BI2.1-5 | Enzyme — Glossary

Without enzymes, digesting a single meal would take about 50 years.

Enzymes speed up these reactions by factors of millions to billions.

As a doctor, you'll use enzyme knowledge in three ways every single day: 1.

Diagnosis — when you order a blood test for a patient with chest pain, you're looking at cardiac enzymes (troponin, CK-MB).

Diagnosis — when you order a blood test for a patient with chest pain, you're looking at cardiac enzymes (troponin, CK-MB).

When you suspect liver disease, you check liver enzymes (ALT, AST).

Treatment — many drugs you'll prescribe work by inhibiting enzymes.

Treatment — many drugs you'll prescribe work by inhibiting enzymes.

Understanding disease — inborn errors of metabolism (like phenylketonuria, galactosaemia) are caused by missing or defective enzymes.

Understanding disease — inborn errors of metabolism (like phenylketonuria, galactosaemia) are caused by missing or defective enzymes.

From school chemistry, you learned that a catalyst speeds up a chemical reaction without being consumed.

You also learned that reactions need activation energy — a minimum energy barrier that must be overcome.

From school biology, you know that enzymes are proteins (most of them) and that they are specific — each enzyme acts on a particular substrate.

From school biology, you know that enzymes are proteins (most of them) and that they are specific — each enzyme acts on a particular substrate.

An enzyme is a biological catalyst — a molecule (usually a protein) that speeds up a specific chemical reaction without itself being permanently altered.

• Specificity — each enzyme catalyses a particular reaction on a particular substrate.

The enzyme sucrase only breaks down sucrose; it won't touch lactose.

This specificity comes from the shape of the enzyme's active site — the pocket or cleft where the substrate binds.

• Efficiency — enzymes accelerate reactions by factors of 10⁶ to 10¹⁴.

• Regulation — enzyme activity can be turned up or down by the cell.

• Mild conditions — enzymes work at body temperature (~37°C) and physiological pH.

Most enzymes are proteins, but a few are RNA molecules called ribozymes (e.g., the ribosome's peptidyl transferase activity).

Many enzymes need a helper molecule to function.

• Apoenzyme — the protein part of the enzyme alone (inactive without its helper) • Cofactor — the non-protein helper.

• Cofactor — the non-protein helper.

An inorganic ion (e.g., Zn²⁺ in carbonic anhydrase, Mg²⁺ in kinases, Fe²⁺ in catalase) An organic molecule — called a coenzyme (e.g.

An organic molecule — called a coenzyme (e.g., NAD⁺, FAD, coenzyme A, thiamine pyrophosphate) • Holoenzyme = apoenzyme + cofactor (the complete, active enzyme) Apoenzyme + Cofactor → Holoenzyme (active) A coenzyme that is tightly and permanently bound to the enzyme is called.

An organic molecule — called a coenzyme (e.g., NAD⁺, FAD, coenzyme A, thiamine pyrophosphate) • Holoenzyme = apoenzyme + cofactor (the complete, active enzyme) Apoenzyme + Cofactor → Holoenzyme (active) A coenzyme that is tightly and permanently bound to the enzyme is called.

• Holoenzyme = apoenzyme + cofactor (the complete, active enzyme) Apoenzyme + Cofactor → Holoenzyme (active) A coenzyme that is tightly and permanently bound to the enzyme is called a prosthetic group (e.g., FAD in succinate dehydrogenase — it never leaves the enzyme).

Apoenzyme + Cofactor → Holoenzyme (active) A coenzyme that is tightly and permanently bound to the enzyme is called a prosthetic group (e.g., FAD in succinate dehydrogenase — it never leaves the enzyme).

A coenzyme that is tightly and permanently bound to the enzyme is called a prosthetic group (e.g., FAD in succinate dehydrogenase — it never leaves the enzyme).

A coenzyme that binds loosely and acts as a co-substrate (carrying chemical groups between enzymes) is a co-substrate (e.g., NAD⁺ shuttles hydrogen atoms between dehydrogenases).

Why do coenzymes matter? Many coenzymes are derived from vitamins: • NAD⁺ and NADP⁺ ← from niacin (vitamin B₃) • FAD ← from riboflavin (vitamin B₂) • Coenzyme A ← from pantothenic acid (vitamin B₅) • Thiamine pyrophosphate (TPP) ← from thiamine (vitamin B₁) • Pyridoxal phosphate.

Many coenzymes are derived from vitamins: • NAD⁺ and NADP⁺ ← from niacin (vitamin B₃) • FAD ← from riboflavin (vitamin B₂) • Coenzyme A ← from pantothenic acid (vitamin B₅) • Thiamine pyrophosphate (TPP) ← from thiamine (vitamin B₁) • Pyridoxal phosphate (PLP) ← from pyridoxine.

