Biomedical Engineering Reference
In-Depth Information
dei ned as the quantity of enzyme required to consume one micromole of
substrate per minute at a given temperature and pH, under the conditions
of substrate saturation:
International Unit (I.U.) = 1-μmol substrate consumed/min (7.11)
h e concentrations are ot en given as milliunits (mU) or microunits
(μU) per liter (L) or milliliter (mL) as 1 I.U is usually a very large quantity
of enzyme.
To convert these units to molar concentrations, it is important to know
the turn over number,
cat
, of the particular enzyme which is used. For a
simple one-substrate enzyme,
cat
=
2
. h is value is are the fundamental
characteristic of an enzyme which indicates the maximum rate at which
substrate are consumed, and is generally given in reciprocal units of sec-
onds. (
s
-1
) under the conditions of substrate saturation.
=
V
max
=
K
cal
[
E
]
0
(
mol . S . converted
×
×
L
-1
)
(7.12)
s
-1
V
max
is obtained from the I.U. of enzyme activity present in a given vol-
ume (
V
T
)
of enzyme solution:
V
max
= (10
-6
mol
/
mol
)
× (1.
min/60.
s
)
× (
I
.
U./V
T
)
(7.13)
and
V
max
= (1.67
×
10
-8
)
× (
I
.
U./V
T
)
(7.14)
Eq. 7.14 has units of the molar concentration of an enzyme solution
which is found by combinings Eqs. 7.14 and 7.12
[
E
]
0
(1.67
×
10
-8
) =
(
I
.
U./V
T
) × (1/
k
cat
)
(7.15)
while Eq. 7.15 is preferred for the calculation of
[
E
0
]
, which requires the
value of
k
cat
to be known for the enzyme of interest under the conditions
required for its use.It is also assumed that this value is not always available
in the literature form.
Specii c activity
is dei ned as I.U. of enzyme activity with respect to unit
weight ,under the conditions of T and pH), and is considered for the mea-
surement of enzyme purity. h e purity is greater with the higher specii c
activity of a enzyme preparation. h e I.U per milligram determines the
specii c activity. h e specii c activity can be used to estimate through
k
cat
if
