⚗️ Enzymes
Learn enzyme structure, classification and action for CUET Agriculture. Lock-and-key, induced fit, cofactors and Michaelis-Menten kinetics.
Definition
An enzyme is an organic substance found in living organisms that catalyzes (speeds up) biochemical reactions without being consumed in the process. Enzymes are nature's biological catalysts — without them, most biochemical reactions would occur too slowly to sustain life.
- Edward Buchner discovered enzymes in yeast extract in a cell-free system — proving that enzymes can work outside living cells (a groundbreaking finding at the time).
- The word "enzyme" was coined by Wilhelm Kuhne (from Greek en = "in" + zyme = "leaven/yeast").
- J.B. Sumner crystallized the enzyme urease for the first time, proving that enzymes can work in crystalline form and are chemical entities.
- Sumner and Northrop established that enzymes are proteins in their chemical nature.
- T. Cech and Altman discovered that RNA can also catalyze reactions — these RNA catalysts are called ribozymes. This discovery challenged the dogma that all enzymes are proteins.
IMPORTANT
Key distinction: All enzymes are generally proteins, but not all proteins are enzymes. Also, ribozymes (catalytic RNA) are a notable exception to the "enzymes are proteins" rule.
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Definition
An enzyme is an organic substance found in living organisms that catalyzes (speeds up) biochemical reactions without being consumed in the process. Enzymes are nature's biological catalysts — without them, most biochemical reactions would occur too slowly to sustain life.
- Edward Buchner discovered enzymes in yeast extract in a cell-free system — proving that enzymes can work outside living cells (a groundbreaking finding at the time).
- The word "enzyme" was coined by Wilhelm Kuhne (from Greek en = "in" + zyme = "leaven/yeast").
- J.B. Sumner crystallized the enzyme urease for the first time, proving that enzymes can work in crystalline form and are chemical entities.
- Sumner and Northrop established that enzymes are proteins in their chemical nature.
- T. Cech and Altman discovered that RNA can also catalyze reactions — these RNA catalysts are called ribozymes. This discovery challenged the dogma that all enzymes are proteins.
IMPORTANT
Key distinction: All enzymes are generally proteins, but not all proteins are enzymes. Also, ribozymes (catalytic RNA) are a notable exception to the "enzymes are proteins" rule.
Structure of Enzymes
An enzyme consists of two parts that work together:
- Apoenzyme (Protein part): The protein portion of the enzyme. It contains the active site — the specific region where the substrate binds and the reaction takes place.
- Cofactor (Non-protein part): The non-protein component that assists in catalysis. Without the cofactor, many enzymes cannot function. Cofactors can be organic or inorganic.
Holoenzyme = Apoenzyme + Cofactor
Think of it like a lock mechanism: the apoenzyme is the lock body, and the cofactor is the spring inside that makes it work.
Types of Cofactors
| Type | Description | Examples |
|---|---|---|
| Prosthetic Group | Organic substance permanently and tightly bound to the apoenzyme (covalent bonds) | Cytochrome, Flavoprotein |
| Coenzyme | Organic substance loosely bound to the apoenzyme (can detach and reattach) | NAD, NADP, FAD, Vitamins |
| Metal Ion Activator | Inorganic substance tightly bound to the apoenzyme | Iron (Fe), Zinc (Zn), Mg²⁺ |
Important Coenzymes
- NAD — Nicotinamide Adenine Dinucleotide (carries electrons in respiration)
- NADP — Nicotinamide Adenine Dinucleotide Phosphate (carries electrons in photosynthesis)
- FAD — Flavin Adenine Dinucleotide (carries electrons in Krebs cycle)
TIP
Many vitamins function as coenzymes or coenzyme precursors. This is why vitamin deficiency can disrupt enzyme function and cause disease — for example, niacin (vitamin B₃) is a precursor of NAD.
Properties of Enzymes
1. Protein Nature
Enzymes are generally proteins. When a protein functions as an enzyme, it always exists in a tertiary (3D) structure — this three-dimensional folding creates the precise shape of the active site needed for substrate binding.