• NAD⁺ and NADP⁺ ← from niacin (vitamin B₃) • FAD ← from riboflavin (vitamin B₂) • Coenzyme A ← from pantothenic acid (vitamin B₅) • Thiamine pyrophosphate (TPP) ← from thiamine (vitamin B₁) • Pyridoxal phosphate (PLP) ← from pyridoxine (vitamin B₆) This is why vitamin.

• FAD ← from riboflavin (vitamin B₂) • Coenzyme A ← from pantothenic acid (vitamin B₅) • Thiamine pyrophosphate (TPP) ← from thiamine (vitamin B₁) • Pyridoxal phosphate (PLP) ← from pyridoxine (vitamin B₆) This is why vitamin deficiencies cause disease — without the vitamin,.

• Coenzyme A ← from pantothenic acid (vitamin B₅) • Thiamine pyrophosphate (TPP) ← from thiamine (vitamin B₁) • Pyridoxal phosphate (PLP) ← from pyridoxine (vitamin B₆) This is why vitamin deficiencies cause disease — without the vitamin, the coenzyme can't be made, the enzyme.

An organic molecule — called a coenzyme (e.g., NAD⁺, FAD, coenzyme A, thiamine pyrophosphate) • Holoenzyme = apoenzyme + cofactor (the complete, active enzyme) Apoenzyme + Cofactor → Holoenzyme (active) A coenzyme that is tightly and permanently bound to the enzyme is called.

• Pyridoxal phosphate (PLP) ← from pyridoxine (vitamin B₆) This is why vitamin deficiencies cause disease — without the vitamin, the coenzyme can't be made, the enzyme can't function, and the metabolic pathway stalls.

This is why vitamin deficiencies cause disease — without the vitamin, the coenzyme can't be made, the enzyme can't function, and the metabolic pathway stalls.

Isoenzymes (isozymes) are different molecular forms of the same enzyme that catalyse the same reaction but differ in their structure, tissue distribution, or kinetic properties.

The classic example is lactate dehydrogenase (LDH).

It exists as 5 isoforms (LDH1–LDH5), each a combination of two subunit types (H for heart, M for muscle): • LDH1 (H₄) — predominates in heart muscle • LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and.

It exists as 5 isoforms (LDH1–LDH5), each a combination of two subunit types (H for heart, M for muscle): • LDH1 (H₄) — predominates in heart muscle • LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and.

It exists as 5 isoforms (LDH1–LDH5), each a combination of two subunit types (H for heart, M for muscle): • LDH1 (H₄) — predominates in heart muscle • LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and.

• LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH.

• LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH.

• LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction.

• LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction.

• LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

• LDH4 (HM₃) — found in kidneys and placenta • LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

It exists as 5 isoforms (LDH1–LDH5), each a combination of two subunit types (H for heart, M for muscle): • LDH1 (H₄) — predominates in heart muscle • LDH2 (H₃M) — found in RBCs and heart • LDH3 (H₂M₂) — found in lungs and lymphoid tissue • LDH4 (HM₃) — found in kidneys and.

• LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

• LDH5 (M₄) — predominates in liver and skeletal muscle Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

Clinical use: If a patient's blood shows elevated LDH1 > LDH2 (the 'flipped' LDH pattern), it suggests myocardial infarction (heart attack).

If LDH5 is elevated, think liver disease.

Another important isoenzyme: creatine kinase (CK): • CK-MM — skeletal muscle (elevated after muscle injury) • CK-MB — heart muscle (elevated in myocardial infarction) • CK-BB — brain (rarely measured in serum)

• CK-MM — skeletal muscle (elevated after muscle injury) • CK-MB — heart muscle (elevated in myocardial infarction) • CK-BB — brain (rarely measured in serum)

• CK-MB — heart muscle (elevated in myocardial infarction) • CK-BB — brain (rarely measured in serum)

• CK-BB — brain (rarely measured in serum)

The International Union of Biochemistry and Molecular Biology (IUBMB) classifies all enzymes into six main classes based on the type of reaction they catalyse.

Each enzyme gets an EC number (Enzyme Commission number) with four digits.

Oxidoreductases | Transfer electrons (oxidation-reduction) | Lactate dehydrogenase, Cytochrome oxidase | "Ox-Red" — they do redox 2.

Transferases | Transfer a functional group from one molecule to another | Transaminases (ALT, AST), Kinases | "Transfer" — move a group 3.

