Biomolecules — NEET notes

Chemical Composition of Living

Living organisms are made of the same elements found in non-living matter but in vastly different proportions. C, H, O, and N make up ~96% of living tissue. Small molecules form the metabolite pool; large ones (proteins, nucleic acids, polysaccharides) are macromolecules built by polymerization.

C, H, O, N 96 % of living mass

Elemental Composition

Living organisms contain about 27 elements. Four of them — C, H, O, N — dominate. The rest are present in trace amounts but are equally essential.

If you burned a living organism and analyzed the ash, you'd find about 27 elements. But the distribution is wildly uneven. Carbon, Hydrogen, Oxygen, and Nitrogen together account for ~96% of living tissue mass. The remaining ~4% includes Ca, P, K, S, Na, Cl, Mg, Fe, and trace elements.

• C, H, O, N → ~96% of living mass. Carbon is the backbone of all organic molecules
• Ca, P, K, S, Na, Cl, Mg → major minerals, present in moderate amounts
• Fe, Mn, Cu, Zn, Co, Mo, I, B → trace elements, needed in tiny quantities but absolutely critical
• Oxygen is the most abundant element by mass in living organisms (~65%)
• Carbon is the most abundant element by number of atoms in organic molecules

Ash Analysis vs Acid-Soluble Pool

When we analyze living tissue, we can separate compounds into an acid-soluble fraction (micromolecules in the cytoplasmic pool) and an acid-insoluble fraction (macromolecules that form cellular structures).

To study what living tissue is made of, grind it and treat with trichloroacetic acid (TCA). This separates the homogenate into two fractions: the acid-soluble pool (filtrate containing small molecules) and the acid-insoluble fraction (retentate containing macromolecules).

• Acid-soluble pool: amino acids, sugars, nucleotides, lipids, vitamins, hormones — molecular weight < 18 daltons threshold → typically < 800 daltons
• Acid-insoluble fraction: proteins, nucleic acids, polysaccharides, lipid complexes — molecular weight in thousands to millions of daltons
• Water is the most abundant compound in the acid-soluble pool and in living organisms overall
• On a dry weight basis (after removing water), proteins are the most abundant macromolecules in a cell

Micromolecules vs Macromolecules

Micromolecules (monomers) are the building blocks; macromolecules (polymers) are the assembled structures. Polymerization joins monomers via condensation, and hydrolysis breaks them apart.

A macromolecule is a giant molecule made by linking many small subunits (monomers) through condensation reactions (removing water). Breaking them back down requires hydrolysis (adding water). This monomer → polymer logic applies to proteins, nucleic acids, and polysaccharides — but NOT to lipids.

• Amino acids → Proteins (via peptide bonds)
• Monosaccharides → Polysaccharides (via glycosidic bonds)
• Nucleotides → Nucleic acids (via phosphodiester bonds)
• Lipids are NOT true macromolecules — they are small molecules that aggregate but are NOT polymers of repeating monomeric units
• Molecular weight: micromolecules < 800 Da, macromolecules > 10,000 Da

Bonds in Living Systems

Covalent bonds form the backbone of macromolecules. Non-covalent forces (hydrogen bonds, ionic interactions, van der Waals, hydrophobic interactions) maintain 3D shapes and molecular associations.

The primary structure of every biomolecule is held by covalent bonds — peptide bonds in proteins, glycosidic bonds in polysaccharides, phosphodiester bonds in nucleic acids. But the higher-order folding and interactions rely on weaker non-covalent forces acting in large numbers.

• Peptide bond: –CO–NH– link between amino acids. Formed by condensation (loss of H₂O)
• Glycosidic bond: C–O–C link between sugar units. or orientation matters hugely
• Phosphodiester bond: links nucleotides in DNA/RNA via sugar-phosphate backbone
• Hydrogen bonds: hold DNA double helix together; stabilize protein secondary structure
• Hydrophobic interactions: drive protein folding (nonpolar side chains cluster inward, away from water)

Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones — the primary energy currency of living cells. Classified by size: monosaccharides (glucose, fructose, ribose), disaccharides (sucrose, maltose, lactose), and polysaccharides (starch, glycogen, cellulose, chitin). The reducing vs non-reducing distinction is a NEET favorite.

C_n(H_2O)_n — 'hydrates of carbon'

Monosaccharides

Monosaccharides are single sugar units. Based on the number of carbons: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C). Based on the functional group: aldoses (–CHO) or ketoses (C=O).