2. Colloidal Nature
Enzyme particles are large molecules, which gives them a large surface area. This allows substrates to attach easily, increasing the rate of reaction. Their colloidal nature also means they cannot pass through semipermeable membranes.
3. Specificity
Enzymes show remarkable specificity — each enzyme typically catalyzes only one type of reaction. There are three types of specificity:
| Type | Description | Example |
|---|---|---|
| Absolute Specificity | A specific enzyme catalyzes only one specific reaction with one specific substrate | Urease acts only on urea hydrolysis — no other substrate |
| Group Specificity | An enzyme acts on a specific group of related substrates | Hexokinase acts on all hexose sugars (6C sugars like glucose, fructose) |
| Bond Specificity | A specific enzyme breaks a specific type of chemical bond | Ribonuclease breaks phosphodiester bonds in RNA |
4. Thermolability (Heat Sensitivity)
Enzymes are sensitive to temperature changes. They can work with both acidic and alkaline substrates, but excessive heat destroys their 3D structure (denaturation), rendering them non-functional.
5. Optimum Activity
Every enzyme works best under specific conditions — optimal temperature, light intensity, pH, and substrate concentration. Deviations from these optimal conditions reduce enzyme efficiency.
Factors Affecting Enzyme Activity
1. Temperature
- Optimum temperature for most enzymes: 20-35°C
- Van't Hoff Rule: Q₁₀ = 2 — for every 10°C rise in temperature, the rate of reaction doubles (within the normal range)
- If temperature is raised excessively, the hydrogen bonds and disulfide (S-S) bonds that maintain the tertiary structure break, converting the 3D structure to a flat 2D structure — this is called denaturation, and the enzyme stops working
- If temperature is reduced significantly, the enzyme becomes inactive (but is not destroyed — it can regain activity when warmed)
2. pH
- Enzymes work most efficiently at pH 5-7.5 (slightly acidic to neutral)
- Each enzyme has its own optimum pH — for example, pepsin works best at pH 2 (highly acidic), while trypsin works best at pH 8 (slightly alkaline)
- Extreme pH changes cause denaturation by disrupting ionic bonds in the enzyme
3. Substrate Concentration
- As substrate concentration increases, the rate of reaction increases — more substrate molecules encounter enzyme active sites
- Eventually, all active sites are occupied (saturation) and the rate plateaus at V_max — adding more substrate beyond this point has no effect
4. Enzyme Concentration
- When enzyme concentration is low, the reaction rate is limited by the number of available enzymes
- When enzyme concentration increases, the reaction rate increases proportionally (assuming excess substrate)
5. Product Concentration
- When product concentration increases, the reaction rate decreases — this is called product inhibition (the reverse reaction is favored, or products block the active site)
Activation Energy
The energy required to initiate a biochemical reaction is called activation energy. It is the "energy barrier" that must be overcome for a reaction to proceed. Enzymes work by lowering the activation energy required, thereby dramatically increasing the rate of reaction.
- Enzymes lower the activation energy but do not change the equilibrium of the reaction — the same products are formed, just much faster
- The enzyme is not consumed in the reaction — it is released unchanged and can catalyze the same reaction again and again
NOTE
An enzyme can catalyze thousands to millions of reactions per second. For example, the enzyme catalase can decompose about 5 million molecules of hydrogen peroxide per second!
Enzyme Inhibition
Inhibitors are molecules that reduce or stop enzyme activity. They compete with or interfere with substrate binding at the active sites of enzymes.
