2027 · High School Specialised Subject · Biochemistry

2.4 Water as a Reactant

Hydrolysis, condensation and water in energy metabolism

Water is not only a solvent: it is a reactant in hydrolysis, a product of condensation, a participant in ATP hydrolysis, a partner in hydration reactions, and the electron donor oxidised in photosynthesis.

7 SECTIONS19 FIGURESEnglish edition

01

Water is both solvent and reactant

Most biochemical reactions occur in water, but some consume or produce it
The distinction matters for balancing equations and for free-energy accounting
Water activity is effectively constant in dilute solution, which simplifies kinetics

Figure 2-4-1Water is both solvent and reactant — reference figure

Table 2.4-2Hydrolytic cleavage of the main biopolymer classes

Table 2.4-5Gel results by treatment (record sheet)

02

Water is a reactant in hydrolysis and a product of condensation

Hydrolysis cleaves a bond with water; condensation forms a bond and releases water
Peptide, glycosidic and phosphodiester bonds are made and broken this way
The overall stoichiometry is simple, though the enzymatic mechanisms are not

Figure 2-4-2Water is both solvent and reactant — reference figure

Table 2.4-4Hydrolysis experiment time course (record sheet)

03

Water contributes atoms when biopolymers are degraded

Proteases, glycosidases and nucleases add water across the bond being cut
One water per bond cleaved; the products gain the elements of water
Complete hydrolysis of a polypeptide yields free amino acids

Figure 2-4-3Water is both solvent and reactant — reference figure

04

Water changes the overall free energy of ATP hydrolysis

ATP + H₂O → ADP + Pi has ΔG°′ ≈ −30.5 kJ/mol under standard biochemical conditions
The favourable value arises from resonance stabilisation, hydration and ionisation of products
Actual ΔG depends on concentrations of ATP, ADP, Pi and Mg²⁺

Figure 2-4-4Water is a reactant in hydrolysis and a product of condensation — reference figure

Table 2.4-6Free energy of ATP hydrolysis as [ADP] varies (record sheet)

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04

Water changes the overall free energy of ATP hydrolysis

ATP + H₂O → ADP + Pi has ΔG°′ ≈ −30.5 kJ/mol under standard biochemical conditions
The favourable value arises from resonance stabilisation, hydration and ionisation of products
Actual ΔG depends on concentrations of ATP, ADP, Pi and Mg²⁺

Figure 2-4-4Water is a reactant in hydrolysis and a product of condensation — reference figure

05

Enzymes accelerate reactions in which water participates

Hydrolases position water and the susceptible bond for in-line attack
Metal ions or acid–base residues polarise the water molecule
Rate enhancements of many orders of magnitude are common

Figure 2-4-5Water is a reactant in hydrolysis and a product of condensation — reference figure

Table 2.4-1Four roles of water in biological reactions

Table 2.4-7Common misconceptions about water in metabolism

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05

Enzymes accelerate reactions in which water participates

Hydrolases position water and the susceptible bond for in-line attack
Metal ions or acid–base residues polarise the water molecule
Rate enhancements of many orders of magnitude are common

Figure 2-4-5Water is a reactant in hydrolysis and a product of condensation — reference figure

06

Hydration reactions help carry carbon dioxide in blood

Carbonic anhydrase catalyses CO₂ + H₂O ⇌ H₂CO₃
The carbonic acid dissociates to bicarbonate and a proton
This is the fastest known enzyme and central to acid–base transport

Figure 2-4-6Water is a reactant in hydrolysis and a product of condensation — reference figure

07

Water is oxidised in photosynthesis

Photosystem II extracts electrons from water, releasing O₂ and protons
The electrons replace those lost by the reaction centre chlorophyll
The resulting proton gradient contributes to ATP synthesis

Figure 2-4-7Water contributes atoms when biopolymers are degraded — reference figure

Table 2.4-3Water, electron flow and the main site of reaction

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07

Water is oxidised in photosynthesis

Photosystem II extracts electrons from water, releasing O₂ and protons
The electrons replace those lost by the reaction centre chlorophyll
The resulting proton gradient contributes to ATP synthesis

Figure 2-4-7Water contributes atoms when biopolymers are degraded — reference figure

