Now this is kinda complicated so I wanna step
you through it. But these are considerations
for doing reactions and what are
called Michaelis-Menten Kinetics.
We can see here that on the y-axis again, we have
concentration and on the x-axis we have time.
And before we saw simply the accumulation of
product as shown in the orange icon here,
at the very beginning of a reaction what
are the circumstances. Well, we have
four different things to think about that can be measured.
We have the concentration of substrate.
We have the concentration of the enzyme.
We have the concentration of the enzyme substrate complex,
and ultimately we are gonna have concentration of product
which is what we are interested in studying, okay?
At the beginning, the concentration of
product is low, as you can see.
And that's not surprising, because the reaction
hasn't had a chance to get started.
The concentration of ES is low because
there hasn't been an opportunity for the substrate
to really encounter the enzyme very much.
The concentration of the free enzyme, that
is the enzyme not bound to substrate
is relatively high. You see
it's coming down from the y-axis.
And finally the concentration of substrate is high
because none of the substrate has reacted.
So at the time 0, we have these circumstances going on.
And these circumstances turn out not to be ideal
for us to measure the enzymatic reaction.
Now as the reaction proceeds we see changes to these entities.
We see first of all that the concentration of substrate
by the end reaction is low and it's following during the entire process.
The concentration of the ES substrate which started out at 0
is going higher and we will see that
it will eventually sort of level off.
We also see that the concentration of the free enzyme
which started out to relatively high position
is falling and it too will sort a level off in time.
And finally we see of course that the concentration of product
is going to start at the low
and go to high by the very end.
Well, I show you this graph not to
complicate the picture too much hopefully,
but rather to demonstrate what we tried
to do in studying enzymatic reactions.
In the very initial phase,
I hope I have made the case for you,
that we are in a set of conditions called pre-steady state.
Now I will explain what's steady state means in a minute,
but we have a circumstance where the reaction
hasn't had a change to get started.
The enzyme isn't doing its thing and everything
is there changing pretty rapidly.
The change in substrate, the change in enzyme, the change
in enzyme substrate complex and the change in product.
This is gonna give us a lot of variability in a reaction.
Now I said we want to study initial velocity,
but we wanna be careful if we do it too soon
we may not get what we are after here.
So it's important to think about really studying a reaction at a
place where these things have sort of leveled off.
Now in a conditions of steady state, what's actually happening is
that these other quantities that were varying fairly rapidly
in the very initial phases of the reaction will start to even out,
and that's very important for our consideration.
So we can see for example that, in that early state
the concentration of free enzyme and ES complex are changing.
The concentration of E is falling very rapidly
and the concentration of ES is rising very rapidly.
However, under steady state conditions, as we
can see here, they have started to flatten out.
They are not exactly linear but they are much closer to
linear then they were in that pre-steady state condition.
That turns out to be important for us;
because, what we are interested in studying
is the conversion of enzyme substrate complex into product.
And so if we have a relatively constant concentration
of enzyme substrate complex, then that decay or that folly
into product that has actually happened
is happening at a relatively constant rate.
That's the place we wanna be and that's
why it's important for us to be
studying these reactions
under steady state conditions.
Steady state conditions, of course, again, meaning that
these quantities are not varying significantly.
Now, we can see now the overall plot of what's happening on here.
The steady state conditions are where we make our measurements
and we see that this relatively linear portion
of the plot for the concentration of free
enzyme and concentration of ES complex
is happening under the conditions
as we measure enzymatic reactions.
So in this presentation, we have seen some details now
about how enzymes interact with substrate,
how enzymes can manipulate energy to
make things happen the way they want,
how at the electron level, enzymes can accomplish what they do.
And we have also learned something about
the way that we study the kinetics or the speeds of reactions.
In another presentation, we will put these together
or use this information to study
some specifics in enzyme kinetics.