their active sites. This effect is sufficient to
decrease the responses of these receptors, leading to an
anesthetic state.
Key words: Inhaled anesthetics, ligand diffusion, mechanism of action, protein landscape, water chreodes.
anesthetic molecule.12-14 There is no evidence of high
affinity binding, characteristic of a specific ligandreceptor encounter. Indeed, inhaled anesthetic molecules alter the function of many receptors, enzymes,
transporters, and structural proteins.14
The interaction of anesthetics with certain receptors
has come under study. In particular, gamma-aminobutyric acid (GABA) has been studied and the profile of
response documented.15,16 The behavior of GABA
receptors does not conform to what is observed clinically in the presence of anesthetics.17-20 As an alternative, the involvement of multiple binding sites, one for
each drug, on the same target has been postulated21,22
and supported by some studies.23,24 The evidence for
multiple binding sites also comes from the observation
of a steep dose-response relationship,25 characteristic of
low variability of responsiveness. The responsiveness to
anesthetics is ubiquitous among mammals, and even
plants.26 Tolerance is not common with anesthetics, an
attribute expected of drugs acting at specific receptors.
A general view emerges from these studies that relatively weak binding, not characteristic of a specific
drug-receptor encounter, is occurring to produce the
anesthetic effect. One idea supporting this view is that
cavities on proteins are targets of anesthetic molecules, disrupting their normal function.27,28 To summarize, the current level of wisdom on the mechanism of inhaled anesthetics points to small encounters
at many sites on a protein, most likely not at the
receptor on its surface.1,2 This conclusion relates to
our recent studies of the landscape around receptors
on proteins and their role in facilitating ligand diffusion to the receptors.29 This report will reveal the significance of that work and a postulate of an inhalation
anesthetic mechanism that arises from it.
A theory of ligand diffusion across a protein
landscape
• Background. An early view considered the
approach of a ligand to be through bulk water to a target site. A 3-dimensional random walk throughout the
entire approach is now postulated to be too slow to
permit coordination of the heterogeneous complex
processes in living systems.30 A current view considers
some period of residence of the ligand near the surface
of a protein on the way to an encounter with a target
site. The central idea is that the approach of a ligand to
a target site is a process assumed to be 2-dimensional diffusion, acting as a facilitating condition for the ligandreceptor events. This diffusion has been the subject of
a number of studies and models over the years.
Several models of a 2-dimensional passage on cell
and protein surfaces have been described. A model by
Hasinoff describes the nonspecific binding of a ligand
to the charged surface of an enzyme.31 The ligand then
follows a 2-dimensional walk to the active site. This
effectively increases the size of the target on the
enzyme. A model by Chou and Zhou describes an
enzyme molecule with multiple receptor sites.32 A ligand
molecule encounters the enzyme surface and diffuses
to a nearby receptor to initiate the reaction. The
expectation of a significant accumulation of ligands
on the enzyme surface is an outcome of the calculations based on this model. The entire surface of the
enzyme is regarded as a “sink” where the ligand diffuses to an active site. The conclusion was drawn that
amino acid side chains outside of the active site play a
major role as “promoters,” facilitating a rapid diffusion to the active site. The participation of the water
near the protein surface was discussed by Welch who
theorized that the microviscosity of near water was
influential on the rate of catalytic reactions.33 The possibility that the viscosity of water near the protein surface is significantly higher than bulk water creates an
ordering in a layer that might facilitate faster diffusion
of a solute near the protein surface.
A functional model was proposed by Birch and
Latymer who did not specify the nature of the ligand
approach to the receptor but described a model of the
residence of several ligands near the active site.34 An
experimental observation was made that upon administration of sweet-tasting molecules to observers followed by a washout of the compound, a persistence of
the sweet taste results. Birch and Latymer considered
several possible models including a nonspecific localized pooling of the compound in the vicinity of the
receptor. Based on experimental work they proposed a
model in which the sweet molecules are located in a
queue organized near the surface of the protein. After
washout, the molecules are retained in the queue, and
so they continue to move toward the receptor, activating it as long as the queue is occupied. When the
queue is empty, the sweet taste vanishes. This model
invokes a directional effect focused on the receptor as
well as the possibility of sustained residence of several
molecules near the receptor.
In summary, experimental and modeling evidence
may be interpreted as describing a facilitation of reaction rates and receptor activation due to the rapid diffusion of ligands to target sites in essentially a 2dimensional domain. This diffusion most likely
occurs near the surface of a protein involving water
and structural features not part of the target site. The
forces guiding the ligand to the target site would be
expected to result in several effects. These include
some period of retention on the protein surface and
some influence directing the ligand to the target site.
These considerations and the demonstrated role of
water lead us to consider a model involving the
immediate layers of water enshrouding the protein.
• Chreodes. We have recently proposed that ligand
molecules encounter the surface of a protein molecule
and are captured within the first few layers of water
on the surface.29 They are then guided to the active
site over a series of evanescent cavity paths created by
the relative hydrophobic effect responding to the
hydropathic state of each amino acid side chain.
