# Biomolecules (NEET Biology Class 11): Proteins, Enzymes, Nucleic Acids & Cofactors Source: https://tracknation.in/story/biomolecules-neet-biology-class-11-proteins-enzymes-nucleic-acids-cofactors (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Biomolecules, mined from NCERT: chemical composition of living tissue, primary and secondary metabolites, micro vs macromolecules, the four levels of protein structure (with haemoglobin/collagen/RuBisCO), polysaccharides (cellulose/starch/glycogen/chitin, the iodine test), nucleic acid structure (nucleoside vs nucleotide, purines/pyrimidines, DNA vs RNA), and a deep dive into enzymes — active site, activation energy, the catalytic cycle, the 6 enzyme classes, factors affecting acti ## Chemical Composition: Primary & Secondary Metabolites 🎯 **NEET priority: High-yield.** *4–5 questions every year. **Enzymes** alone has produced more NEET questions over the years than any other single sub-topic in this chapter — give it your sharpest attention.* Grind any living tissue in trichloroacetic acid and strain it: you get an **acid-soluble pool** (the filtrate — thousands of small organic compounds) and an **acid-insoluble fraction** (the retentate — the macromolecules). Elemental analysis shows living tissue is NOT chemically different from non-living matter in *which* elements are present — only in their **relative abundance**: carbon and hydrogen are far more abundant in living tissue (18.5% and 9.5% of the human body by weight) than in the earth's crust (0.03% and 0.14%). ### Primary vs secondary metabolites - **Primary metabolites** (amino acids, sugars, etc.) have identifiable functions and known roles in normal physiology — found across all organisms. - **Secondary metabolites** are found in plants, fungi and microbes and include **pigments** (carotenoids, anthocyanins), **alkaloids** (morphine, codeine), **terpenoids**, **essential oils**, **toxins** (abrin, ricin), **lectins** and **drugs** (vinblastine, curcumin) — their role in the host organism is often not fully understood, but many are valuable to human welfare (rubber, spices, drugs). ## Biomacromolecules: Proteins & Polysaccharides ### Micromolecules vs macromolecules - Compounds in the acid-soluble pool have molecular weights **18 to ~800 daltons** — **micromolecules**. The acid-insoluble fraction has only **four classes**: **proteins, nucleic acids, polysaccharides and lipids** — **macromolecules**, with weights ≥10,000 Da (lipids are the exception: they're small, but end up in the insoluble fraction because grinding shatters cell membranes into insoluble vesicles). - By average cellular mass: **water is the most abundant** (70–90%), then **proteins (10–15%)**, **nucleic acids (5–7%)**, **carbohydrates (~3%)**, **lipids (~2%)**. ### Proteins — four levels of structure - Proteins are **heteropolymers** of amino acids (not homopolymers — **20 types** of amino acids occur in proteins, joined by **peptide bonds**). The **primary structure** is the amino acid sequence (N-terminal → C-terminal). Portions fold into a **right-handed helix** or other shapes — the **secondary structure**. The chain then folds on itself like a hollow woollen ball — the **tertiary structure**, essential for biological activity. Multiple polypeptide subunits arranged together give the **quaternary structure** — e.g. adult **human haemoglobin has 4 subunits** (2 α-type + 2 β-type). - **Collagen** is the most abundant protein in the animal world; **RuBisCO** is the most abundant protein in the entire biosphere. ### Polysaccharides - **Cellulose** (plant cell walls; homopolymer of glucose only), **starch** (plant energy store, forms a helix that binds I₂ giving a **blue-black colour**) and **glycogen** (the animal equivalent of starch) are all glucose polymers. **Cellulose has no helix, so it does NOT bind iodine.