We'll begin by taking a little closer look at the fats.
First of all, fats you'll recall are non-polymer
macromolecules and they're broken into their constituent
fatty acids and glycerol molecules. So glycerol will
enter the pathway for metabolism in glycolysis.
One of the intermediates in glycolysis that you don't
really need to know. Just that it comes in in glycolysis
and then will go through and be made into pyruvate
and in the future will become Acetyl CoA.
The fatty acids on the other hand undergo a process called
beta oxidation to make them into two carbon molecules
which will then enter as Acetyl CoA.
So in beta oxidation, we are taking a fatty acid chain
which could have single bonds or double bonds,
but we're cleaving it in two carbon subunits. So this
image represents, green dots representing carbons.
They're cleaved in two carbon subunits which is an
acetyl group, and then added to coenzyme A.
Coenzyme A joins it and these can freely join the Krebs
cycle and we can generate much ATP from these fatty acid tails.
So now let's take a look at how proteins are catabolized.
Proteins are composed of amino acids and proteins that
we consume get broken down into amino acids
during the process of digestion
Those amino acids then become absorbed and we will break
them down to enter into the process of cell respiration
but only really if we're starving. Because as I said
earlier, we really don't need to be just breaking down
any old proteins. But if we need to use them in the process
of metabolism then we will first deaminate them,
take the amino groups off. And then they will enter at
various points during glycolysis or pyruvate oxidation
and the Krebs cycle. So they're not as easy or straight
lined as carbohydrates or fats are in this whole process.
Lets look at deamination for a moment. For example if we take
glutamate and we remove the amino group, then we can
form α-Ketoglutarate. α-Ketoglutarate is one of the intermediates
we see in the Krebs cycle and so it fits right in there
The problem with breaking down amino acids is also we
generate a lot of these nitrogeneous bases or NH3's
and so NH3's can be sort of toxic to the system.
They are actually broken down and form urea
and are excreted in the urine. So, a lot of protein
breakdown results in a lot of extra urea in urine.
And proteins will or is their amino acids are going to
enter as we saw at various points during the Krebs cycle
or glycolysis or pyruvate oxidation. And here you can see
an example of how some of these amino acids might
go into the cycle. For example you'll see a group that
enters as pyruvate. We see a group that enters as acetyl CoA.
We can see the α-Ketoglutarate here.
As well as at Fumarate or Succinyl-CoA.
So each amino acid has a specific place that it could
enter the Krebs cycle. Again, it takes a few conversions,
few groups being cut off for them to be able to enter as
those intermediates that you see in this image.