Lesson 2 – Video 2b
675 [00:00:01.09] PROFESSOR: All right. In this video, we will resume our survey of
676 And we'll talk about proteins first, which are made up of amino acids. Here, we can see
677 structure of amino acid-- amino acid because it has an amino group on one end and a
678 acid on the other, amino acid.
679 [00:00:25.20] In between those two groups, there's an alpha carbon. And connected to
that is the
680 side chain often talked about just as an R group. Now, every amino acid is going to have
681 same backbone. And every amino acid is going to have a different side chain. So that's
682 going to give them their different properties.
683 [00:00:48.28] Now, in aqueous solution, amino acids exist as zwitterions, meaning
charged molecules with a positively-charged amino group and a negatively-charged
685 group. And in the next couple of videos, we'll be talking about some of the
consequences for its
686 zwitterionic form. This is, again, two other depictions of the same molecule, ball and
chain and a
687 space filling model with the amino group, the alpha carbon, and the carboxylic acid.
688 [00:01:22.53] Now, proteins are linear chains of amino acids. And so that we're going to
689 connect amino acid one to amino acid two, we're going to do that by a type of reaction
690 condensation reaction, where we'll lose a molecule of water but we're going to connect
691 carboxylic group to an amino group of the second protein. So here is amino acid one.
692 amino acid two. They're joined between those groups in this gray box by a bond called
the 693 peptide bond.
694 [00:01:57.35] This peptide bond, there's nothing special about peptide bonds compared
695 bonds. But it's a convenient way to refer to that. Now, peptide bonds do have some
696 that some of these other bonds don't. Even though this is a single bond, it has partial
697 character. So these four atoms will be planar. We will not rotate around these bonds,
698 these single bonds we can rotate freely.
699 [00:02:21.79] Down here, we see three amino acids connected to it. This is the amino
700 because we have the free amino end. This is the carboxy terminus. We have a histidine,
701 cystine, and a valine amino acid. Now, you won't have to memorize the amino acids. As
702 see, I struggled a bit on that. But we will need to know a little bit about amino acids, and
703 get to that in a second.
704 [00:02:50.18] Important point about the synthesis of proteins-- we'll see this a couple
705 that proteins are synthesized from the amino terminus to the carboxy terminus,
706 depicted as N to C. Here, we have the amino group. This amino acid was before searing
707 glutamine and lysine. If we're going to add another amino acid, we're going to connect it
to the C
708 terminus, so that the amino terminus is always the first part of a peptide that is
[00:03:23.99] Now I'm going to run through different types of amino a 709 cids. We'll talk
710 just flip through charged amino acids, uncharged polar amino acids, and nonpolar or
711 hydrophobic. So we're going to have to remember these three groups. Here are the
charged 712 amino acids. Lysine, arginine, and histidine-- these are all positively charged on their
713 Histidine can be neutral or positively charged.
714 [00:03:52.37] Acidic side chains, aspartic acid, and glutamic acid-- these are all charged
715 will tend to have certain properties that you will need to be able to deduce and reason
716 when talking about that. For instance, just as a preview, you would never find a charged
717 acid buried inside the hydrophobic interior of a lipid membrane. But we'll get to that.
718 [00:04:25.78] The second main category of amino acids are polar, like the charged
719 But they themselves are not charged. We have asparagine and glutamine that have
720 side groups. And we have three types of amino acids that have hydroxyl groups-- serine,
721 threonine, tyrosine. These also tend to be exposed towards the aqueous solvent. These
will not be
722 buried inside the hydrophobic interior of proteins.
723 [00:05:02.41] These three amino acids with their hydroxyl groups could all be
724 And that will play a major role in cell signaling. We'll see those.
725 [00:05:12.95] The third type of amino acids are the hydrophobic amino acids. There are
726 them. They are primarily hydrocarbons-- as you can see here, methyl groups,
727 have a methionine with the sulfur group, and the largest amino acid, tryptophan, with an
728 functionality. There are two special types of amino acids here.
729 [00:05:39.49] Proline-- this is technically not an amino acid, you don't need to know this.
730 called an imino acid. And this has a side chain that is connected to both the alpha
carbon and the
731 amino group. So we're not going to be able to rotate a