** **Chitin** forms arthropod exoskeletons, built from amino-sugars. ## Nucleic Acids & Enzyme Mechanism/Classification ### The four biomolecule families — monomer, bond, example | **Family** | **Building block (monomer)** | **Bond that links them** | **Key examples** | | Carbohydrates | Monosaccharide (glucose, fructose) | Glycosidic bond | Starch and glycogen (storage); cellulose and chitin (structural) | | Proteins | Amino acid — 20 standard types | Peptide bond | Collagen (most abundant animal protein); RuBisCO (most abundant enzyme on Earth) | | Nucleic acids | Nucleotide = base + sugar + phosphate | Phosphodiester bond | DNA (deoxyribose, thymine); RNA (ribose, uracil) | | Lipids | Glycerol + fatty acids | Ester bond | Triglycerides, phospholipids (membranes), steroids | **Exam trap.** A **nucleoside is base + sugar**; add a phosphate and it becomes a **nucleotide**. Lipids are NOT strictly macromolecules — their molecular weight is under 800 Dalton, so they appear in the acid-soluble fraction even though they behave like macromolecules in the cell. ### Nucleic acids — nucleoside vs nucleotide - A nucleotide has three components: a **nitrogenous base** + a **pentose sugar** + a **phosphate group**. Base + sugar alone (no phosphate) = a **nucleoside** (e.g. adenosine); base + sugar + phosphate = **nucleotide** (e.g. adenylic acid). - **Adenine and guanine** are **purines**; **cytosine, uracil and thymine** are **pyrimidines**. A nucleic acid with **deoxyribose** is **DNA**; one with **ribose** is **RNA**. ### Enzymes — mechanism & classification - Almost all enzymes are proteins (a few catalytic RNAs are called **ribozymes**). The **active site** is the crevice/pocket where the **substrate** binds. Enzymes accelerate reactions enormously — **carbonic anhydrase** speeds up CO₂ + H₂O ⇌ H₂CO₃ by about **10 million times** (200 molecules/hour uncatalysed vs 600,000/second catalysed). - Enzymes lower the **activation energy** — the extra energy the substrate needs to reach the unstable **transition state** before becoming product. The catalytic cycle: (1) substrate binds the active site, (2) the enzyme changes shape to fit more tightly (induced fit), (3) bonds break and the **enzyme-product (EP) complex** forms, (4) product is released and the free enzyme repeats the cycle. - Enzymes are sorted into **6 classes** by reaction type, numbered in this exact order (a common "match the class to its reaction" NEET question): | **Class** | **Reaction catalysed** | **Example** | | Oxidoreductases | Oxidation-reduction between two substrates | Dehydrogenases | | Transferases | Transfer of a group (other than hydrogen) between substrates | Transaminases | | Hydrolases | Hydrolysis of ester, ether, peptide, glycosidic bonds | Amylase, lipase, protease | | Lyases | Non-hydrolytic removal of groups, leaving a double bond | Decarboxylase | | Isomerases | Inter-conversion of optical, geometric or positional isomers | Isomerase | | Ligases | Joining two compounds (e.g. C-O, C-S, C-N, P-O bonds) | Synthetase | ## Enzymes: Factors, Inhibition & Cofactors ### Factors affecting enzyme activity - Each enzyme has an **optimum temperature and pH**; activity falls on either side. **Low temperature** only makes an enzyme temporarily inactive, but **high temperature denatures** it (proteins unfold) — this is why most enzymes fail above ~40°C, though enzymes from **thermophilic organisms** (hot springs, sulphur vents) stay stable up to 80–90°C. - As **substrate concentration** rises, reaction velocity rises too, then plateaus at **Vmax** — beyond this, all enzyme molecules are saturated, so adding more substrate does nothing. - A **competitive inhibitor** structurally resembles the substrate and competes for the active site, blocking the true substrate — e.g. **malonate** inhibits **succinic dehydrogenase** because it resembles the real substrate, succinate. This principle is used to control bacterial pathogens with drugs. ### Protein structure — the four levels side by side | **Level** | **What it is** | **Held together by** | **Example** | | Primary | The linear sequence of amino acids | Peptide bonds | The N-terminal to C-terminal chain | | Secondary | Local coiling or pleating of the chain | Hydrogen bonds | Alpha-helix and beta-pleated sheet | | Tertiary | The overall three-dimensional fold | Disulphide, ionic, hydrophobic bonds | The hollow woollen-ball shape; essential for enzyme activity | | Quaternary | Two or more polypeptide chains assembled | The same non-covalent forces | Haemoglobin — four subunits, 2 alpha and 2 beta | **Exam trap.