Biology Reference
In-Depth Information
R is the gas constant
T is the temperature in
K
n is the solute charge (
þ
1 for protons)
F is the Faraday
D
pH is the trans-membrane pH gradient
It is the force on an H
þ
(called the proton motive force,
) that drives lactose uptake.
Note that the ability to take up lactose is a combination of the electrical gradient and the
pH gradient. Although lactose uptake is directly coupled to H
þ
trans-membrane movement,
it may be possible to take up lactose even if the pH gradient is zero (i.e. the
Dm
H
þ
DJ
is sufficiently
large).
Vectorial Metabolism
Over fifty years ago Peter Mitchell recognized the importance of what he termed
'vectorial metabolism'
[23,24]
. Water-soluble enzymes convert substrate to product
without any directionality. Mitchell proposed that many enzymes are integral membrane
proteins that have a unique trans-membrane orientation. When these enzymes convert
substrate to product they do so in one direction. This enzymatic conversion is therefore
'vectorial' or unidirectional. Mitchell expanded this basic concept into his famous chemi-
osmotic mechanism for ATP synthesis in oxidative phosphorylation
[25,26]
. For this idea
Mitchell was awarded the 1997 Nobel Prize in Chemistry. Vectorial metabolism has been
used to describe several membrane transport systems. For example, it has been reported
in some cases the uptake of glucose into a cell may be faster if the external source of
glucose is sucrose rather than free glucose. Through a vectorial trans-membrane reac-
tion, membrane-bound sucrase may convert external sucrose into internal glucose
þ
fructose more rapidly than the direct transport of free glucose through its transport
system.
Mitchell defined one type of vectorial transport as Group Translocation, the best example
being the PTS (phosphotransferase system) discovered by Saul Roseman in 1964. PTS is
a multicomponent active transport system that uses the energy of intracellular phosphoenol
pyruvate (PEP) to take up extracellular sugars in bacteria. Transported sugars may include
glucose, mannose, fructose, and cellobiose. Components of the system include both plasma
membrane and cytosolic enzymes. Energy to drive the system comes from PEP (
D
G of hydro-
lysis is
61.9 KJ/mol). The high energy phosphoryl group is transferred through an enzyme
bucket brigade from PEP to glucose producing glucose-6-phosphate (PEP
EI
HPr
/
/
/
EIIA
glucose-6-phosphate). The sequence is depicted in more detail in
Figure 14.17 [27]
. HPr stands for heat-stable protein that carries the high energy ~P from EI
(enzyme-I) to EIIA. EIIA is specific for glucose and transfers ~P to EIIB that sits next to the
membrane where it takes glucose from the trans-membrane EIIC and phosphorylates it
producing glucose-6-phosphate. Although it is glucose that is being transported across the
membrane, it never actually appears inside the cell as free glucose, but rather as glucose-6-
phosphate. Free glucose could leak back out of the cell, but glucose-6-phosphate is trapped
inside, where it can be rapidly metabolized through glycolysis. Group translocation is defined
by a transported solute appearing in a different form immediately after crossing the
membrane.
EIIB
EIIC
/
/
/