A monosaccharide is the simplest carbohydrate — it cannot be broken into smaller sugar units by hydrolysis. The general formula is (CH_2O)_n where n = 3 to 7 . They are sweet, crystalline, and water-soluble.

• Triose (3C): glyceraldehyde, dihydroxyacetone — key intermediates in glycolysis
• Pentose (5C): ribose (in RNA, ATP), deoxyribose (in DNA) — the sugars of nucleic acids
• Hexose (6C): glucose (blood sugar, universal fuel), fructose (fruit sugar, sweetest natural sugar), galactose (part of lactose)
• Aldoses have an aldehyde group (–CHO) at C-1. Example: glucose
• Ketoses have a ketone group (C=O) at C-2. Example: fructose
• Glucose and fructose are isomers — same formula C_6H_ 12 O_6 , different structure

Disaccharides

Disaccharides form when two monosaccharides are linked by a glycosidic bond with the loss of one water molecule. The three biologically important ones: sucrose, maltose, and lactose.

A disaccharide is formed when two monosaccharides join via a glycosidic bond (a condensation reaction that releases one H_2O ). Hydrolysis reverses this — adding water breaks the disaccharide back into its two monosaccharide units.

• Sucrose (table sugar) = glucose + fructose. Found in sugarcane, beet. Non-reducing sugar — both anomeric carbons locked in glycosidic bond
• Maltose (malt sugar) = glucose + glucose. Formed during starch digestion by amylase. Reducing sugar
• Lactose (milk sugar) = glucose + galactose. Found in mammalian milk. Reducing sugar
• Glycosidic bond = C–O–C link between the anomeric carbon of one sugar and an –OH of the other

Polysaccharides

Polysaccharides are polymers of hundreds to thousands of monosaccharide units. Storage polysaccharides (starch, glycogen) provide energy reserves. Structural polysaccharides (cellulose, chitin) provide mechanical support.

Polysaccharides are long polymers containing hundreds to thousands of monosaccharide units linked by glycosidic bonds. Unlike mono- and disaccharides, polysaccharides are tasteless, mostly insoluble in water, and do NOT have a sweet taste. They serve two main roles: energy storage and structural support.

• Starch = plant energy reserve. Two components: amylose (linear, -1,4) and amylopectin (branched, -1,4 + -1,6). Gives blue with iodine
• Glycogen = animal energy reserve (liver + muscles). More branched than amylopectin. Called 'animal starch'
• Cellulose = structural polymer in plant cell walls. Linear -1,4 linked glucose. Most abundant organic compound on Earth. Humans can't digest it (no cellulase enzyme)
• Chitin = structural polymer in arthropod exoskeletons and fungal cell walls. Polymer of N-acetylglucosamine (a modified glucose). Second most abundant organic polymer after cellulose
• Inulin = polymer of fructose. Storage polysaccharide in some plants (dahlia tubers). Used to measure GFR in kidney physiology

Reducing vs Non-Reducing Sugars

A reducing sugar has a free anomeric carbon that can open to its aldehyde/ketone form and reduce oxidizing agents. All monosaccharides are reducing. Among disaccharides, sucrose is the classic non-reducing exception.

A reducing sugar can reduce mild oxidizing agents (Tollens', Benedict's, Fehling's reagents) because it has a free anomeric carbon that can exist in the open-chain aldehyde or ketone form. This open-chain form acts as a reducing agent — it gets oxidized while reducing the reagent.

• All monosaccharides are reducing sugars — glucose, fructose, galactose, ribose, ALL of them
• Maltose and lactose are reducing disaccharides — one anomeric carbon remains free
• Sucrose is non-reducing — both anomeric carbons are locked in the glycosidic bond
• Polysaccharides are essentially non-reducing (technically one free end exists but negligible among thousands of units)
• Test reagents: Fehling's (blue → red precipitate), Benedict's (blue → green/yellow/red), Tollens' (forms silver mirror)

Quick recall

Fructose is a ketose. Can it still be a reducing sugar?

Yes! In alkaline conditions (used in Fehling's/Tollens' tests), fructose undergoes keto-enol tautomerism and converts to glucose (an aldose). So it CAN reduce the reagent. All monosaccharides — aldoses AND ketoses — are reducing sugars.