** Denaturation destroys the secondary and tertiary structure but leaves the **primary structure intact** — that is why a boiled egg cannot be un-boiled yet its amino-acid sequence is unchanged. ### Cofactors — three kinds - The protein part of an enzyme needing a cofactor is called the **apoenzyme**. **Prosthetic groups** are organic and **tightly, permanently bound** (e.g. **haem** in peroxidase/catalase). **Coenzymes** are organic but bind only **transiently** during catalysis — many are **vitamin-derived** (e.g. **NAD/NADP** contain the vitamin niacin). **Metal ions** form coordination bonds with both enzyme and substrate — e.g. **zinc** is a cofactor for carboxypeptidase. Remove the cofactor and catalytic activity is lost. ### Why this matters for NEET - High-value one-liners: 20 amino acid types; haemoglobin's 4 subunits (2α+2β); starch binds iodine (blue-black), cellulose does NOT; DNA=deoxyribose, RNA=ribose; 6 enzyme classes; 3 cofactor types. - **Trap:** a **nucleoside** (base+sugar) is NOT the same as a **nucleotide** (base+sugar+phosphate) — don't swap them. Cellulose and starch are BOTH glucose homopolymers, but only starch's helical shape binds iodine. Prosthetic groups are **permanently** bound; coenzymes are only **transiently** bound — this exact distinction is a repeat NEET question. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** Compounds found in the acid-soluble pool of a ground tissue are generally called: - (a) Macromolecules - (b) Nucleic acids only - (c) Polysaccharides only - (d) Micromolecules **Answer: (d)** — The acid-soluble pool has small organic compounds (18–800 Da), called micromolecules. **Q2.** Which of these is NOT one of the four classes of macromolecules in the acid-insoluble fraction? - (a) Amino acids - (b) Nucleic acids - (c) Proteins - (d) Polysaccharides **Answer: (a)** — The four macromolecule classes are proteins, nucleic acids, polysaccharides and lipids; free amino acids are micromolecules. **Q3.** The amino acid sequence of a protein is called its: - (a) Quaternary structure - (b) Primary structure - (c) Tertiary structure - (d) Secondary structure **Answer: (b)** — Primary structure is simply the linear sequence of amino acids from N-terminal to C-terminal. **Q4.** Adult human haemoglobin is made up of: - (a) 1 single polypeptide - (b) 6 identical subunits - (c) 4 subunits: 2 alpha and 2 beta - (d) 2 subunits, both alpha **Answer: (c)** — Human haemoglobin has 4 subunits — 2 of the alpha type and 2 of the beta type — its quaternary structure. **Q5.** Which polysaccharide forms a helix and gives a blue-black colour with iodine? - (a) Chitin - (b) Starch - (c) Glycogen only - (d) Cellulose **Answer: (b)** — Starch's helical secondary structure can trap I2 molecules, producing the blue-black starch-iodine colour; cellulose has no helix. **Q6.** A nucleoside differs from a nucleotide in that a nucleoside lacks: - (a) Both base and sugar - (b) A phosphate group - (c) A pentose sugar - (d) A nitrogenous base **Answer: (b)** — A nucleoside is base + sugar only; adding a phosphate group makes it a nucleotide. **Q7.** Which of the following is a pyrimidine? - (a) Guanine - (b) Both adenine and guanine - (c) Adenine - (d) Cytosine **Answer: (d)** — Cytosine, uracil and thymine are pyrimidines; adenine and guanine are purines. **Q8.** The crevice or pocket on an enzyme into which the substrate fits is the: - (a) Transition state - (b) Apoenzyme - (c) Prosthetic group - (d) Active site **Answer: (d)** — The active site is the pocket formed by the enzyme's folded tertiary structure where the substrate binds. **Q9.