These paths are preferences reminiscent of the chreodes envisioned by Waddington in his description of
preferences on an epigenetic landscape.35 The word
“epigenetic” was coined from the Greek words for
“necessary” and “route” or “path.” He defined it as a
representation of a temporal succession of states of a
system. That system is characterized by a property
with ingredients in the system that tend to respond to
perturbations by preferring a presence in the chreode.
Because this definition is close to the dynamic character of the paths created by the hydrophobic effect of
the field of surface side chains, we have adopted this
term to characterize the system in our model.
A number of attributes are associated with the
chreodes proposed in this report that have an impact
on diffusion events that they influence. These chreodes are distinct for each type of receptor (or enzyme
active site). This arises from the fact that the landscape of amino acid side chains around a receptor is
just as defined as the receptor itself. The landscape is
therefore an extension of the receptor with some
degree of specificity. This is true because the pattern
of amino acid side chains in the landscape is reproducible from one specific receptor protein to the next,
just as is the receptor itself. In a sense, when we speak
AANA Journal/December 2003/Vol. 71, No. 6
423
of a receptor, we should be including the landscape as
a part of the whole. The chreodes created in the landscape of a receptor are invoked to account for a facilitated diffusion of a ligand to that receptor. It is possible that the exiting of a ligand from the receptor may
be facilitated by another set of chreodes.
A second attribute of these chreodes is that they are
evanescent, constantly coming into existence, fading,
and returning. They exist only as a “most probable”
pattern of passages through water. Collectively, and
over time, they meet the definition of a chreode (discussed above), always returning to a pattern favoring a
directed and facilitated diffusion influence. A third
attribute is found in the detailed structure of each
chreode, governed by local patterns of amino acids,
their hydropathic states, and their topology. The
arrangement of amino acids on a protein is such that
there would be a size and structure limitation for a
molecule traversing the chreode. This is expected
since the surface side chains are relatively close to each
other and because the water structured by their hydropathic states is only 2 or 3 molecule layers out from the
protein surface. The protein structure has evolved
along with the ligand specific for a receptor on its surface. Therefore the specific ligand for the receptor
experiences an optimal relationship with the chreode.
The local structure and topology of segments of a
chreode present to surrounding bulk water asymmetric
patterns of side chains. The consequences of this are a
different orientation and binding affinity of each segment to chiral isomers captured within the segment.
This effect translates into a possible stereoselectivity
Figure 1. A cellular automata model of a protein surface with randomly placed amino acid side chains
S
C
C
D
D
B
E
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D
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©
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C
The letters depict a particular polar state of the side chain. The letter A stands for a polar side chain, while the letter E
stands for a nonpolar side chain. The letters B, C and D encode intermediate degrees of polarity. These side chain
models are stationary. The surface is bathed with water molecules, not shown, that move over the surface, interacting
in a characteristic way with each side chain. A ligand molecule, S, is allowed to diffuse throughout the system,
responding to the influences of the side chains on the water molecules. The dynamics are run until the S molecule
contacts the receptor, the heart, in the center of the grid. The heart,©, is the target of the ligand, S, in its diffusion
across the protein surface. The average rate of this diffusion is calculated over many runs.
424
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Figure 2. A cellular automata model of a protein surface with organized placement of amino acid side chains to
create a chreode
S
C
C
D
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©
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This figure is the same as Figure 1 except that the side chains are not randomly placed but are organized to create
chreodes focused on the receptor. The dynamics are run as before with the average diffusion time recorded. The
underlined letters are the side chains that constitute the chreodes. The heart,©, is the receptor. It is the target of the
ligand, S, in its diffusion across the protein surface.
associated with each segment of a chreode. The sum of
these effects along the chreode may result in a modest
stereoselectivity manifested before the molecule
reaches the receptor. The credit for stereoselectivity has
traditionally been ascribed exclusively to the receptor.
A final attribute of the chreodes is their potential
fragility in the presence of other molecules that are of
optimal size and hydropathic state. Such molecules
may enter, interfere with, and disrupt the structure of
the chreode at any segment along their path. Just as
the amino acid side chains are the source of the
hydrophobic effect creating the chreodes, so another
optimally structured molecule may reorganize the
water near the protein surface to create a “nonchreode” pattern. This would have some effect on the diffusion of the specific ligand associated with that
receptor and its chreode system.
In our study of chreodes created by protein surface
amino acid side chains, we modeled the effect of chreodes using cellular automata dynamics.29 This study
was conducted using a simulated surface with a random distribution of side chain models. A model simulating the absence of any chreode pattern, shown in
Figure 1, led to an average diffusion rate of a ligand
model to be about 3.1 ¥ 10-3 cell spaces per iteration.