(a) Competitive Inhibition
- The inhibitor and substrate have similar structures — the inhibitor fits into the active site like a "wrong key" in a lock
- Example: Malonic acid is a competitive inhibitor of succinic acid (both have similar shapes, so malonate blocks succinate dehydrogenase)
- To overcome competitive inhibition: increase the substrate concentration — more substrate molecules will outcompete the inhibitor for active sites
(b) Non-competitive Inhibition
- The inhibitor binds to a site other than the active site, causing a permanent change in the enzyme's structural conformation that distorts the active site
- Examples: Pb²⁺, Ag²⁺, Hg²⁺, CN⁻ (heavy metals and cyanide)
- CN⁻ (cyanide) inhibits cytochrome oxidase enzyme in the ETS — this is why cyanide poisoning is so deadly (it blocks cellular respiration)
- To overcome non-competitive inhibition: increase the enzyme concentration (since damaged enzymes cannot be fixed, you need more functional enzyme molecules)
WARNING
Cyanide is one of the most potent enzyme inhibitors known. It binds irreversibly to cytochrome oxidase (Complex IV of ETS), completely shutting down aerobic respiration and causing rapid death.
Mechanism of Enzyme Action
The basic mechanism follows this pattern:
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
The enzyme is never consumed — it emerges unchanged after each reaction cycle.
(1) Lock and Key Model (Emil Fischer)
- Proposed by Emil Fischer
- According to this model, the active site of the enzyme is rigid and pre-shaped — only a specific substrate with the exact complementary shape can bind
- Like a lock that only fits a specific key — the shapes must match perfectly
- This model explains absolute specificity well but cannot explain how some enzymes work on multiple related substrates
Classification of Enzymes
Enzymes are classified into 6 classes by the International Union of Biochemistry (IUB):
| Class | Name | Function | Examples |
|---|---|---|---|
| 1 | Oxidoreductases | Catalyze oxidation-reduction reactions (transfer of electrons) | Cytochrome oxidase, Alcohol dehydrogenase, Reductase |
| 2 | Transferases | Transfer functional groups (amino, phosphate, methyl, etc.) from one substrate to another | Transaminase, Transmethylase, Transphosphorylase, Transketolase |
| 3 | Hydrolases | Break bonds by adding water (hydrolysis reactions) | Esterase, Amylase, Nuclease, Protease, Lipase |
| 4 | Lyases | Break bonds without water or form bonds; remove/add groups by non-hydrolytic means | Aldolase, Decarboxylase |
| 5 | Isomerases | Convert one isomer to another; rearrange molecular structure without adding or removing atoms | Phosphoisomerase, Epimerase |
| 6 | Ligases | Join two molecules using ATP energy (form covalent bonds) | Pyruvate carboxylase, Citrate synthase, DNA ligase |
TIP
Memory aid for the 6 enzyme classes: "Old Trees Have Long Irregular Limbs" — Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.
Michaelis-Menten Kinetics
The Michaelis-Menten equation describes the relationship between reaction rate and substrate concentration — it is the fundamental equation of enzyme kinetics:
V = (V_max × [S]) / (K_m + [S])
Where:
- V = reaction velocity (rate)
- V_max = maximum reaction velocity (when all enzyme active sites are saturated with substrate)
- [S] = substrate concentration
- K_m (Michaelis constant) = the substrate concentration at which reaction rate is half of V_max
Key Points about K_m:
- A low K_m means the enzyme has high affinity for its substrate — it reaches half-maximum speed at a low substrate concentration (very efficient)
- A high K_m means the enzyme has low affinity for its substrate — it needs a lot of substrate to work efficiently
- K_m is a characteristic constant for each enzyme-substrate pair — it does not change with enzyme or substrate concentration
Allosteric Regulation
- Some enzymes have additional regulatory sites called allosteric sites, which are separate from the active site
- When regulatory molecules (activators or inhibitors) bind to allosteric sites, they change the enzyme's conformation, affecting its activity at the active site — like a remote control for the enzyme
- Allosteric activators enhance enzyme activity by making the active site more receptive to substrate