** The extra energy needed to bring the substrate to its transition state is called the: - (a) Kinetic energy - (b) Activation energy - (c) Potential energy of product - (d) Free energy **Answer: (b)** — Activation energy is the energy gap between the substrate's stable state and the unstable transition state; enzymes lower this barrier. **Q10.** Enzymes are classified into how many major classes based on reaction type? - (a) 6 - (b) 5 - (c) 8 - (d) 4 **Answer: (a)** — Enzymes are divided into 6 classes: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. **Q11.** Malonate inhibits succinic dehydrogenase by acting as a: - (a) Coenzyme - (b) Prosthetic group - (c) Metal ion cofactor - (d) Competitive inhibitor **Answer: (d)** — Malonate structurally resembles succinate (the true substrate) and competes for the active site — a competitive inhibitor. **Q12.** Haem, in enzymes like catalase and peroxidase, is an example of a: - (a) Prosthetic group - (b) Metal ion cofactor - (c) Substrate - (d) Coenzyme **Answer: (a)** — Haem is tightly, permanently bound to the apoenzyme, making it a prosthetic group, not a transiently-bound coenzyme. **Q13.** Beyond the substrate concentration that produces Vmax, the reaction rate: - (a) Keeps increasing linearly - (b) Does not increase further - (c) Becomes negative - (d) Decreases sharply **Answer: (b)** — At Vmax all enzyme molecules are saturated with substrate, so further substrate increase does not raise the rate. **Q14.** The bond that links two amino acids in a protein is a: - (a) Peptide bond - (b) Glycosidic bond - (c) Ester bond - (d) Phosphodiester bond **Answer: (a)** — Amino acids join by peptide bonds. Learn the set together: glycosidic links sugars, phosphodiester links nucleotides, and ester links glycerol to fatty acids in a lipid. **Q15.** A nucleoside differs from a nucleotide in that a nucleoside: - (a) Lacks the sugar - (b) Contains two phosphate groups - (c) Lacks the phosphate group - (d) Lacks the nitrogenous base **Answer: (c)** — A nucleoside is nitrogenous base + sugar. Adding a phosphate group converts it into a nucleotide, which is the actual building block of nucleic acids. **Q16.** Which of the following is the most abundant protein in the whole biosphere? - (a) Haemoglobin - (b) RuBisCO - (c) Insulin - (d) Collagen **Answer: (b)** — RuBisCO is the most abundant protein in the entire biosphere, because of the sheer mass of photosynthesising plants. Collagen is the most abundant protein in the ANIMAL world — NEET tests this pair against each other constantly. **Q17.** Denaturation of a protein by heat destroys which level(s) of structure? - (a) All four levels including the primary - (b) Only the quaternary structure - (c) Only the primary structure - (d) Secondary and tertiary, leaving the primary intact **Answer: (d)** — Heat breaks the hydrogen and other weak bonds holding the secondary and tertiary folds, so the protein loses its shape and its function. The peptide bonds of the primary sequence survive. **Q18.** Haemoglobin, with two alpha and two beta chains, illustrates which level of protein structure? - (a) Tertiary - (b) Quaternary - (c) Primary - (d) Secondary **Answer: (b)** — Quaternary structure is the assembly of two or more separate polypeptide chains into one functional protein. Haemoglobin's four subunits are the standard NCERT example. **Q19.** A competitive inhibitor acts by: - (a) Permanently destroying the enzyme - (b) Raising the enzyme's optimum temperature - (c) Binding to a site away from the active site and changing the enzyme's shape - (d) Resembling the substrate and binding to the enzyme's active site **Answer: (d)** — A competitive inhibitor is structurally similar to the substrate and occupies the active site. Malonate inhibiting succinate dehydrogenase is the classic example — and because the block is competitive, raising the substrate concentration can overcome it. **Q20.