Metabolic
Regulation
A
microbial
cell
contains
a
large
number
of
micro
and
macro
molecules.
To
ensure
that
every
molecule
is
produced
in
right
numbers
the
cell
must
possess
a
control
device
for
their
synthesis.
Microorganisms
are
also
extremely
versatile
in
their
nutrient
requirement;
for
example, E.
coli can
grow
in
a
medium
containing
only
an
inorganic
source
of
nitrogen
and
a
simple
carbon
source
as
well
as
in
a
complex
medium
containing
meat
extract,
peptone
and
glucose.
When
grown
in
a
minimal
medium,
the
organism
converts
the
simple:
materials
into
complex
cellular
building
blocks
through
enzyme
mediated
reactions.
Such
cells
therefore
contain
a
large
number
of
enzymes
necessary
for
the
synthesis
of
various
metabolic
intermediates.
On
the
other
hand,
cells
growing
in
a
complex
medium
synthesize
only
a
few
enzymes
and
shut
off
the
synthesis
of
enzymes
that
are
no
longer
required.
Also,
when
cells
growing
in
a
simple
synthetic
medium
are
transfered
to
a
rich
nutrient
medium
or
vice
versa,
they
react
quickly
to
the
new
surroundings
and
readjust
the
rates
of
macromolecular
synthesis.
When
transferred
from
a
poor
medium
to
a
rich
medium
they
stop
synthesising
enzymes
that
are
no
longer
required
and
this
allows
the
cells
to
conserve
energy
which
otherwise
would
have
been
wasted.
Thus,
microorganisms
have
the
ability
to
regulate
the
synthesis
of
macromolecules
depending
on
environmental
conditions
and
thus
prevent
waste
of
energy.
Each
enzyme
represents
the
product
of
one
or
more
microbial
genes.
Since
the
amount
of
DNA
per
cell
is
limited,
the
number
of
enzymes
that
a
bacterial
cell
can
produce
is
also
limited.
Some
enzymes
are
produced
in
fixed
amounts
regardless
of
the
environmental
conditions
and
these
are
known
as "constitutive
enzymes".
Most
organisms
possess
the
capacity
to
produce
enzymes
only
under
certain
environmental
conditions.
These
are
known
as
the "incucible
enzymes" whose
rate
of
production
can
be
increased
by
the
presence
of
inducers.
Their
synthesis
can
also
be
repressed
and
these
are
also
called
as "repressible
enzymes".
For
example, E.
coli produces
ßgalactosidase
(an
enzyme
that
catalyses
the
hydrolysis
of
lactose
to
glucose
and
galactose)
only
when
grown
in
a
medium
containing
lactose.
Lactose
therefore,
is
an
inducer
of
this
enzyme.
The
synthesis
of
this
enzyme
is
repressed
by
glucose
which
is
the
end
product
of
the
reaction.
Both
these
mechanisms
(induction
and
repression)
are
now
known
to
operate
at
the
transcriptional
level.
well
known
mechanism
of
regulation
of
enzyme
activity
is
the "feed
back
inhibition",
in
which
the
concentration
of
the
end
product
regulates
the
functioning
of
enzymes
involved
in
its
production.
Basically
therefore,
induction,
repression
and
feed
back
inhibition
of
enzymes
are
three
major
regulatory
mechanisms
that
operate
in
microorganisms.
In
recent
years,
two
other
mechanisms
that
regulate
enzyme
activity
have
also
been
recognized
and
these
are:
i)
regulation
through
enzyme
modification
and
(ii)
regulation
through
enzyme
inactivation.
Most
of
our
knowledge
about
regulation
of
macromolecular
synthesis
has
come
from
the
studies
with
the
bacteria
and
the
subsequent
description
will
therefore
be
mostly
confined
to
these
organisms.
The
above
regulatory
mechanisms
are
mediated
by
low
molecular
weight
compounds
which
ffiay
be
either
formed
inside
the
cell
as
intermediatory
products,
end
products
or
enter
the
cell
from
the
environment.
These
compounds
interact
with
a
class
of
proteins
called
the
allosteric
proteins,
whose
properties
are
altered
in
the
presence
of
specific
small
molecular
weight
compounds
(effectors).
Two
classes
of
allosteric
proteins
are
known.
One,
allosteric
protein" which
are
devoid
of
catalytic
activity
but
control
the
synthesis
of
specific
enzymes
and
tbe
second,
allosteric
enzymes
whose
activity
is
either
enhanced
or
inhibited'by
effectors.
The
repressor
protein
that
regulates
the "functioning
of
the
lactose
operon
is
an
example
of
the
former
class
while
asparatate
transcarbamylase
is
an
example
of
the
latter.
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