Hydrolases | Break bonds using water (hydrolysis) | Lipase, Amylase, Trypsin | "Hydro" — they use water 4.

Lyases | Break bonds without water or oxidation (or form double bonds) | Aldolase, Decarboxylases | "Lyse" — they cleave 5.

Isomerases | Rearrange atoms within a molecule (isomerisation) | Phosphoglucose isomerase, Racemases | "Iso" — they shuffle 6.

Ligases | Join two molecules using ATP energy | DNA ligase, Glutamine synthetase | "Ligate" — they join (like a surgical ligature) | Mnemonic for the order: "Over The Hill, Liz Is Lazy" (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases).

Mnemonic for the order: "Over The Hill, Liz Is Lazy" (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases).

1. Lock-and-Key Model (Emil Fischer, 1894) The substrate fits into the enzyme's active site like a key into a lock — the shapes are perfectly complementary before binding.

1. Induced Fit Model (Daniel Koshland, 1958) — the currently accepted model The active site is not rigid — it's flexible.

The active site is not rigid — it's flexible.

The induced fit model explains why some enzymes can act on similar substrates (the hand can grip different-sized balls) and why binding actually promotes catalysis (the conformational change contributes to lowering activation energy).

Think of it this way: Lock-and-key = a rigid lock; Induced fit = a smart lock that adjusts its shape when the right key approaches.

1. Temperature • Enzyme activity increases with temperature (molecules move faster, more collisions) up to an optimum temperature (~37°C for human enzymes).

• Enzyme activity increases with temperature (molecules move faster, more collisions) up to an optimum temperature (~37°C for human enzymes).

• Above the optimum, the enzyme denatures — the protein unfolds, the active site loses its shape, and activity drops to zero.

• Clinical link: This is why fever above 41°C is dangerous — enzymes start to denature.

1. pH • Each enzyme has an optimum pH where it works best.

• Each enzyme has an optimum pH where it works best.

1. Substrate concentration • At low substrate concentration, adding more substrate increases the rate (more active sites are occupied).

• At high substrate concentration, all active sites are occupied — the enzyme is saturated — and the rate reaches a maximum (Vmax).

• At high substrate concentration, all active sites are occupied — the enzyme is saturated — and the rate reaches a maximum (Vmax).

1. Enzyme concentration • If substrate is in excess, doubling the enzyme concentration doubles the rate (more active sites available).

1. Inhibitors and activators — covered in Part 3.

When you plot reaction rate (v) against substrate concentration [S], you get a characteristic hyperbolic curve that levels off.

When you plot reaction rate (v) against substrate concentration [S], you get a characteristic hyperbolic curve that levels off.

When you plot reaction rate (v) against substrate concentration [S], you get a characteristic hyperbolic curve that levels off.

This curve is described by the Michaelis-Menten equation, but you need to understand just two numbers: Vmax — the maximum rate of the reaction, achieved when every enzyme molecule has a substrate bound to it (the enzyme is fully saturated).

Vmax — the maximum rate of the reaction, achieved when every enzyme molecule has a substrate bound to it (the enzyme is fully saturated).

Vmax — the maximum rate of the reaction, achieved when every enzyme molecule has a substrate bound to it (the enzyme is fully saturated).

Km (Michaelis constant) — the substrate concentration at which the reaction rate is half of Vmax (½Vmax).

Km (Michaelis constant) — the substrate concentration at which the reaction rate is half of Vmax (½Vmax).

• Low Km = the enzyme reaches half-speed at a low substrate concentration = it binds substrate tightly = high affinity • High Km = the enzyme needs a lot of substrate to reach half-speed = it binds substrate loosely = low affinity Analogy: Imagine a taxi (enzyme) picking up.

• Low Km = the enzyme reaches half-speed at a low substrate concentration = it binds substrate tightly = high affinity • High Km = the enzyme needs a lot of substrate to reach half-speed = it binds substrate loosely = low affinity Analogy: Imagine a taxi (enzyme) picking up.

• High Km = the enzyme needs a lot of substrate to reach half-speed = it binds substrate loosely = low affinity Analogy: Imagine a taxi (enzyme) picking up passengers (substrates) on a street.

• High Km = the enzyme needs a lot of substrate to reach half-speed = it binds substrate loosely = low affinity Analogy: Imagine a taxi (enzyme) picking up passengers (substrates) on a street.

Analogy: Imagine a taxi (enzyme) picking up passengers (substrates) on a street.

What changes Vmax? • Increasing enzyme concentration increases Vmax (more taxis = more capacity) • Vmax is NOT changed by adding more substrate — it's already at maximum What changes Km?

