Past JEE Questions: Proteins and Enzymes

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Boil an egg, and the clear, runny egg white turns into a firm white solid. So what did the heat destroy, and what did it leave untouched? Keep that question in mind. JEE Main has tested this very idea again and again. So today we solve past JEE questions on proteins and enzymes. Let's begin.

0:26Idea 1: the four levels of protein structure

So, idea number one is the four levels of protein structure, left to right in the figure. Level one, the primary structure, is the sequence of amino acids in each chain, the beads in the figure, joined by peptide bonds, which are covalent. Okay? Level two is the secondary structure, the regular shape of a length of chain. Its forms are the alpha-helix, the orange spiral, and the beta-pleated sheet, the zigzag; alpha and beta are Greek letters. JEE Main twenty twenty five asked just this, and both are secondary. Right? Now JEE Main tested this idea twice in twenty twenty one, once about the alpha-helix itself. What stabilises the secondary structure, like the alpha-helix: peptide bonds, glycosidic bonds, hydrogen bonding, or van der Waals forces? Yes, exactly, it is hydrogen bonding. These hydrogen bonds, written H-bonds, join the C double bond O and N H groups of peptide bonds; the equals sign is a double bond, the dash a single bond. Peptide bonds is the trap option: they hold the primary structure. Clear? Level three, the tertiary structure, is the curled chain in the figure: the overall folding of the whole chain in space. Okay? And level four, the quaternary structure, is the arrangement of two or more chains. The four round shapes in the figure are like the four subunits of haemoglobin. Got it?

1:56Idea 2: what holds the tertiary structure

Idea number two: what holds the tertiary folding? It is held by four forces: hydrogen bonds, disulphide links, van der Waals forces, and ionic attractions. The disulphide link is written dash S dash S dash, two sulphur atoms bonded together. With me? JEE Main twenty twenty three made this list a question. Which does not stabilise the secondary and tertiary structures: hydrogen bonding, S S linkages, van der Waals forces, or O O linkages, two oxygen atoms bonded together? That is correct, the O O linkage is the odd one out; it is simply not on our list. Right?

2:41Idea 3: denaturation, what heat leaves behind

Now idea number three, and back to our egg. Heating, or a change of p H, disturbs the hydrogen bonds of a protein. The helices uncoil, and it loses its biological activity; this is denaturation. Okay? So the secondary and tertiary structures are destroyed, the spiral and the folded chain in the figure. That is why, in a twenty twenty six JEE Main paper, the statement that the tertiary structure stays intact when the p H changes was the incorrect one. Right? Now the egg question itself. JEE Main asked it about heating in twenty twenty two, and about boiled egg white in twenty twenty four. After egg white is boiled, which level of structure stays intact? Yes, exactly, it is the primary structure. Heat does not break the covalent peptide bonds, so the amino acid sequence stays the same. That solves our egg puzzle. With me?

3:38Idea 4: enzymes lower the activation energy

Now idea number four, enzymes. They are biological catalysts, and almost all are globular proteins. In the figure, the enzyme E has an active site that fits only its own substrate S, like a lock and key. Okay? The graph plots energy against the progress of the reaction; the height of the hump above the starting energy is the activation energy, E with a small a below it. For the acid-catalysed hydrolysis of sucrose, E a is six point two two kilojoules per mole; the minus one on mol means per mole. Right? With the enzyme sucrase, it drops to only two point one five kilojoules per mole, the lower, orange curve. So a lower barrier means a faster reaction. Keep these numbers in mind. Got it? Here is one from JEE Main twenty twenty one. Enzyme A turns sucrose and water into glucose and fructose; enzyme B turns glucose into ethanol and carbon dioxide. Can you name both? Enzyme A is invertase, which is another name for sucrase. It turns sucrose into glucose plus fructose; the arrow means gives. Okay? And enzyme B is zymase, which turns glucose into ethanol and C O two, carbon dioxide. So it is invertase first, then zymase. Clear?

4:57Example 1: minimum molar mass from 0.30% glycine

Time for Example one, from JEE Main twenty twenty two. Protein A contains zero point three zero percent of glycine, molar mass seventy five, given in the brackets. Find the minimum molar mass of the protein. Pause the video and try it first. Done? Okay, let's check. Many students just divide seventy five by zero point three zero, as the division sign shows, and get two hundred and fifty. So it is two hundred and fifty grams per mole, right? Careful now, that forgets the percent. Zero point three zero percent means zero point three zero grams of glycine per hundred grams of protein, with at least one glycine per molecule. So M min, the small min below the M meaning minimum, is one hundred times small m, over p, where small m is the molar mass of glycine and p is its percentage. Okay? Here m is seventy five, and p is zero point three zero. So M min is seventy five times one hundred, over zero point three zero. Right? Seventy five times one hundred is seven thousand five hundred. Seven thousand five hundred over zero point three zero gives twenty five thousand. So the minimum molar mass is twenty five thousand grams per mole, a high value, as a protein should have. Got it?

6:15Example 2: denaturation, enzymes and protein structure

Example two is harder, from JEE Main twenty twenty six. Statement A says sucrose hydrolysis has a lower activation energy with the enzyme than with acid. Statement B says denaturation destroys the secondary and tertiary structures, and the primary stays intact. Statement C says nucleotides are joined by a glycosidic linkage between C one and C four of the pentose sugar; the small numbers are carbon numbers. Statement D says the quaternary structure is the overall folding of the polypeptide chain. Which statements are correct? Pause the video and try it first. Done? Okay, let's check. Statement A uses the numbers from idea four: two point one five kilojoules per mole with the enzyme, against six point two two with the acid. The enzyme's value is lower, so statement A is correct. Okay? Statement B is our egg puzzle again: the secondary and tertiary structures are lost, and the primary stays intact. So statement B is correct too. Right? Statement C is from the next chapter, on nucleic acids. Nucleotides are joined by phosphodiester links, from C five prime of one sugar to C three prime of the next; the small mark is a prime. So it is not a glycosidic linkage, and statement C is incorrect. Clear? Now statement D, where many students slip. Quaternary is the highest level, so surely it is the overall folding, right? Careful now, that is exactly the trap. The overall folding of the whole chain is the tertiary structure, while quaternary is the arrangement of two or more chains. So statement D is incorrect. With me? So the correct statements are A and B, okay? If you got both, very good, because that was the hardest one in this set.

8:12Quick revision: structure levels, denaturation, enzymes

So, quick revision. The secondary structure, the alpha-helix or the beta-pleated sheet, is held by hydrogen bonds; the overall folding is tertiary. Denaturation destroys the secondary and tertiary structures, and the primary stays intact. Enzymes lower the activation energy, and the minimum molar mass is one hundred times m, over p. Now try the other past questions on this page. All the best!