- Allosteric inhibitors reduce enzyme activity by distorting the active site
- This is a key mechanism for feedback inhibition in metabolic pathways — the end product of a pathway inhibits an earlier enzyme, preventing overproduction
Key Facts for Exam Revision
| Fact | Detail |
|---|---|
| First enzyme crystallized | Urease (by J.B. Sumner) |
| Enzymes are made of | Proteins (generally) |
| RNA acting as enzyme | Ribozyme (discovered by T. Cech & Altman) |
| Enzyme word coined by | Wilhelm Kuhne |
| Lock and Key model | Emil Fischer |
| Induced Fit model | Daniel Koshland (more widely accepted) |
| Optimum pH | 5-7.5 |
| Optimum temperature | 20-35°C |
| Q₁₀ value (Van't Hoff rule) | 2 |
| Number of enzyme classes (IUB) | 6 |
| Holoenzyme formula | Apoenzyme + Cofactor |
| Activation energy | Lowered by enzymes, not consumed |
| Competitive inhibitor example | Malonic acid (inhibits succinic acid) |
| Non-competitive inhibitor example | CN⁻ (inhibits cytochrome oxidase) |
Practice Questions (from PDF)
Practice Questions with Answers (Click to expand)
- Enzymes are made of? — Protein (generally)
- Which enzyme is found in yeast? — Zymase
- True statement: All enzymes are generally proteins (but not all proteins are enzymes)
- Enzyme activity is affected by: Substrate concentration, Temperature, pH — all of these
- First scientist to identify enzyme as biological catalyst: Buchner
- Prosthetic group is of which nature? — Organic (carbon-based)
- Activation energy of enzyme reaction: First increases then decreases
- Enzyme that changes its properties is called: Allosteric enzyme
- Naturally occurring plant hormone that is also an enzyme: 2,4-D (note: this is a synthetic auxin, not naturally occurring)
- Isomerase shows: Equal molecular weight representation
- Enzymes are: Protein in nature (not carbohydrate polymer)
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Enzyme definition | Organic biological catalysts that speed up biochemical reactions without being consumed |
| Enzyme discovery | Edward Buchner — in yeast extract (cell-free system) |
| Enzyme word coined by | Wilhelm Kuhne (Greek: en + zyme = "in leaven") |
| First enzyme crystallized | Urease by J.B. Sumner |
| Enzymes are proteins | Established by Sumner and Northrop |
| Ribozymes | Catalytic RNA; discovered by T. Cech and Altman |
| Holoenzyme | Apoenzyme (protein) + Cofactor (non-protein) |
| Prosthetic group | Organic, permanently bound to apoenzyme (e.g., Cytochrome, Flavoprotein) |
| Coenzyme | Organic, loosely bound (e.g., NAD, NADP, FAD, Vitamins) |
| Metal ion activator | Inorganic, tightly bound (e.g., Fe, Zn, Mg²⁺) |
| Enzyme specificity types | Absolute (urease — one substrate only), Group (hexokinase — all hexoses), Bond (ribonuclease — phosphodiester bonds) |
| Optimum temperature | 20-35°C |
| Van't Hoff Rule (Q₁₀) | Q₁₀ = 2 — rate doubles per 10°C rise |
| Denaturation | Excessive heat breaks H-bonds and S-S bonds → 3D → 2D → enzyme non-functional |
| Optimum pH | 5-7.5 (pepsin: pH 2; trypsin: pH 8) |
| Substrate saturation | At V_max, all active sites occupied; more substrate has no effect |
| Activation energy | Energy barrier for reaction; enzymes lower it (not consumed) |
| Competitive inhibition | Inhibitor has similar structure to substrate; binds active site; overcome by increasing substrate |
| Competitive inhibitor example | Malonic acid inhibits succinic acid (succinate dehydrogenase) |
| Non-competitive inhibition | Binds other than active site; permanent conformational change; overcome by increasing enzyme |
| Non-competitive inhibitors | Pb²⁺, Ag²⁺, Hg²⁺, CN⁻; CN⁻ inhibits cytochrome oxidase (lethal) |
| Lock and Key model | By Emil Fischer; active site is rigid and pre-shaped |
| Induced Fit model | By Daniel Koshland; active site is flexible and molds around substrate; more widely accepted |
| 6 enzyme classes (IUB) | Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases |
| Michaelis-Menten equation | V = (V_max × [S]) / (K_m + [S]) |
| K_m meaning | Substrate concentration at half V_max; low K_m = high affinity |
| Allosteric regulation | Regulatory molecules bind at allosteric sites (not active site); key for feedback inhibition |
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