Proteins

Proteins are polymers of amino acids linked by peptide bonds. With 20 standard amino acids combining in endless sequences, proteins are the most diverse macromolecules — they serve as enzymes, antibodies, hormones, structural elements, and transport molecules. Their function depends critically on their 3D shape (primary → secondary → tertiary → quaternary structure).

Amino acid: H_2N–CHR–COOH

Amino Acids

Amino acids have a central carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R-group (side chain). The R-group determines the identity and properties of each amino acid.

An amino acid has a central -carbon bonded to four groups: an amino group ( -NH_2 ), a carboxyl group ( -COOH ), a hydrogen atom, and a variable R-group (side chain). There are 20 standard amino acids in proteins, each with a unique R-group.

• All 20 amino acids share the same backbone — only the R-group (side chain) differs
• Glycine is the simplest (R = H) and the only amino acid that is NOT chiral (not optically active)
• Amino acids are amphoteric — they can act as both acid (donate H⁺ from –COOH) and base (accept H⁺ on –NH₂)
• At physiological pH (~7.4), amino acids exist as zwitterions: ^+H_3N–CHR–COO^- (both groups ionized)
• Based on R-group: non-polar (Ala, Val, Leu, Ile), polar uncharged (Ser, Thr, Cys), positively charged (Lys, Arg, His), negatively charged (Asp, Glu)

Peptide Bond

A peptide bond forms between the –COOH of one amino acid and the –NH₂ of the next, releasing one water molecule. This is a condensation (dehydration synthesis) reaction.

A peptide bond is formed when the carboxyl group ( -COOH ) of one amino acid reacts with the amino group ( -NH_2 ) of the next, releasing one molecule of water. The resulting -CO–NH- linkage is planar and rigid due to partial double-bond character.

-CO–NH- (peptide bond — formed by loss of H_2O)

• Dipeptide = 2 amino acids joined by 1 peptide bond
• Tripeptide = 3 amino acids, 2 peptide bonds
• Polypeptide = many amino acids (typically 10+), many peptide bonds
• A polypeptide has an N-terminus (free -NH_2 ) and a C-terminus (free -COOH )
• For n amino acids, there are n-1 peptide bonds and n-1 water molecules released

Quick recall

How many peptide bonds in a polypeptide of 50 amino acids?

49 peptide bonds. Always n - 1 where n is the number of amino acid residues.

Protein Structure

Proteins have four levels of structural organization: primary (amino acid sequence), secondary (local folding into α-helix or β-sheet), tertiary (overall 3D shape), and quaternary (multi-subunit assembly). Each level builds on the previous one.

The function of a protein depends entirely on its 3D shape, which is determined by four hierarchical levels of structure. Disrupt the shape (denaturation) and the protein loses function — even though the primary sequence is intact.

• Primary structure: the linear sequence of amino acids. Held by peptide bonds. Determines all higher levels
• Secondary structure: local folding into -helix (coiled) or -pleated sheet (zigzag). Stabilized by hydrogen bonds between backbone –CO and –NH groups
• Tertiary structure: the overall 3D shape of a single polypeptide. Stabilized by R-group interactions — disulfide bonds, hydrophobic interactions, ionic bonds, H-bonds
• Quaternary structure: arrangement of 2+ polypeptide subunits. Example: hemoglobin has 4 subunits ( 2 + 2 ). Not all proteins have this level
• Denaturation: loss of secondary/tertiary/quaternary structure by heat, pH change, or chemicals. Primary structure remains intact

Simple vs Conjugated Proteins

Simple proteins yield only amino acids on hydrolysis. Conjugated proteins have a non-protein component (prosthetic group) essential for function. Classification also by shape: fibrous vs globular.

Simple proteins are made entirely of amino acids — hydrolysis gives only amino acids. Conjugated proteins have a protein part (apoprotein) plus a non-protein prosthetic group that is essential for biological activity.

• Nucleoproteins: protein + nucleic acid. Example: chromosomes (histones + DNA)
• Glycoproteins: protein + carbohydrate. Example: mucin (in saliva), blood group antigens
• Lipoproteins: protein + lipid. Example: HDL, LDL (cholesterol transport in blood)
• Chromoproteins: protein + colored prosthetic group. Example: hemoglobin (protein + heme with Fe²⁺)
• Metalloproteins: protein + metal ion. Example: ferritin (Fe storage), cytochromes

Lipids

Lipids are a heterogeneous group of water-insoluble, organic-solvent-soluble compounds. They include fats/oils (energy storage), phospholipids (cell membranes), steroids (hormones), and waxes (waterproofing). Unlike proteins and polysaccharides, lipids are NOT true polymers — they are NOT macromolecules by NCERT's definition.

