Biology Reference
In-Depth Information
down in order to harvest its chemical energy in the form of ATP (in the destructive
processes collectively called
catabolism
). Glucose is the preferred energy source for
all cells, as it is used in
glycolysis
, the first and most fundamental energy-providing
pathway of both anaerobic and aerobic cellular respiration.
The interactions between sugars and their transport proteins and the enzymes
involved in their utilization within the cell are very specialized and cells must have
the capacity to make specific proteins for every sugar they take up and use. However, it
would not make sense for the
E. coli
to make the lactose transport protein if no lactose
is present in its environment. It would also be inefficient tomake the enzyme necessary
to break lactose into glucose and galactose if there were no lactose inside the cell. If a
cell had tomake all of its proteins all of the time, it would be expending a lot of cellular
energy in the making of proteins for which it has no use. Instead, cells have the ability
to make certain of their proteins only when the environmental conditions warrant.
Such proteins are called
inducible proteins
, because their synthesis is induced as a
consequence of a certain cellular condition. The proteins needed for lactose uptake
and utilization are inducible proteins, as they are only made if lactose is present and
glucose, the preferred energy source, is not. Their concentrations increase 1000-fold
under these conditions. Inducible genes belong to the group of regulated genes, in
that they are only transcribed under certain specific conditions.
In
E. coli
, when lactose is the only sugar present,
the transporter protein
lactose
(
lac
)
permease
and the enzyme
-
galactosidase
are both required in order to
utilize it.
Lac
permease is a transmembrane protein which binds the disaccharide and
brings it across the plasma membrane into the cytoplasm of the cell. The
lac
permease
mediated transport of lactose through the cellular membrane is reversible, meaning
that high levels of lactose inside the cell result in reverse transport of lactose to the
outside of the cell.
β
-galactosidase catalyzes the hydrolysis of lactose into glucose
and galactose. It also catalyzes the rearrangement of lactose into allolactose. These
two proteins, lactose permease and
β
-galactosidase are produced by a tightly coor-
dinated mechanism described by Francois Jacob and Jacques Monod and termed the
lac operon
.Inthe
lac
operon, due to the arrangement of its genes
LacZ
,
LacY
, and
LacA
, a single messenger RNA encodes both
β
-galactosidase and lactose permease,
as well as a third enzyme,
transacetylase
, not involved in the metabolism of lactose
(see Figure
1.1
). Thus, when the
lac
promoter is active, all three proteins are produced.
Adjacent to the
lac
genes
lacZ
,
lacY
, and
lacA
, is the gene
LacI
that encodes a reg-
ulatory protein, the
lac repressor
(see Figure
1.2
A). When there is no lactose present
in the cell's environment, the
lac
repressor protein will bind to the operator, prevent-
ing the RNA polymerase from producing the
lac
mRNA. When lactose is present
in the medium outside of the cell, a small amount of it will be transported into the
cell by the few lactose permease molecules found in the plasma membrane. Once
the lactose is inside the cell, it is converted to allolactose by the few
β
-galactosidase
molecules present. Allolactose binds the
lac
repressor and causes it to undergo a con-
formational change, such that it can no longer bind to the operator (see Figure
1.2
B).
As a result, the RNA polymerase is able to read right through, produce the
mRNA, and the three proteins are produced. This causes more lactose to be brought
β
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