A pattern of side chains was then modeled, creating a
chreode that is focused on the receptor (Figure 2). The
average rate of diffusion of a ligand was found to be 4.5
¥ 10-3 cell spaces per iteration. These results were part
of the evidence used to construct the theory of ligand
passage to a receptor via chreodes in water that are
organized by the surface amino acid side chains.29
In a second study, we have repeated these simulations and have modified the simulation containing the
chreode pattern36 (see Figure 2). In this case we have
introduced a number of solute molecules in addition
AANA Journal/December 2003/Vol. 71, No. 6
425
Table 1. Comparison of molecular volumes (V) of amino acid side chains and volatile anesthetic drugs
Side chain
3
V (cm /mole)
Tryptophan
Arginine
Tyrosine
Phenylalinine
80
69
64
56
Lysine
51
Glutamine
Methionine
Leucine
Isoleucine
Glutamic acid
Histidine
Asparagine
Valine
Aspartic acid
Serine
Threonine
Cystine
46
45
45
45
43
36
36
36
33
27
25
25
Alanine
Glycine
14
3
Anesthetic
Sevoflurane
Methoxyflurane
Enflurane
Desflurane
Halothane
Isoflurane
Ether
69
66
63
57
57
53
52
Chloroform
45
Nitrous oxide
to the ligand. It was found that the additional molecules interfered with the diffusion rate. The rate of diffusion of the ligand was reduced to 3.0 ¥ 10-3 cell
spaces per iteration. The presence of the modeled
solute molecules interfered with the chreodes, thereby
reducing their influence on the diffusion process. This
model, along with the other evidence described, has
led us to propose a general theory of the action of the
inhaled anesthetics.
A theory of volatile anesthetic mechanism
The current mechanistic view of weak encounters of
volatile anesthetic molecules at many sites on or near
protein surfaces appears compatible with the theory
of chreodes facilitating and directing ligands to receptors (or enzyme active sites). Each system of chreodes
associated with a receptor has its own structure,
which is governed by the landscape of side chains.
Each dynamic system of chreodes has evolved to
accommodate a specific ligand. The chreodes are
formed by water molecules and hydrogen bonding in
426
3
V (cm /mole)
AANA Journal/December 2003/Vol. 71, No. 6
3
Alkane
V (cm /mole)
Decane
Nonane
Octane
Heptane
Hexane
109
99
88
79
68
Pentane
58
Butane
48
Propane
38
Ethane
27
Methane
17
19
places and forming cavities in other places in response
to the relative hydropathic states of surface side
chains. This phenomena has been demonstrated by
the orientation effects of nonpolar solutes studied by
vibrational spectroscopy37 and the flickering, fluctuating pattern of water and cavities observed between
close polar and nonpolar surfaces, which are revealed
by shear measurements.38
Inhalational anesthetic molecules have sizes
approximating those of amino acid side chains (Table
1) and lipophilicities approximating those of the
lipophilic side chains (Table 2). These 2 molecular
properties are most influential in controlling the creation of the hydrophobic effect. Thus their presence
in or near a chreode could alter the original pattern,
thereby disrupting the normal diffusion of the ligand
to the receptor. This effect depends upon a significant
concentration and would vary in its intensity from
one receptor-chreode complex to another. Because of
the diversity of landscape structures associated with
different receptors and the differences among the
Table 2. Comparison of octanol/water partition coefficients (Log P) of lipophilic amino acid side chains and
volatile anesthetic drugs
Side chain
Tryptophan
Log P
2.25
Isoleucine
Phenylalanine
Leucine
Cystine
Valine
Methionine
1.80
1.80
1.70
1.54
1.23
1.23
Tyrosine
0.96
Alanine
Threonine
Histidine
0.31
0.26
0.13
Anesthetic
Log P
Alkane
Log P
Heptane
Hexane
3.5
3.0
Sevoflurane
2.34
Pentane
2.5
Methoxyflurane
Enflurane
Isoflurane
2.21
2.10
2.06
Butane
2.0
Desflurane
Halothane
1.80
1.70
Propane
1.5
Ethane
1.0
Methane
0.5
Ether
Nitrous oxide
structures of the anesthetic molecules, it is understandable that there would be some differences in the
clinical profiles of each anesthetic drug. The interactions of inhaled anesthetic molecules with the chreodes are weak and nonspecific. The patterns of the
side chains forming the chreode create local asymmetries that may produce different responses to chiral
isomers of an anesthetic agent.
A recent study has modeled segments of chreodes
using cellular automata dynamics.36 When the diffusion behavior of chiral molecules was compared, it
was found that there were modest differences in the
rate of diffusion out of these segments, between
stereoisomers. This corresponds to the modest stereospecificity observed with some anesthetics.7,8 Circular
dichroism studies have revealed that chiral molecules
produce chiral solvent structure in their vicinity.39
Since chreodes may be associated with many receptors and enzymes, such an interference in their function and influence is expected to be widespread. Their
influence may be inhibitory, as at receptor-chreode
systems, or reinforcing as at reuptake sites supported
by a chreode network.
In summary, the presence of an inhalation anesthetic agent in a chreode system supporting a ligand
diffusion to a receptor may alter the structure and
0.89
0.43
hence the function of the chreode thereby reducing
the diffusion and the receptor response. The summation of these numerous diffusion disruption events
leads to the manifestation of clinical anesthesia.
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AUTHOR
Lemont B. Kier, PhD, is professor of Medicinal Chemistry and Nurse
Anesthesia and a senior fellow in the Center for the Study of Biological
Complexity, Virginia Commonwealth University, Richmond, Va.