The α-Amino Acids

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Your hair, your muscles, the enzymes digesting your lunch: all of these are made of proteins, built from only about twenty small building blocks. Now here is the strange part. About half of these building blocks, your body cannot make. You have to eat them! So which ones are they? That is today's lesson, the alpha-amino acids. Questions on them do come up in JEE chemistry, so let's begin.

0:30What makes it an α-amino acid?

So, let us start with the general formula on screen. H two N, with a small lowered two, is the amino group, and COOH is the carboxyl group. The short dashes are bonds, and both groups sit on the same carbon, the one in the middle. The capital R on that carbon stands for the side chain. Okay? Now, the name of these compounds starts with alpha, a Greek letter. So tell me, what does that alpha tell us about the amino group? Yes, exactly, it tells us where the amino group sits. The carbon right next to the carboxyl group is called the alpha carbon, and the amino group sits on it. Clear? Now, hydrolysis of proteins, breaking them with water, gives only alpha-amino acids. About twenty of them occur in proteins, and they differ in the side chain. So, know the side chain, and you know the amino acid. With me?

1:25Names and codes

Now, every amino acid has two short codes. For glycine, the brackets hold GLY, its three-letter code, and G, its one-letter code. Its side chain is just a hydrogen atom, so R equals capital H, okay? Alanine has the codes ALA and the letter A, and its side chain is C H three, a methyl group; the lowered three counts the hydrogen atoms. Got it? Now let us take lysine. Its three-letter code is LYS, easy. And its one-letter code? It must be the first letter, L, right? Careful here, because that is a trap. The letter L already belongs to leucine. So the one-letter code is not always the first letter. Lysine gets the letter K, glutamic acid gets E, tyrosine gets Y, and tryptophan gets W, next to their three-letter codes on screen. Learn these four. Okay?

2:22Neutral, acidic or basic?

Now, how do we sort these twenty? We simply count groups. Neutral amino acids have equal numbers of amino groups, N H two, and carboxyl groups, COOH, on screen. So one amino group and one carboxyl group means neutral. Right? Acidic amino acids have more carboxyl groups than amino groups. The brackets hold the codes of aspartic acid and glutamic acid; each of them carries an extra COOH in its side chain. Clear? And basic amino acids have more basic groups. In the brackets you see the three of them: lysine, arginine and histidine. With me? Now look at asparagine in the badge. Its side chain ends in CONH two, so it has a second N H two group. So asparagine must be a basic amino acid, right? Careful, because that is the trap. That N H two is part of an amide group, CONH two, and an amide is not basic. So asparagine is neutral. Glutamine ends in the same amide group, so it is neutral too. Got it?

3:31Essential amino acids

Now, back to our puzzle. Some amino acids cannot be made in the body, so they must come from the diet. These are the essential amino acids. The rest are non-essential. Okay? Here are the essential ones from the course, in their three-letter codes: valine, leucine, isoleucine, phenylalanine, tryptophan, threonine, methionine, lysine, arginine and histidine. Ten out of about twenty, so about half, as our puzzle said. Right? Now look at this list carefully. Remember our three basic amino acids, lysine, arginine and histidine? Are they in this list? Yes, exactly, all three are there, at the end of the list; the brackets hold their codes. And the two acidic ones, aspartic acid and glutamic acid, are not there at all. So here is a small trick for these twenty: the basic ones are all essential, and the acidic ones are not.

4:30Four amino acids with a special feature

With me? Now, a few amino acids have something special. Remember, an asymmetric carbon carries four different groups. The alpha carbon carries an amino group, a carboxyl group, a hydrogen and the side chain. So which amino acid has no asymmetric carbon? Yes, exactly, it is glycine. Its side chain is just a hydrogen atom. So its alpha carbon carries two hydrogens, and only three different groups. There is no asymmetric carbon at all. Clear? Next is proline. Its side chain bends back and joins the nitrogen, closing a five-membered ring. So its nitrogen is bonded to two carbon atoms, which makes proline a secondary amine. Okay? Cysteine and methionine both contain sulphur. The brackets hold the side chain of cysteine, C H two S H; that S H group is called a thiol. Right? And tyrosine has a phenolic O H group, which means an O H on a benzene ring. C six H four, with the lowered six and four, is that ring, and the O H sits on it. Got it?

5:39Example 1: classify amino acids as neutral, acidic or basic

Time for the first example. Classify these seven as neutral, acidic or basic: alanine, lysine, aspartic acid, valine, arginine, glutamic acid and serine. Pause the video and try it first. Done? Okay, let's check, starting with the tricky one. Serine's side chain, in the badge, is C H two O H, so it carries an extra O H group. Does that extra O H make serine acidic? No, it does not, and that is the trap. An O H group is neither a carboxyl group nor an amino group, so C H two O H adds neither, as the first line says. Okay? So serine is neutral. Alanine and valine have side chains of only carbon and hydrogen, so they are neutral too. Right? Aspartic acid and glutamic acid carry an extra COOH in the side chain, as the brackets remind us, so both are acidic. Clear? And lysine and arginine have an extra basic group in the side chain, so both are basic. That gives three neutral, two acidic and two basic. That's it!

6:52Example 2: chiral carbons in alanine, threonine, isoleucine

Now a harder one. How many asymmetric carbon atoms are there in alanine, threonine and isoleucine? Pause the video and try it first. Done? Okay, let's check, and let us take alanine first. Its alpha carbon carries an amino group, a carboxyl group, a hydrogen and a methyl group, four different groups. The methyl carbon has three hydrogens, so it is not asymmetric. The double arrow means gives, so alanine has one asymmetric carbon. Okay? Now let us take threonine. Its alpha carbon is asymmetric too, so the answer is one again, right? Careful, because we must check the side chain as well. Beta, the next Greek letter, marks the next carbon along the chain. In threonine's side chain, with the O H in brackets, the beta carbon carries a hydrogen, an O H, a methyl group and the alpha carbon, four different groups again! So threonine has two asymmetric carbons. Clear? Isoleucine works the same way. Its beta carbon carries a hydrogen, a methyl group, an ethyl group, the C two H five on screen, and the alpha carbon. So isoleucine also has two. The answers are one, two and two. See how fast that was?

8:13Quick revision: amino acid classes, essential ones, glycine

So, quick revision. Alpha-amino acids have the amino and carboxyl groups on the same carbon; about twenty occur in proteins, differing in the side chain. Count the groups to call them neutral, acidic or basic, and remember, an amide is not basic. Ten are essential and come from the diet, and glycine has no asymmetric carbon. Now go and try the practice problems on this page. All the best!