Organic Chemistry · Biomolecules: Amino Acids, Peptides, and Proteins

Enzymes and Coenzymes

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Enzymes are protein catalysts that accelerate biochemical reactions by factors of a million to a trillion or more without being consumed. Each enzyme recognizes specific substrates, binds them at an , and stabilizes the transition state so the reaction follows a lower-energy path. Many enzymes need a non-protein partner: a (usually a metal ion such as Zn²⁺ or Mg²⁺) or a — a small organic molecule, often vitamin-derived, that carries chemical groups between reactions. This topic covers how enzymes catalyze reactions, how activity is measured (Michaelis–Menten kinetics), how enzymes are classified, and what coenzymes do.

Why this matters

Nearly every reaction in metabolism is catalyzed: without enzymes, glycolysis, the citric acid cycle, and DNA replication would take years instead of milliseconds. Enzymes are also the targets of about half of all drugs — aspirin inhibits cyclooxygenase, statins inhibit HMG-CoA reductase, and HIV protease inhibitors block viral maturation. Measuring enzyme activity is a core lab skill: elevated liver enzymes (ALT, AST) signal tissue damage. And coenzymes explain vitamin deficiencies: less niacin means less NAD⁺, less B₁ means less thiamine pyrophosphate — the vitamin is the raw material for the coenzyme.

The college version

Core Concepts

What an enzyme does: lowering the activation barrier

An enzyme does not change the thermodynamics of a reaction: the equilibrium constant and the overall free-energy change ΔG are the same with or without the catalyst. What the enzyme changes is the path — it binds the substrate(s), positions catalytic groups, and stabilizes the transition state, lowering the activation energy ΔG^‡. The result is a much faster approach to the same equilibrium. In the simplest picture, catalysis proceeds through an enzyme–substrate complex:

$E + S ⇌ ES \to E + P$

The active site: specificity and catalysis

The active site is a three-dimensional pocket lined with side chains that bind the substrate through hydrogen bonds, ionic interactions, and hydrophobic contacts; specificity comes from complementarity between pocket and substrate. The older lock-and-key model pictured a rigid fit; the induced-fit model holds that both enzyme and substrate change shape on binding, tightening the fit and straining bonds near the reaction center. Catalytic side chains act as general acids or bases, as nucleophiles forming covalent intermediates, and by stabilizing developing charges in the transition state (oxyanion holes).

Michaelis–Menten kinetics: measuring enzyme activity

For a simple single-substrate enzyme, the initial rate v follows the Michaelis–Menten equation:

$v = V max [S] K m + [S]$

V_max is the maximum rate, reached when the enzyme is saturated with substrate.
K_m (the Michaelis constant) is the substrate concentration at which v = V_max/2; a smaller K_m means the enzyme binds/processes the substrate more avidly.
At low [S] the rate is first order in substrate, v ≈ (V_max/K_m)[S]; at high [S] it approaches V_max (zero order).

The turnover number k_cat = V_max/[E]_t reports how many substrate molecules one enzyme molecule converts per unit time.

How enzymes are classified

Enzymes are named for the reaction they catalyze, usually with the suffix -ase, in six classes: oxidoreductases (redox), transferases (group transfer), hydrolases (cleavage with water), lyases (cleavage without water), isomerases (rearrangement), and ligases (ATP-driven bond formation). The class tells you the reaction type: a kinase is a transferase moving a phosphoryl group from ATP to a substrate.

Cofactors and coenzymes: the non-protein partners

Cofactors are metal ions needed for activity: Zn²⁺ in carbonic anhydrase, Mg²⁺ in kinases, Fe²⁺/Fe³⁺ in cytochromes.
Coenzymes are organic molecules that shuttle chemical groups. Many are derived from vitamins:

Coenzyme	Carries	Vitamin source
NAD⁺/NADH	hydride (2 e⁻ + H⁺)	niacin (B₃)
FAD/FADH₂	electrons/hydrogens	riboflavin (B₂)
CoA (CoA–SH)	acyl groups	pantothenic acid (B₅)
Thiamine pyrophosphate	activated aldehydes	thiamine (B₁)
Pyridoxal phosphate	amino groups	pyridoxine (B₆)
Tetrahydrofolate	one-carbon units	folate (B₉)
Biotin	CO₂	biotin (B₇)

A coenzyme that binds and releases each turnover is a cosubstrate (NAD⁺); one that stays bound is a prosthetic group (FAD in succinate dehydrogenase). The enzyme without its partner is an inactive apoenzyme; the complete active form is the holoenzyme.

Regulation: keeping catalysis under control

Enzyme activity is regulated by concentration, covalent modification (phosphorylation), allosteric effectors that bind away from the active site, and feedback inhibition, where a pathway's end product inhibits its first committed enzyme. Competitive inhibitors resemble the substrate and are overcome by raising [S]; noncompetitive inhibitors bind elsewhere and are not. Some enzymes are made as inactive zymogens (trypsinogen, pepsinogen) and activated by cleavage, protecting tissues from premature digestion.