** Which of the following is a non-protein constituent tightly bound to an enzyme and essential for its activity? - (a) Substrate - (b) Product - (c) Apoenzyme - (d) Prosthetic group **Answer: (d)** — A prosthetic group is a cofactor tightly bound to the apoenzyme — haem in peroxidase and catalase is the standard example. Coenzymes, such as NAD and FAD, associate only transiently, and metal ions form the third class of cofactor. **Q21.** The polysaccharide that forms the exoskeleton of arthropods is: - (a) Chitin - (b) Glycogen - (c) Starch - (d) Cellulose **Answer: (a)** — Chitin is a nitrogen-containing structural polysaccharide forming the arthropod exoskeleton and fungal cell walls. Cellulose is the plant structural polysaccharide, while starch and glycogen are the plant and animal storage forms. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q22.** The most abundant protein in the biosphere is: (NEET PYQ) - (a) RuBisCO - (b) Haemoglobin - (c) Collagen - (d) Insulin **Answer: (a)** — RuBisCO is the most abundant protein in the entire biosphere; collagen is the most abundant in the animal world specifically. **Q23.** Which statement about coenzymes is correct? (NEET PYQ) - (a) They are inorganic and permanently bound - (b) They are organic and bind only transiently during catalysis - (c) They are always metal ions - (d) They are identical to prosthetic groups **Answer: (b)** — Coenzymes are organic cofactors that associate with the apoenzyme only transiently, often during catalysis; many are vitamin-derived. **Q24.** A nucleic acid containing deoxyribose sugar is called: (NEET PYQ) - (a) RNA - (b) A ribozyme - (c) DNA - (d) A nucleoside only **Answer: (c)** — A nucleic acid built on deoxyribose is DNA; one built on ribose is RNA. **Q25.** The reaction catalysed by carbonic anhydrase is accelerated by approximately how many times compared to the uncatalysed reaction? (NEET PYQ) - (a) 2 times - (b) 10 million times - (c) 100 times - (d) 10,000 times **Answer: (b)** — Carbonic anhydrase speeds up CO2 + H2O to carbonic acid by roughly 10 million times. **Q26.** Proteins are described as heteropolymers because: (NEET PYQ) - (a) They contain only sugars - (b) They contain only one type of amino acid - (c) They are not polymers at all - (d) They contain 20 different types of amino acids **Answer: (d)** — Proteins are built from 20 different amino acid types, unlike a homopolymer which repeats a single monomer. **Q27.** Zinc acts as a cofactor for the enzyme: (NEET PYQ) - (a) Carbonic anhydrase - (b) Carboxypeptidase - (c) Catalase - (d) Succinic dehydrogenase **Answer: (b)** — Zinc is a metal ion cofactor required by the proteolytic enzyme carboxypeptidase. **Q28.** Which polysaccharide is a homopolymer of glucose found in the cell wall of plants? (NEET PYQ) - (a) Inulin - (b) Cellulose - (c) Chitin - (d) Glycogen **Answer: (b)** — Cellulose, a homopolymer of glucose only, forms the plant cell wall. **Q29.** The tertiary structure of a protein is essential for its: (NEET PYQ) - (a) Biological activity - (b) Solubility in acid only - (c) Colour - (d) Molecular weight **Answer: (a)** — The 3-dimensional tertiary structure, formed when the protein chain folds on itself, is absolutely necessary for a protein's biological activity. ### Active Recall Prompt Write everything you can recall about **Biomolecules**, naming each part first: **chemical composition** (acid-soluble vs acid-insoluble, primary vs secondary metabolites); **micromolecules vs macromolecules** and average cell composition; **protein structure** (primary through quaternary, haemoglobin, collagen, RuBisCO); **polysaccharides** (cellulose, starch, glycogen, chitin, the iodine test); **nucleic acids** (nucleoside vs nucleotide, purines vs pyrimidines, DNA vs RNA); and **enzymes** (active site, activation energy, the catalytic cycle, the 6 classes, factors affecting activity, competitive inhibition, and the 3 types of cofactors). Begin each fact with its topic and end it with a full stop.