What changes Km? • Km is an intrinsic property of the enzyme-substrate pair — it doesn't change with enzyme concentration • Inhibitors can alter the apparent Km (we'll see how in Part 3)

Lower Km = higher affinity. Enzyme X reaches half its maximum speed at just 0.1 mM substrate, meaning it grabs onto the substrate very efficiently.

An enzyme inhibitor is a molecule that reduces enzyme activity.

Reversible Inhibition (the inhibitor binds and releases): 1.

1. Competitive Inhibition • The inhibitor resembles the substrate and competes for the active site • It's like a wrong key stuck in a lock — the real key (substrate) can't get in • Effect on kinetics: Km increases (you need more substrate to outcompete the inhibitor), but Vmax.

• Effect on kinetics: Km increases (you need more substrate to outcompete the inhibitor), but Vmax is unchanged (with enough substrate, you can overwhelm the inhibitor) • Drug example: Methotrexate — resembles folic acid and competitively inhibits dihydrofolate reductase (used.

• Drug example: Methotrexate — resembles folic acid and competitively inhibits dihydrofolate reductase (used in cancer chemotherapy).

Statins — competitively inhibit HMG-CoA reductase (used to lower cholesterol).

1. Non-competitive Inhibition • The inhibitor binds to a site other than the active site (an allosteric site) • It changes the enzyme's shape so the active site no longer works properly • It's like someone bending your lock — even with the right key, the lock won't turn •.

• The inhibitor binds to a site other than the active site (an allosteric site) • It changes the enzyme's shape so the active site no longer works properly • It's like someone bending your lock — even with the right key, the lock won't turn • Effect on kinetics: Vmax decreases.

• Drug example: Heavy metals (lead, mercury) — bind to sulfhydryl groups on enzymes, distorting their shape.

1. Uncompetitive Inhibition (less common) • The inhibitor binds only to the enzyme-substrate complex (not to the free enzyme) • Effect: Both Km and Vmax decrease Irreversible Inhibition (the inhibitor binds permanently, destroying the enzyme): • Drug example: Aspirin —.

• Effect: Both Km and Vmax decrease Irreversible Inhibition (the inhibitor binds permanently, destroying the enzyme): • Drug example: Aspirin — irreversibly acetylates cyclooxygenase (COX), blocking prostaglandin synthesis.

Irreversible Inhibition (the inhibitor binds permanently, destroying the enzyme): • Drug example: Aspirin — irreversibly acetylates cyclooxygenase (COX), blocking prostaglandin synthesis.

• Drug example: Aspirin — irreversibly acetylates cyclooxygenase (COX), blocking prostaglandin synthesis.

• Organophosphates (nerve agents, some pesticides) — irreversibly inhibit acetylcholinesterase, causing a buildup of acetylcholine at nerve junctions.

Enzymes aren't just drug targets — some are used as drugs themselves: • Streptokinase and tissue plasminogen activator (tPA) — dissolve blood clots by activating plasminogen → plasmin, which digests fibrin.

• Streptokinase and tissue plasminogen activator (tPA) — dissolve blood clots by activating plasminogen → plasmin, which digests fibrin.

Used in acute myocardial infarction and stroke (the 'clot-buster' drugs).

Used in acute myocardial infarction and stroke (the 'clot-buster' drugs).

• L-Asparaginase — breaks down asparagine (an amino acid).

Some cancer cells (especially in acute lymphoblastic leukaemia, ALL) cannot synthesise asparagine and depend on blood supply.

• Pancreatic enzyme supplements (lipase, amylase, protease) — given to patients with chronic pancreatitis or cystic fibrosis who cannot produce enough digestive enzymes.

• Pancreatic enzyme supplements (lipase, amylase, protease) — given to patients with chronic pancreatitis or cystic fibrosis who cannot produce enough digestive enzymes.

• Pancreatic enzyme supplements (lipase, amylase, protease) — given to patients with chronic pancreatitis or cystic fibrosis who cannot produce enough digestive enzymes.

• α-Galactosidase (Beano®) — breaks down oligosaccharides in beans that humans can't digest, reducing flatulence.

Enzymes are also used therapeutically in enzyme replacement therapy for genetic enzyme deficiencies (e.g., imiglucerase for Gaucher disease, agalsidase for Fabry disease).

This is competitive inhibition — the drug competes with the real substrate for the active site.

Km increases (you need more substrate to achieve ½Vmax because the inhibitor is blocking some active sites) but Vmax is unchanged (with enough substrate, you can outcompete the inhibitor and saturate all enzymes).