Common Confusions

Do not confuse	With	Difference
Catalyst changes rate	Catalyst changes equilibrium	Enzymes speed approach to equilibrium; ΔG and K_eq are unchanged
K_m	Affinity	K_m is a proxy for affinity (smaller = tighter), but it mixes rate constants, so it is not a true binding constant
Competitive inhibition	Noncompetitive inhibition	Competitive is overcome by raising [S]; noncompetitive is not
Cofactor	Coenzyme	Cofactor = metal ion; coenzyme = organic (usually vitamin-derived) molecule
Cosubstrate	Prosthetic group	Cosubstrate binds and leaves each turnover (NAD⁺); prosthetic group stays bound (FAD)
Enzyme class	Enzyme name	Class = reaction type (transferase, hydrolase); name often adds substrate, e.g., hexokinase

Eli explains

The same idea, in plain words

Explain it like I’m 10

An enzyme is like a factory worker whose station only one kind of piece fits. The worker grabs the piece, snaps it into the right shape, and lets it go — over and over, without ever getting used up. Some workers need a helper tool (a coenzyme) that carries parts between stations; if the factory runs out of the material those tools are made from, the whole line slows down.

Worked example

Example 1: Using the Michaelis–Menten equation

An enzyme has K_m = 2.0 mM and V_max = 40 μmol/min.

(a) At [S] = 2.0 mM (equal to K_m):

$v = V max [S] K m + [S] = 40 \times 2.0 2.0 + 2.0 = 80 4.0 = 20 μmol/min$

Half of V_max, as K_m predicts. (b) At [S] = 6.0 mM:

$v = 40 \times 6.0 2.0 + 6.0 = 240 8.0 = 30 μmol/min$

The rate rises toward V_max but is not there yet — partial saturation.

Example 2: Turnover number with dimensional analysis

The same enzyme is assayed at [E]_t = 1.0 μmol. The turnover number is

$k cat = V max [E] t = 40 μmol/min 1.0 μmol = 40 min -1 = 0.67 s -1$

The μmol units cancel, leaving a frequency: about 0.67 substrate molecules per enzyme per second — modest; carbonic anhydrase, one of the fastest enzymes, reaches ~10⁶ s⁻¹.

Example 3: Why a vitamin deficiency looks like an enzyme deficiency

A diet lacking niacin limits NAD⁺. Because NAD⁺ is a cosubstrate for hundreds of dehydrogenases, all of those reactions slow — the "one vitamin, many enzymes" pattern. That is why pellagra (niacin deficiency) has widespread metabolic and neurological symptoms.

Key takeaways

Enzymes lower ΔG^‡ (activation energy) but never change ΔG or K_eq of a reaction.
Catalysis goes through an ES complex: E + S ⇌ ES → E + P.
K_m = [S] at half Vmax; small K_m = high affinity. k_cat = V_max/[E]_t.
Six enzyme classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Coenzymes are vitamin-derived shuttles: NAD⁺ (niacin), FAD (riboflavin), CoA (pantothenate), TPP (thiamine), PLP (B₆).
Apoenzyme + cofactor/coenzyme = holoenzyme; NAD⁺ is a cosubstrate, FAD can be a prosthetic group.
Competitive inhibitors are overcome by high [S]; noncompetitive inhibitors are not.
Zymogens (trypsinogen) are inactive precursors activated by cleavage.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Does an enzyme change the equilibrium constant of a reaction? Explain.
Show answer
No — enzymes lower the activation energy and speed up the reaction, but ΔG and K_eq are unchanged.
Define K_m in words, and state what a small K_m implies.
Show answer
K_m is the substrate concentration at which v = V_max/2; a small K_m implies high affinity (low [S] needed to half-saturate).
A dehydrogenase needs NAD⁺. Which vitamin is the source, and what does NAD⁺ carry?
Show answer
Niacin (B₃); NAD⁺ carries a hydride (2 electrons + H⁺) in oxidations.
Why is a competitive inhibitor less effective when substrate concentration is high?
Show answer
Substrate and inhibitor compete for the same active site; high [S] outcompetes the inhibitor, restoring most of the rate.
An enzyme with V_max = 60 μmol/min and [E]_t = 2.0 μmol: what is k_cat?
Show answer
k_cat = 60/2.0 = 30 min^-1 = 0.50 s^-1.
What is the difference between an apoenzyme and a holoenzyme?
Show answer
An apoenzyme lacks its cofactor/coenzyme and is inactive; the holoenzyme includes it and is the active form.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Active site: The pocket where the substrate binds and reacts
Induced fit: Enzyme and substrate change shape on binding
Michaelis constant (Km): [S] at half of V_max
Turnover number (kcat): Substrates converted per enzyme per time
Cofactor: Metal ion needed for activity (Zn²⁺, Mg²⁺)
Coenzyme: Vitamin-derived organic group shuttle (NAD⁺, CoA)
Apoenzyme / holoenzyme: Inactive enzyme / enzyme plus its cofactor
Zymogen: Inactive precursor activated by cleavage

Sources & references

openstax.org — Organic Chemistry

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