In order to be alive, cells must generate
energy and use that energy to make molecules.
That's the function of what we call metabolism.
In this presentation I will go through the
processes of catabolism and anabolism, I'll
deal with energy considerations as they regard
biochemical reactions and finally talk about
regulation and mechanisms.
First I need to define some terms. Catabolism
is a process whereby larger molecules that
the cell takes in, are broken down into smaller
components. You can see on the screen for
example that proteins, polysaccharides and
fats are broken respectively into amino acids,
sugars and fatty acids. These precursors or
these building blocks that were used to make
the larger molecules are in turn broken down
into smaller components and though there are
a variety of these components that are made,
a common one that they all converge to is
acetyl-CoA. Acetyl-CoA can be oxidized readily
in the citric acid cycle to generate ATP energy.
The process of oxidation of course generates
electrons and those electrons in the cell
must be dealt with. The electrons in cells
are placed on electron carriers, either NAD+
or FAD typically, NAD+ is shown here.
When NAD+ accepts electrons and a proton,
it becomes NADH as you can see on the presentation.
The process of anabolism is essentially the
opposite of catabolism, that is, smaller molecules
are built into precursors and those precursors
are made into the building blocks of the individual
larger components. So we can see here for
example that the precursors are made into
amino acids, sugars and fatty acids to make
respectively, proteins, polysaccharides and
fats. Because this process requires energy,
ATP input is necessary and the ATP generated
by the catabolic processes is used to make
the molecules here in the anabolic processes.
Anabolism usually is reductive, meaning that
it requires electrons and so electron sources
are needed to make these larger molecules.
Electron sources come from a variety of things,
but typically NADH can be used.
Now as I noted in the previous presentation,
cells are governed by the rules of the universe.
They can't violate the ways that energy is
used and stored, so they have to work within
those constraints. Now consider for example
what cells must do in oxidizing glucose.
This metabolic pathway is known as glycolysis and
in glycolysis, the very first reaction involves
the addition of a phosphate to glucose. If
the cell were to try to simply put a phosphate
on to glucose, it would encounter the reaction
shown at the very top, where glucose is added
to a phosphate, creating a molecule known
as glucose-6-phosphate. Now delta G
0 prime (ΔG0') for that reaction is +15 kJ per mol.
It's possible to make that reaction go, but
that very positive ΔG0' value makes it somewhat
of a barrier and makes it difficult for the
cell to make glucose-6-phosphate. So rather
than try to push that reaction excessively,
cells have another alternative that they can use
to make the reaction favorable. The consideration
that the cells have, is that they have an
energy source and the energy source that they
have is ATP. Now ATP as you see in the second
line, can be hydrolyzed using water, to create
ADP plus phosphate. Now the ΔG0' for that
reaction is very negative, -31 kJ per mol.
So what cells do to overcome the barrier of
the first reaction is to couplet with the
second reaction. When these two reactions
are put together, then what happens is the
reaction overall becomes energetically favorable.
In this case glucose, plus ATP, is used to
generate glucose-6-phosphate plus ADP, and
for this reaction the ΔG0' is -16 kJ per
mol, that is the sum of the two previous reactions.
So by pairing an energy releasing reaction,
with an energy requiring reaction, also called
coupling, cells can overcome energy barriers.