Now the hormonal systems that I have talked about
it, epinephrine, glucagon and insulin
that apply for the sugar metabolism
also apply for a fat metabolism
in ways that you could probably
predict, as we will see.
Let's start with the epinephrine glucagon
stimulation of that same cell
and understand what happens to fat metabolism.
The hormone binds first and that
causes the G protein to be activated.
We see the activation of the
adenylate cyclase to make cyclic AMP.
The cyclic AMP interacts with protein
kinase A to activate it.
Now in this case what protein
kinase A is phosphorylating,
is an enzyme that's essential
for the breakdown of fat.
In the breakdown of fat, the most important
enzyme is an enzyme known as triacylglycerol lipase.
And like the glycogen breakdown
enzymes, phosphorylating it
favors its activity. Now triacylglycerol lipase works
in conjunction with another protein called
perilipin and the phosphorylation of it
also favors the breakdown of fat.
Now we see that happening below where the
triacylglycerol fat is being converted
into a diacylglycerol and then beneath
that you see the other lipases.
Well, what those lipases are doing,
is that they are breaking fatty
acids off of the triacylglycerol.
Once the first fatty acid is
removed from the triacylglycerol
to make the diacylglycerol, then the other enzymes that
breakdown the diacylglycerol, don't require the regulation.
So in this way the phosphorylation cascade that's
made by the binding of epinephrine or glucagon
is favoring the release of fatty
acids or the breakdown of fat.
The other by product of the breakdown of fat,
of course, is glycerol, as can be seen here.
So non-regulated enzymes don't require phosphorylation
and as a consequence, fat is broken down.
Now when insulin binds to
the cell surface receptor
not surprisingly, we are going to see
exactly the opposite thing happen.
When insulin bind to the receptor there is
our friend phosphoprotein phosphatase,
it is activated. It will take phosphates off of
the proteins that had phosphates put onto them earlier.
In addition there is an important
protein that's necessary and
important enzyme is necessary for
the synthesis of fatty acids.
This enzyme is known as acetyl-CoA-carboxylase
and this enzyme is regulated by phosphorylation.
In the phosphorylated state
that enzyme is inactivated.
So the removal of a phosphate from
acetyl-CoA-carboxylase causes it to become active.
Well when acetyl-CoA-carboxylase is activated
what it does, is it makes fatty acids.
And when you start making fatty acids they
start getting attached to glycerol.
And when they start getting attached
to glycerol, you make fat.
Well on the other side of this, what the removal
of phosphates is doing from these enzymes is
triacylglycerol lipase, the enzyme that
broke down the fat in the first phase
has it's phosphate removed,
as does the perilipin.
These two then become inactive so
fat no longer is being broken down.
Under conditions of insulin
stimulation, what's happening is
the body is making fat and it has
stopped the breakdown of the same fat.