Classification and Structure of Proteins

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. The keratin in your hair and the insulin your body makes are both proteins, built from the same alpha-amino acids. Now, you wash your hair every day, and it never dissolves in water. But insulin dissolves in water easily. Same building blocks, so why do they behave so differently? Keep that question in mind, because we will answer it in a few minutes. That is today's lesson, the classification and structure of proteins. It is a regular part of JEE chemistry, so let's begin.

0:36Proteins: the chief materials of living cells

So, what is a protein? Proteins are the chief structural and functional materials of living cells. Chemically, they are polypeptides of high molar mass, usually more than a hundred alpha-amino acid residues; alpha is the Greek letter on screen. Each amino acid unit in the chain is called a residue. Okay? And which elements do they contain? All proteins contain C, H, O and N, that is carbon, hydrogen, oxygen and nitrogen. Many also contain S, sulphur, and in a few minutes you will see what that sulphur does. Right?

1:10Two classes: fibrous and globular

Now, by shape, proteins fall into two classes. First come the fibrous proteins. Their polypeptide chains run parallel, side by side, held together by H-bonds, short for hydrogen bonds, and by disulphide bonds. So the chains form fibres. Clear? The examples are keratin, which the brackets tell you is found in hair, wool and silk; myosin, found in muscle; and collagen. With me? The second class is globular. Here the chains coil into a compact, roughly spherical shape, like a small ball. Okay? The examples are insulin, albumin, haemoglobin and the enzymes. And look, insulin from our puzzle sits in this class. Right? So, back to the puzzle. Keratin is fibrous, and insulin is globular. Which shape would you expect to dissolve in water, long fibres or small compact balls? Yes, exactly, it is the compact balls that dissolve. Fibrous proteins are insoluble in water, and globular proteins are usually soluble in water. So your hair stays put while insulin dissolves; the shape decides it. And notice the word usually. Got it?

2:22Level 1: primary structure

Now let us look at the structure itself. The figure shows four levels, from left to right. Level one, the primary structure, is the sequence of amino acids in each chain. On the left, each bead carries a three-letter code: glycine, alanine, serine, valine and lysine, in that order. The short lines joining them are peptide bonds, which are covalent bonds. Okay? Now let me ask you this. Out of hundreds of amino acids in a chain, suppose just one is changed. Is it still the same protein? No, it is not, because any change in the primary structure makes a different protein. So the sequence is like the identity card of a protein. Clear?

3:06Level 2: secondary structure, regular shapes

Level two is the secondary structure: the regular shape that a length of chain takes. It is held by H-bonds between the C double bond O group and the N H group of the peptide bonds. The small angle sign before the C and before the N stands for their other two bonds, and the dash between N and H is a single bond. Okay? The first shape is the alpha-helix, the orange wave in the figure. It is a right-handed spiral. Right? Inside the helix, each C double bond O is hydrogen bonded to the N H of the fourth residue further along the chain, in the next turn. So these hydrogen bonds stay within the same chain; we call them intramolecular. With me? The second shape is the beta-pleated sheet, the zigzag below the helix; beta is the Greek letter after alpha. Here the chains are stretched almost fully and laid side by side. So where do you think the hydrogen bonds form now? Yes, exactly, between neighbouring chains, so they are intermolecular. Remember the contrast: in the helix, within one chain; in the sheet, between chains. Got it?

4:14Levels 3 and 4: tertiary and quaternary

Level three is the tertiary structure, the curled line in the figure. It is the overall folding of the whole chain, which means the secondary structure folds further. Okay? And this folding gives the protein its fibrous or globular shape. So the two classes we met earlier are decided right here. Right? What holds this folding together? Hydrogen bonds, disulphide links, shown as two S atoms between dashes, van der Waals forces, and ionic attractions between side chains. There is our sulphur again! Clear? Level four is the quaternary structure: how two or more polypeptide chains, called subunits, are arranged with respect to one another. In the figure, those are the four round shapes packed together. With me? Now here is a quick check for you. Suppose a protein has only one polypeptide chain. Does it have a quaternary structure? No, it does not, because quaternary structure needs two or more chains. Haemoglobin has four subunits, so it does have a quaternary structure. Okay?

5:19Example 1: classify six proteins as fibrous or globular

Time for the first example. Classify these six proteins as fibrous or globular: keratin, insulin, myosin, albumin, collagen and haemoglobin. Pause the video and try it first. Done? Okay, let's check. Keratin, myosin and collagen are fibrous, with parallel chains forming fibres. Keratin is the hair protein from our puzzle, and myosin is the muscle protein. Right? Insulin, albumin and haemoglobin are globular: compact and roughly spherical. So that is three fibrous and three globular, and that's it!

5:57Example 2: primary, secondary, tertiary or quaternary

Now let us try a harder one. Which level of structure does each statement describe? The letters in brackets just number them. First, glycine, isoleucine, valine, glutamic acid, written in three-letter codes with dashes, is the beginning of a chain of insulin. Second, a chain of keratin is coiled into a helix. Third, haemoglobin consists of four chains packed together. And fourth, a chain of myoglobin is folded into a compact ball. Pause the video and try it first. Done? Okay, let's check. The first statement gives the order of amino acids in the chain, so it describes the primary structure. Okay? Now the second one. Keratin is a fibrous protein, and we said the fibrous shape comes from tertiary folding. So the answer is tertiary, right? Careful, because that is the trap. The statement is only about one chain coiled into a helix. That regular shape is the alpha-helix, so it is the secondary structure. Clear? The third statement has four chains packed together, so it is about how chains are arranged with respect to one another. That is the quaternary structure. With me? And in the fourth, one chain is folded into a compact ball, the overall folding of the whole chain, so it is the tertiary structure. The answers are primary, secondary, quaternary and tertiary. See how fast that was?

7:25Quick revision: fibrous vs globular, the four levels

So, quick revision. Fibrous proteins have parallel chains and are insoluble in water; globular proteins are compact and usually soluble. Primary structure is the sequence; secondary is the alpha-helix or the beta-pleated sheet, held by hydrogen bonds. Tertiary is the folding of the whole chain, and quaternary needs two or more chains. Now go and try the practice problems on this page. All the best!