Fat = Glycerol + 3 Fatty Acids ester bonds Triglyceride + 3H_2O

Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group at one end. Saturated fatty acids have no C=C double bonds (straight chains, solid at room temp). Unsaturated fatty acids have one or more C=C double bonds (kinked chains, liquid at room temp).

A fatty acid is a long-chain carboxylic acid, typically with 12–24 carbon atoms. The hydrocarbon chain is hydrophobic (water-repelling), while the –COOH head is hydrophilic (water-attracting). This amphipathic nature is key to membrane structure.

• Saturated fatty acids: no C=C double bonds. Straight chains pack tightly → solid at room temperature (fats). Example: palmitic acid (C16), stearic acid (C18)
• Unsaturated fatty acids: one or more C=C double bonds. Kinks prevent tight packing → liquid at room temperature (oils). Example: oleic acid (C18, 1 double bond), linoleic acid (C18, 2 double bonds)
• MUFA = monounsaturated (1 double bond). PUFA = polyunsaturated (2+ double bonds)
• Most naturally occurring fatty acids have an even number of carbon atoms (biosynthesis adds 2C at a time)
• Essential fatty acids (body can't synthesize): linoleic acid and linolenic acid — must come from diet

Fats & Oils

Fats and oils are esters of glycerol with three fatty acids (triglycerides). Fats are solid (saturated) and oils are liquid (unsaturated) at room temperature. They store more energy per gram than carbohydrates.

A triglyceride (triacylglycerol) forms when glycerol (a 3-carbon alcohol) reacts with 3 fatty acid molecules via ester bonds (esterification). Each bond releases one water molecule. Triglycerides are the body's most concentrated energy reserve — they yield 9 kcal/g vs 4 kcal/g for carbohydrates or proteins.

• Simple lipids: triglycerides with all 3 fatty acids identical
• Mixed lipids: triglycerides with 2 or 3 different fatty acids (more common in nature)
• Fats = triglycerides with predominantly saturated fatty acids → solid at room temperature
• Oils = triglycerides with predominantly unsaturated fatty acids → liquid at room temperature
• Fats are hydrophobic — stored in adipose tissue as energy reserves and insulation
• Hydrogenation: adding H₂ to unsaturated oils converts them to solid fats (vanaspati ghee). Produces harmful trans fats

Phospholipids & Steroids

Phospholipids are the structural basis of all biological membranes (bilayer). Steroids (like cholesterol) have a characteristic four-ring structure and serve as hormones and membrane components.

Phospholipids are like triglycerides but with one fatty acid replaced by a phosphate group (often linked to a nitrogen base). This gives them a hydrophilic head (phosphate + base) and two hydrophobic tails (fatty acid chains) — making them amphipathic. In water, they spontaneously form a bilayer — the basis of all cell membranes.

• Phospholipid structure: glycerol + 2 fatty acids + 1 phosphate group (± a head group like choline, ethanolamine, serine)
• Lecithin (phosphatidylcholine) is the most abundant membrane phospholipid
• Cell membranes = phospholipid bilayer — hydrophilic heads face water (outside and inside cell), hydrophobic tails face each other (interior of membrane)
• Steroids: 4 fused carbon rings (3 cyclohexane + 1 cyclopentane). NO fatty acid chains
• Cholesterol: most common animal steroid. Component of cell membranes + precursor for steroid hormones (estrogen, testosterone, cortisol), vitamin D, and bile salts
• Waxes: esters of long-chain fatty acids with long-chain alcohols. Waterproofing function (cuticle of leaves, earwax, beeswax)

Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides — the molecules of heredity and gene expression. DNA stores genetic information as a double helix; RNA reads and executes that information. Understanding nucleotide structure, purine vs pyrimidine bases, and the 5 key DNA-RNA differences is essential for NEET.

Nucleotide = Nitrogenous Base + Pentose Sugar + Phosphate Group

Nucleotide Structure

A nucleotide has three components: a nitrogenous base (purine or pyrimidine), a pentose sugar (ribose or deoxyribose), and a phosphate group. Nucleotides link via phosphodiester bonds to form nucleic acid chains.