References

**Table 2.4-1**Four roles of water in biological reactions
Role	Criterion	Biochemical example
Solvent	May not appear in the reaction stoichiometry	Hydration layer around a protein
Reactant	H₂O is consumed in the net reaction	Maltose hydrolysis, CO₂ hydration
Product	H₂O is produced in the net reaction	Overall condensation equation, oxygen reduction
Electron donor	Electrons come from the oxidation of water	Water oxidation at photosystem II

**Table 2.4-2**Hydrolytic cleavage of the main biopolymer classes
Biomolecule	Bond cleaved	Typical catalyst	Feature
Maltose and starch	Glycosidic bond	Maltase, amylase and others	Yields monosaccharides or short chains
Protein	Peptide bond	Pepsin, trypsin and others	Yields amino acids and peptides
Triacylglycerol	Ester bond	Lipase	Positional specificity in digestion
DNA and RNA	Phosphodiester bond	Nucleases	The cleaved end depends on the enzyme

**Table 2.4-3**Water, electron flow and the main site of reaction
Reaction	Role of water	Electron movement	Main site
Water oxidation in photosynthesis	Reactant	H₂O supplies electrons	Photosystem II in the thylakoid membrane
Oxygen reduction in aerobic respiration	Product	O₂ accepts electrons	Complex IV of the inner mitochondrial membrane
CO₂ hydration	Reactant (reversible)	Mainly hydration and proton transfer	Red blood cells and many tissues

**Table 2.4-4**Hydrolysis experiment time course (record sheet)
Time (min)	A: active enzyme	B: heat-treated enzyme	C: no enzyme	D: no substrate
0
3
6
9
12

**Table 2.4-5**Gel results by treatment (record sheet)
Treatment	15 min	30 min	45 min	Interpretation
Fresh juice A
Boiled juice B
Water C

**Table 2.4-6**Free energy of ATP hydrolysis as [ADP] varies (record sheet)
[ADP] (mmol/L)	Q′ (dimensionless)	ΔG (kJ/mol)
0.10
0.20
0.50
1.00
2.00

**Table 2.4-7**Common misconceptions about water in metabolism
Commonly misstated	Scientifically accurate
Water only provides the place where reactions happen	Water can be solvent, reactant, product or electron donor
Breaking any bond releases energy	Bond cleavage costs energy; the overall ΔG includes products and solvation
ATP first hydrolyses and the released heat drives the reaction	Enzymatic coupling such as phosphate transfer is the key step
Protein synthesis is simply the reverse of hydrolysis, removing one water	Biological peptide synthesis uses aminoacyl-tRNAs
The O₂ of photosynthesis comes from CO₂	In oxygenic photosynthesis the O₂ comes from water
Enzymes change the equilibrium by adding water	Enzymes raise the rate but do not change the equilibrium constant

Water is both solvent and reactant — reference figure — Figure 2-4-1Water is both solvent and reactant — reference figure

Water is both solvent and reactant — reference figure — Figure 2-4-2Water is both solvent and reactant — reference figure

Water is both solvent and reactant — reference figure — Figure 2-4-3Water is both solvent and reactant — reference figure

Water is a reactant in hydrolysis and a product of condensation — reference figure — Figure 2-4-4Water is a reactant in hydrolysis and a product of condensation — reference figure

Water is a reactant in hydrolysis and a product of condensation — reference figure — Figure 2-4-4Water is a reactant in hydrolysis and a product of condensation — reference figure

Water is a reactant in hydrolysis and a product of condensation — reference figure — Figure 2-4-5Water is a reactant in hydrolysis and a product of condensation — reference figure

Water is a reactant in hydrolysis and a product of condensation — reference figure — Figure 2-4-5Water is a reactant in hydrolysis and a product of condensation — reference figure

Water is a reactant in hydrolysis and a product of condensation — reference figure — Figure 2-4-6Water is a reactant in hydrolysis and a product of condensation — reference figure

Water contributes atoms when biopolymers are degraded — reference figure — Figure 2-4-7Water contributes atoms when biopolymers are degraded — reference figure

Water contributes atoms when biopolymers are degraded — reference figure — Figure 2-4-7Water contributes atoms when biopolymers are degraded — reference figure