Km increases (you need more substrate to achieve ½Vmax because the inhibitor is blocking some active sites) but Vmax is unchanged (with enough substrate, you can outcompete the inhibitor and saturate all enzymes).

By measuring these serum enzymes, doctors can identify which organ is damaged and how severely.

Cardiac markers (Myocardial Infarction): • Troponin I and T — the gold standard.

• Troponin I and T — the gold standard.

Useful for detecting re-infarction because it normalises quickly.

Liver function tests (LFTs): • ALT (SGPT) — most specific for liver.

• ALT (SGPT) — most specific for liver.

• AST (SGOT) — found in liver, heart, and muscle.

The AST:ALT ratio (De Ritis ratio) helps differentiate: ratio >2 suggests alcoholic liver disease; ratio <1 suggests viral hepatitis.

The AST:ALT ratio (De Ritis ratio) helps differentiate: ratio >2 suggests alcoholic liver disease; ratio <1 suggests viral hepatitis.

The AST:ALT ratio (De Ritis ratio) helps differentiate: ratio >2 suggests alcoholic liver disease; ratio <1 suggests viral hepatitis.

• ALP (Alkaline Phosphatase) — elevated in obstructive jaundice (bile duct blockage) and bone disease (Paget's disease, rickets, bone metastases).

• ALP (Alkaline Phosphatase) — elevated in obstructive jaundice (bile duct blockage) and bone disease (Paget's disease, rickets, bone metastases).

• ALP (Alkaline Phosphatase) — elevated in obstructive jaundice (bile duct blockage) and bone disease (Paget's disease, rickets, bone metastases).

• GGT (Gamma-glutamyl transferase) — elevated in alcoholic liver disease and biliary obstruction.

• GGT (Gamma-glutamyl transferase) — elevated in alcoholic liver disease and biliary obstruction.

Pancreatic markers: • Amylase — rises within hours of acute pancreatitis, returns to normal in 3–5 days.

• Amylase — rises within hours of acute pancreatitis, returns to normal in 3–5 days.

• Amylase — rises within hours of acute pancreatitis, returns to normal in 3–5 days.

• Lipase — more specific for pancreatitis than amylase, stays elevated longer (7–14 days).

Bone and prostate: • Acid phosphatase — was previously used for prostate cancer (now replaced by PSA, which is not an enzyme).

• Acid phosphatase — was previously used for prostate cancer (now replaced by PSA, which is not an enzyme).

• Acid phosphatase — was previously used for prostate cancer (now replaced by PSA, which is not an enzyme).

• ALP (bone isoform) — elevated in bone diseases with increased osteoblast activity.

Scenario 1: Chest pain + elevated Troponin + elevated CK-MB → Troponin is specific for heart muscle.

→ Very high ALT + AST:ALT ratio <1 = acute viral hepatitis (hepatocyte damage releasing transaminases).

The De Ritis Ratio (AST:ALT) is a simple but powerful diagnostic tool: • Ratio < 1 (ALT > AST): Think viral hepatitis — hepatocytes are damaged, releasing their abundant ALT.

• Ratio < 1 (ALT > AST): Think viral hepatitis — hepatocytes are damaged, releasing their abundant ALT.

• Ratio > 2 (AST >> ALT): Think alcoholic liver disease — alcohol depletes hepatic ALT (via pyridoxal phosphate depletion) while AST from mitochondrial damage remains elevated.

• Ratio 1–2: Non-specific — could be cirrhosis, non-alcoholic fatty liver, or other causes.

Enzyme in your daily life: You ate your last meal several hours ago.

Km in action: Your cells have two enzymes that can phosphorylate glucose: hexokinase (Km = 0.1 mM) and glucokinase (Km = 10 mM).

Km in action: Your cells have two enzymes that can phosphorylate glucose: hexokinase (Km = 0.1 mM) and glucokinase (Km = 10 mM).

Km in action: Your cells have two enzymes that can phosphorylate glucose: hexokinase (Km = 0.1 mM) and glucokinase (Km = 10 mM).

Drug design: If you were designing a drug to block a specific enzyme, would you choose a competitive or non-competitive inhibitor?

Key takeaways — your study checklist: 1.

Enzymes are biological catalysts (mostly proteins) that lower activation energy.

IUBMB classification: 6 classes — Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.

Active site models: Lock-and-key (rigid) vs Induced fit (flexible, currently accepted).

Km and Vmax: Vmax = maximum rate at saturation.

Inhibition: Competitive (active site, ↑Km, Vmax same; e.g., statins).

Clinical enzymes: Troponin + CK-MB = MI.