A nucleotide is the monomer of nucleic acids. It has three parts: (1) a nitrogenous base — either a purine (2 rings) or pyrimidine (1 ring), (2) a pentose sugar — ribose (in RNA) or deoxyribose (in DNA), and (3) a phosphate group ( -PO_4^ 3- ). A nucleotide without the phosphate is called a nucleoside.

Nucleoside = Base + Sugar | Nucleotide = Nucleoside + Phosphate

• Purines (2 rings): Adenine (A) and Guanine (G) — present in BOTH DNA and RNA
• Pyrimidines (1 ring): Cytosine (C) — in both DNA and RNA. Thymine (T) — ONLY in DNA. Uracil (U) — ONLY in RNA
• Base + sugar = nucleoside. Example: adenine + ribose = adenosine
• Nucleoside + phosphate = nucleotide. Example: adenosine + phosphate = AMP (adenosine monophosphate)
• ATP (adenosine triphosphate) is a nucleotide with 3 phosphate groups — the universal energy currency of cells

DNA vs RNA

DNA is double-stranded, contains deoxyribose and thymine, and stores genetic information. RNA is usually single-stranded, contains ribose and uracil, and participates in gene expression. These 5 differences are tested almost every year.

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are both polynucleotides, but they differ in sugar, base composition, strand number, location, and function. These differences are among the most frequently tested facts in NEET Biology.

• DNA double helix: two antiparallel strands running 5'→3' and 3'→5'
• Base pairing: A=T (2 hydrogen bonds) and G≡C (3 hydrogen bonds)
• More G-C pairs → more hydrogen bonds → higher melting temperature (more stable DNA)
• RNA can form secondary structures (hairpin loops) by intramolecular base pairing
• Some viruses have RNA as genetic material (e.g., HIV, SARS-CoV-2, TMV) — exception to the 'DNA is the genetic material' rule

Types of RNA

Three types of RNA work together to translate DNA's instructions into proteins. mRNA carries the message, tRNA delivers amino acids, and rRNA forms the ribosome (the protein-making machine).

All three types of RNA are synthesized from DNA by transcription, but each has a unique structure and role in protein synthesis (translation).

• mRNA (messenger RNA): carries the genetic code from DNA to ribosomes. It's a linear molecule with codons (3-base sequences) that specify amino acids. Shortest-lived of the three
• tRNA (transfer RNA): small, clover-leaf shaped. Carries specific amino acids to the ribosome. Has an anticodon (3 bases) that pairs with mRNA codon. ~75–90 nucleotides long
• rRNA (ribosomal RNA): structural + catalytic component of ribosomes. Most abundant RNA in the cell (~80% of total RNA). The large ribosomal subunit's rRNA has peptidyl transferase activity (a ribozyme!)
• All 3 types are single-stranded but can fold into complex 3D shapes
• Additional types: snRNA (splicing), miRNA/siRNA (gene regulation) — less important for NEET

Quick recall

Which RNA is most abundant in a cell?

rRNA — makes up about 80% of total cellular RNA. It forms the structural and catalytic core of ribosomes, which a cell needs in large quantities for protein synthesis.

Enzymes

Enzymes are biological catalysts — almost all are proteins (exceptions: ribozymes). They accelerate reactions by lowering activation energy, show extraordinary substrate specificity, and their activity depends on temperature, pH, and substrate concentration. Competitive and non-competitive inhibition are NEET staples. Cofactors and coenzymes are essential non-protein helpers.

E + S leftharpoons ES arrow E + P

Nature of Enzymes

Most enzymes are globular proteins. Their catalytic power comes from the active site — a 3D pocket that binds the substrate. Some RNA molecules (ribozymes) also have catalytic activity.

Enzymes are biological catalysts that speed up biochemical reactions by lowering the activation energy ( E_a ) without being consumed. They can accelerate reactions by factors of 10^6 to 10^ 12 . Almost all enzymes are globular proteins, but a few RNA molecules called ribozymes also catalyze reactions (e.g., peptidyl transferase activity of rRNA).

• Enzymes are highly specific — each enzyme acts on a specific substrate. Urease acts only on urea, maltase only on maltose
• The active site is a small 3D pocket (usually made of just 3–12 amino acid residues) where the substrate binds
• Enzymes lower E_a but do NOT change the equilibrium constant or G of the reaction
• Enzymes are NOT consumed — they are regenerated after each catalytic cycle
• Enzyme names usually end in '-ase' added to the substrate name: lipase (lipid), protease (protein), amylase (amylose/starch)

Mechanism

Two models explain enzyme-substrate interaction. The lock-and-key model (Fischer) says the active site is a rigid, perfect fit. The induced-fit model (Koshland) says the active site changes shape to accommodate the substrate. Induced-fit is the currently accepted model.

