Biomedical Engineering Reference
In-Depth Information
If two binding sites with different affinities are present, visible
in a double-sigmoid curve, the equation is modified as follows:
10
n
h
·
x
10
n
h
·
lgEC
50
h
+10
n
h
·
x
10
n
l
·
x
10
n
l
·
lgEC
50
l
+10
n
l
·
x
B
max
h
·
B
max
l
·
=
y
blank +
+
(h and l indicate the respective values for high-affinity and low-
affinity binding sites
3
).
Several computer programs have the opportunity to compare
the goodness of fit obtained by non-linear regression calculations of
different models. This algorithm uses a so-called F test to quantify
the sum of squares of both fits and allows to know which function is
moreappropriateforyourdata.Soitispossibletodecidewhether
only one binding site or two equal sites or two sites with different
affinities are occupied by the ligand. The function types for hyper-
boloid or sigmoid binding characteristics given above must not be
comparedbytheFtestbecauseinthefirstcaseconcentrationsare
used and in the latter logarithms of concentrations are taken.
References
Wells JW (1992) In: Hulme EC (ed.) Receptor-ligand interactions: a prac-
tical approach. IRL Press, Oxford, p 289
Motulsky HJ (1999) GraphPad Prism, version 3.0. GraphPad Software, San
Diego, p 303
9.2.2 Enzyme Kinetics
It is impossible to describe and explain enzyme kinetics unless is
explained by an entire book; therefore, this chapter describes only
briefly some aspects. It is strongly recommended to read once more
a textbook on enzymology and enzyme kinetics. Especially the
reaction kinetics of enzyme oligomeres, multi-enzyme complexes,
and phenomena of cooperation are too complex to explain in just
afewpages.
If enzymes are described under the aspect of reaction mech-
anisms, the maximal rate of turnover v
max
,theMichaelis and
Menten constant K
M
, the half maximal inhibitory concentration
IC
50
, and the specific enzyme activity are keys of characterization
of the biocatalyst. Even though enzymes are not catalysts in a strong
chemical sense, because they often undergo an alteration of struc-
ture or chemical composition during a reaction cycle, theory of
enzyme kinetics follows the theory of chemical catalysis.
In the most simple case an enzymatic reaction is described by
the equation
→
ES
→
EP
→
E + P
E + S
3
According to Zernig et al. (1994) J Pharmacol Expt Therap 269:57
