If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:4:44

Voiceover: So we're going to
talk about enzyme kinetics today, but first let's
review the idea that enzymes speed up reactions by lowering
the delta G of the transition state, or lowering the
activation energy of a reaction. And also remember that for
this to happen the reacting substrate, which I called
S, will bind to the enzyme E to form the enzyme substrate complex ES before being turned into product P. Also remember that enzymes aren't used up when they catalyze
reactions, and that's why we have this E at the end of the equation. Now, there's a general
method of thinking that we want to use when talking
about enzyme kinetics, and that's what I want
to talk about today. Now, when we think about
kinetics, we want to simplify a reaction as
a change from A to B. And you may remember that
the rate of this change from A to B would be equal to sub rate constant K which is
dependent on the environment of the reaction, multiplied
by the concentration of our starting material A. So for our sequence
that I mentioned before, E plus S going to ES going to E plus P we have two reactions
going on, one and two, which would each have
their own rate equation. Rate one would be equal
to rate constant K1 times the two starting
material concentrations, which are E and S, while
rate two would be equal to K2 times the concentration of
one starting material ES. Notice that the reaction one
has two reactants, E and S, while reaction two only has
one, which is the ES complex. So what do we mean when we say "rate"? Well, the rate of a
reaction is the speed that the reaction goes at,
and we could also call it V, which is the symbol for speed. And for our entire
reaction of transitioning S to P, our product, the
speed would be equal to the rate of change of our
concentration of product with respect to time, or for
those of you who aren't really big fans of calculus,
the change in P, delta P, over a change in time, delta T. So to increase the rate that
we get new product, we could do this by either increasing
the substrate concentration or by increasing the enzyme concentration, since we're going to
assume that the K value is constant and can't be changed. Now when we think about enzyme kinetics we like to assume that
we're in a situation where the total concentration
of enzyme is constant. And this is generally the
case when we're looking at enzymes working in different cells. Now if we say that we only
have four enzymes here, and each enzyme can work at
a speed of about 10 reactions per second, that would mean
that the absolute maximum rate or our reaction would be
40 reactions per second. And this rate we would
call "Vmax" or "max speed". And the idea here is that at
really high concentrations of substrate the enzymes will
be saturated and full up with substrate, and won't be able
to react any more quickly. And even if we were to really
increase the concentrations of substrate a lot, there
will still be a Vmax. There's only so much that
we can increase the rate of a reaction by increasing
the substrate concentration. If we were to look at a graph
and plotted the reaction rate V versus our concentration of substrate, we would see that as our
substrate concentration got really, really high,
the rate would level off as it approached our Vmax value. So when we think about enzymes
and their kinetics this way we have made a couple of
assumptions about how our enzymes and substrates are behaving, and I want to talk about
these for a moment. The first assumption we
have made is that our solutions are behaving ideally,
and that we can actually classify our enzymes reaction
into two distinct snaps, the first being the binding
of substrate to enzymes, and the second being the
transitions from substrate to product with the enzymes help. And by assuming that our
solutions are behaving ideally and that we don't have any
external factors messing things up, we can simplify our discussion
of kinetics quite a bit. Our second assumption is that our two big constants stay constant. We're assuming that our enzyme
concentration isn't changing from things like protein
synthesis and degradation, and we're also assuming that our rate constant K isn't changing
from environmental factors like changes in temperature. Our final assumption is
that for our reaction substrate isn't forming
product without the help of enzyme at a big enough
rate for us to consider, and that it's negligible. Remember that enzymes
only speed up reactions, so while it's possible for
the substrate to form product without enzyme, we're going
to assume that it's not really happening when we're talking
about enzyme kinetics. So what did we learn? Well first we learned that we can classify enzyme catalysis into two important steps. The first is that the
enzymes bind the substrate, and then second the formation
of product, and we talked about how each of these
steps has a distinct rate. Second, we learned that if we
keep the enzyme concentration constant, then there
will be a maximum speed, Vmax, for that reaction.