How does an enzyme catalyze a reaction? The substrate binds to the active site, forming an enzyme-substrate (ES) complex. The enzyme then facilitates the chemical transformation, releases the product, and is ready for the next cycle.

E + S [] ES E + P

• The ES complex lowers activation energy by: (1) bringing substrates close together, (2) orienting them correctly, (3) straining bonds in the substrate, (4) providing a favorable microenvironment
• After catalysis, the product has a different shape → it no longer fits the active site → released automatically
• The enzyme returns to its original conformation — ready for another round

Factors Affecting Enzyme Activity

Enzyme activity depends on temperature (optimum ~37°C for humans), pH (each enzyme has its own optimum), substrate concentration (saturation kinetics), and enzyme concentration. Going beyond optimum denatures the enzyme.

Enzyme activity (measured as rate of product formation) is influenced by several factors. Each factor has an optimum value where activity is maximum. Deviating from the optimum — especially toward extremes — reduces or destroys activity.

• Temperature: activity increases with temperature (more collisions) up to the optimum (~25–40°C for most human enzymes, ~37°C specifically). Above this, the enzyme denatures (loses 3D shape) and activity plummets
• pH: each enzyme has an optimum pH. Pepsin: pH ~2 (stomach acid). Trypsin: pH ~8 (alkaline intestine). Salivary amylase: pH ~6.8 (slightly acidic mouth). Extreme pH → denaturation
• Substrate concentration [S]: at low [S], rate increases almost linearly. As [S] increases, rate increases but slows down. At very high [S], all active sites are saturated → rate = V_ (plateau)
• Enzyme concentration [E]: if substrate is in excess, rate is directly proportional to [E]
• K_m (Michaelis constant) = substrate concentration at which rate = V_ /2 . Low K_m = high affinity for substrate

Enzyme Inhibition

Competitive inhibitors mimic the substrate and compete for the active site. Non-competitive inhibitors bind elsewhere (allosteric site) and change enzyme shape. Understanding how they affect Km and Vmax is critical for NEET.

An enzyme inhibitor is a molecule that reduces or eliminates enzyme activity. Inhibition can be reversible (inhibitor binds temporarily) or irreversible (inhibitor binds permanently, destroying the enzyme). NEET focuses on two types of reversible inhibition.

• Malonate is the classic competitive inhibitor — it resembles succinate and blocks succinate dehydrogenase in the Krebs cycle
• Cyanide is an irreversible inhibitor — it permanently binds to cytochrome oxidase in the electron transport chain → stops cellular respiration → death
• Allosteric regulation uses non-competitive binding to regulate enzyme activity in metabolic pathways (feedback inhibition)
• Many drugs work as enzyme inhibitors — aspirin inhibits cyclooxygenase (COX), statins inhibit HMG-CoA reductase

Cofactors & Coenzymes

Many enzymes need a non-protein component for activity. Inorganic ions are cofactors, organic molecules are coenzymes, and tightly bound organic groups are prosthetic groups. The protein alone (apoenzyme) is inactive; the complete enzyme (holoenzyme) is active.

Not all enzymes function with just their protein part. Many require non-protein helpers to be catalytically active. These helpers are broadly called cofactors and fall into three categories.

• Cofactors (inorganic): metal ions that sit in the active site. Examples: Zn^ 2+ (carbonic anhydrase, carboxypeptidase), Mg^ 2+ (kinases), Fe^ 2+ (catalase), Cu^ 2+ (cytochrome oxidase)
• Coenzymes (organic, loosely bound): small organic molecules, often derived from vitamins. Examples: NAD⁺ (from niacin/B3), FAD (from riboflavin/B2), Coenzyme A (from pantothenic acid/B5), TPP (from thiamine/B1)
• Prosthetic groups (organic, tightly bound): permanently attached to the enzyme. Example: heme in cytochromes, FAD in succinate dehydrogenase
• Apoenzyme = protein part alone (inactive)
• Holoenzyme = apoenzyme + cofactor/coenzyme (active)

Apoenzyme (inactive) + Cofactor/Coenzyme = Holoenzyme (active)