Enzymes | Energy and enzymes | Biology | Khan Academy

Enzymes | Energy and enzymes | Biology | Khan Academy


– There are all sorts of
reactions in biological systems that are energetically favorable, but they’re still not
going to happen quickly or even happen on their own, and the phosphorylation of
glucose is an example of that. We go into some detail into that on the video on coupled reactions, and I think we actually called that The Phosphorylation of
Glucose 6-Phosphate, but it’s super important because by putting the phosphate
group on a glucose, it’s ready to be the input to a whole series of
biological mechanisms, it allows the glucose to be tagged so it’s going to be hard for
it to escape the cell again, and it’s fairly straightforward mechanism, where you have a lone pair of electrons on this hydroxyl group right over here, and then it attempts to, if
it’s in the right configuration, it could form a bond with the phosphorus in the phosphate group. Now, the reason why it
doesn’t happen on its own, even though it’s energetically favorable, once you form the bond, you have, electrons are gonna be able
to go to a lower energy state. So it has a negative delta G. If this is the molecules
before the reaction, this is how much free energy
they have before the reaction, after the reaction, they
have less free energy, they have been able to release energy, so this is something
that we would consider to be spontaneous, but for
the reaction to happen, you need a little bit of energy
to be put into the system. We call this our activation energy. You might say, “Well, why is that?” Well, we have electrons
that want to form a bond with this phosphorus, but this phosphorus is
surrounded by negative charges. This oxygen right over
here has a negative charge. This oxygen right over
here has a negative charge, and as you can imagine,
electrons don’t like being around other electrons,
like charges repel each other, so in order for this reaction to occur, or for it to occur more frequently, it has to be catalyzed. A catalyst is anything that
makes a reaction happen faster, or even allows the
reaction to happen at all, and when we talk about
catalysts in biological systems, we’re typically talking about, we’re typically talking about Enzymes. Enzymes. And the way that an Enzyme
might catalyze this reaction, we actually talk about it, and when we talk about coupled reactions, it’ll maybe can provide
some positive charges. It could provide some positive charges around these negative charges
to pull them further away to create space so that you can actually
have the reaction proceed, and so what an Enzyme would do, it would make this curve, instead
of having this hump on it, the curve would more like this, so that the reaction can just proceed. But what are these Enzymes? These things that can maybe, it could place some interesting charge that can allow the reaction
to happen a certain way, it might bend the
molecules in a certain way to expose some bonds, it might have a more
acidic or basic environment that might be more
favorable for the reaction. What are these seemingly magical things? Well, at a very high
level, they tend to be these protein complexes, plus
or minus a few other things, so you can view them as proteins and maybe sometimes, they’ll be
multiple polypeptide chains put together, they might have some other ions associated with them,
but for the most part, they are proteins, and the molecules that are going to react, that are going to bind to the proteins, we call these the Substrates. So these, and this
reaction, (mumbles) glucose and the ATP, these are
going to be the Substrates. So you can imagine the Enzyme that does this, and the general term for the Enzyme that helps phosphorylate a
sugar molecule like this, we call it hexokinase. So it might be this crazy-looking, this crazy-looking protein, we’re gonna take better looks at
this in a few moments, but the ATP might bind to it right over there. ATP is one of the Substrates, and then the glucose might bind to it right over there, and so these two Substrates bind, and the area where all
of this is going on, we call that the Active Site. So the Active Site, because
that’s where all the action is, the Active Site. And often, when you have
the Substrates bind, they’re able to interact with the protein to make the fit even stronger, to make it even more, more suitable for the
reaction to take place, and so the whole protein
might bend a little bit to kind of lock these two
in place a little bit more, and we call that Induced fit. Induced fit. And so, where would these
positive charges come from? Well these would be things
that are the side chains of the different amino
acids on the actual, on the polypeptide chain on the protein, and it could even be other
ions that get involved, in fact, in particular, to facilitate the
phosphorylation of glucose, a magnesium ion might be involved to help draw some positive charge away, but there’s other
positively charged groups that help draw charge
away so that the reaction is more likely to occur. So that’s what enzymes
are, and they tend to be optimally working in
certain pH environments or certain temperatures. In general, the higher temperatures
allow more interactions, things are bumping around more, but if temperatures get
a little bit too high, the protein or the Enzyme
might stop working, it might denature, it might lose its actual structure. And what I want now give
you an appreciation for is how beautiful and complex
these structures are. You should appreciate
what I’m showing you. These are in your cells! These are in your, look at your hand, look at everything around you,
there’s a lot of this stuff going on inside of you,
so hopefully it gives an appreciation for the complexity of you as a biological
system, but frankly, all biological systems. So this right over here,
this is a visualization of a hexokinase, one variety of it, and just to get a sense of scale, this is a glucose molecule, and this right over here is an ATP, and so they will bind, these
are the two Substrates, they will bind at the Active Site. You might have the Induced
fit, where this fits around it. It draws some charge away, it might bend the
molecules in a certain way so that they’re more likely to interact, bring these things close together, and so you’re gonna
have the reaction occur and then once the reaction occurs, they’re not gonna want to bind
to the Substrates anymore. I guess you could say the
products, at that point, and then they’re gonna let go of them, and then the Enzyme has a change, and that’s an important
property of an Enzyme. It’s not like it just has
one use and it goes away, it can keep doing this over
and over and over again. One Enzyme will do this many,
many, many, many, many times in its actual life. And so now what I want
to show you is a little three-dimensional visualization
that I got from a website, so let me go get that. Go ahead and pause my recording so I could get to this little
simulation or this model, and this is actually a hexokinase as well, and hexokinase is come into, in a bunch of different varieties, but this is a pretty neat thing to look at and this has been visualized differently, and when you look up
protein images on the web, or anywhere, you’ll see them sometimes with these ball and stick models, sometimes you’ll see them in
these space-filling model, sometimes you’ll see
them with this kind of, where you the very structures, and you notice the alpha helices here that we studied when we talked
about protein structures, and you can also see some beta sheets, but this gives you an appreciation of the binding sites and how
these things might interact. This right over here, that is a molecule of ATP, and then right next to it, I believe, if I’m looking at that right,
that is a molecule of glucose, and notice they have bound, they are the two Substrates, they have bound at the Active Site, and now, they can
interact with each other, the Enzyme, the hexokinase in this case, can help facilitate the
reaction that we care about, the phosphorylation of glucose. So hopefully, images
like this, and like this, give you an appreciation for how complex and how beautiful these
things actually are.

13 COMMENTS

    Ok, I am by no means an expert, but the picture in the top left (where he draws an arrow to in 1:24) shows that the light green part of the molecule has an oxygen-atom that bonds two two carbon atoms and they each bond to another and those two bond together. So they form that circle of five atoms / four carbon-atoms.
    But the picture he drew a arrow from, which I guess should be a zoomed part/fragment of the larger molecule has a 6 ring / 5 carbon-atoms.

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