Metabolism
Metabolism Cell (from the Greek word
metabole, which means change) is
the totality of an organisms
chemical processes. Metabolic
pathways (each step of which is
catalyzed by an enzyme) are series
of reactions that manage material
and energy resources of the cell.
There are two kinds:
Catabolic pathways degrade or
break down complex molecules
to simpler compounds, to make
energy stored in organic
molecules available to do the
cells work. An example is
cell respiration, where
glucose is broken down into
CO2 and H2O.
Anabolic pathways consume
energy to build complex
molecules from simpler
monomers.
An example is the
synthesis of proteins from
amino acids.
Bioenergetics is the study of how
organisms use energy resources.
Energy is the ability to do work.
There are two kinds of energy:
1. Kinetic- energy of motion.
2. Potential- stored or possessed
energy
To use energy, chemical reactions
must rearrange atoms of molecules
in such a way that the potential
energy stored in the molecules is
converted to kinetic energy.
A spontaneous process is a change
that can occur without outside
help and increases stability.
Systems will most often change
spontaneously if they move from a
high energy and low entropy state
to a low energy and high entropy
state. In any spontaneous
process, the free energy of a
system decreases.
Free Energy, or Gibbs Free Energy
is the portion of a systems
energy that can perform work when
temperature is uniform throughout
the system, or the amount of
energy in a system that is
available for work. It is defined
by
G = H TS
where H is the systems total
energy, T is the absolute
temperature in Kelvins (which is
degrees Celsius plus 273), and S
is entropy. Notice that
Not all the energy stored in a
system (H) is available for
work
As Temperature and Entropy
increase, there is less energy
available for work
Increased temperature
magnifies entropy, because the
intensity of random molecular
motion (heat) disrupts order,
and increases S
Thus we have this equation:
G=Gfinal state-Gstarting
= H-TS
Thermodynamics is the study of
energy transformations that occur
in a collection of matter. There
are two Laws of Thermodynamics.
1. Conservation of EnergyEnergy can be transferred and
transformed, but it can be
neither created nor destroyed.
2.
Entropy- Every energy
transfer or transformation
increases the entropy of the
universe.
Entropy is a measure of chaos,
disorder, or randomness.
The
entropy of a particular system or
organism can decrease as long as
the total entropy of the universe
increases.
There are two types of reactions:
Exergonic, with a spontaneous
net release (or decrease) of
free energy ( G is negative),
and
Endergonic, which
nonspontaneously absorbs free
energy ( G is positive).
In a reversible process, one
direction is exergonic and the
other is endergonic., and Energy
coupling is the use of an
exergonic process to power an
endergonic one.
state
where G is the change in free
energy, H is the change in energy
of a system, and S is the change
in entropy.
At equilibrium, G=0; a reaction
would require help from an outside
energy source to move away from
equilibrium. The idea of
metabolic disequilibrium explains
that cells can never actually
reach metabolic equilibrium,
because G (the capacity to
perform work) would be 0, and the
cells would be dead!
1.
Exergonic reactions move
downhill, by having a negative
change in free energy, and the
energy of the products is less
than that of the reactants, while
endergonic reactions increase free
energy, moving uphill, making
the energy of the products is
greater than that of the
reactants.
ATP, or Adenosine Tri-Phosphate,
is the energy molecule of life,
with the nitrogenous base adenine
bonded to ribose, like an Adenine
RNA nucleotide. RNA has one
phosphate group, while Adenosine
tri-phosphate has three.
ATP can only be used as energy
when bonds between its phosphates
are broken by hydrolysis (the
addition water). of An inorganic
phosphate breaks off, releasing
energy and an ADP (adenosine diphosphate) This is spontaneous
because the tri-phosphate tails
three phosphates are all negative
and repulsive, so ATP is like a
loaded spring, which has potential
energy.
H
Hydrolysis of ATP
Technically, Enzymes are not
usually altered or denatured by
the reactions they catalyze.
Enzymes are proteins, thus their
functionality is reliant on pH and
temperature; an excessive amount
of heat will denature a protein by
breaking its hydrogen or ionic
bonds.
Each enzyme has an optimum
temperature and pH at which it
functions.
The activation energy barrier is
reduced in the presence of an
enzyme;
less
energy
must
be
surpassed.
Activation energy, or free energy
of activation, or Ea, is the
energy (usually in the form of
heat) that reactant molecules must
absorb from their surroundings so
that their bonds will become
unstable and reactive.
The optimum temperature allows the
greatest number of molecular
collisions to occur without
denaturing the enzyme, while the
optimum pH usually falls between 6
and 8 (except with digestive
enzymes, which operate best in the
acidic environment of the
stomach). A bell-shaped graph
usually illustrates the optimal
environment of an enzyme, for
example, Amylase.
Enzymes lower the activation
energy barrier so that a lesser
amount of heat is necessary to
initiate a reaction.
Non-competitive inhibition is not
the only form of allosteric
regulation, or metabolic control
through allosteric sites.
Activators can also bind
allosterically, providing
stability and correct form to an
enzyme.
Similarly, Cofactors encourage
reaction by binding to the enzyme,
but instead bind to the active
site. If the cofactor is an
organic molecule, containing
carbon, it is referred to as a
coenzyme.
Inhibition is highly important to
metabolic control, and feedback
inhibition is the switching-off
of a series of metabolic reactions
by its end product.
In simpler terms, if a metabolic
pathway is started by an enzyme
and results in the formation of a
particular substance, the presence
of that end substance will inhibit
the enzyme, thus stopping the
pathway from continuing
needlessly. This is the cells
mechanism to conserve chemical
resources and not waste energy.
This encourages spontaneous or
exergonic reactions to occur, and,
without enzymes, many cells would
reach deadly temperatures before
they could achieve reactions.
2.
Optimum pH of Amylase
Enzymes are substrate-specific (a
substrate is the reactant on which
an enzyme acts): They cannot just
be used for any reaction.
Instead, they must embrace their
substrate with an induced fit,
like a clasping handshake, thus
requiring that enzymes be uniquely
shaped to accommodate just one
type of substrate. .
An Enzyme-Substrate Complex
and its processes
Enzymes are, as proteins, very
structurally complex. They have
two types of sites:
Active sites, which are the
catalytic centers, and provide
a template for the substrates
to come together in the right
orientation, pH, or
microenvironment to encourage
reaction, and
Allosteric sites, secondary
binding sites on the enzyme
remote from the active site,
used not by specified
substrates, but by inhibitors.
Inhibitors make an enzyme
temporarily useless.
Competitive inhibitors
(through active inhibition)
competes with the substrate
for binding area on the active
site.
Non-competitive inhibitors
bind to allosteric sites,
altering the fit of the enzyme
so the substrate cannot bind
to the active site.
An example of inhibition is
penicillin, which actively
inhibits an enzyme that bacteria
use to form cell walls.
Competitive and Allosteric
Inhibition
1. http://dwb.unl.edu/Teacher/NSF/C10/
C10Links/genbiol.cbs.umn.edu/RWPlecturenotes/
Chemrxnssp98.html
2. http://dc2.unibielefeld.de/dc2/katalyse/e-ph-opt.htm
All information and unlabeled images are from
Campbells Biology, 5th Edition.