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# Hydrolysis

Hydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. This is distinct from a hydration reaction, in which water molecules are added to a substance, but no cleavage occurs.

## Types

In a hydrolysis reaction that involves breaking an ester link, one hydrolysis product contains a hydroxyl functional group, while the other contains a carboxylic acid functional group.

The fragment of the parent molecule that was originally a carboxylate gains a hydrogen ion from the additional water molecule. The fragment that was originally an alkyl group collects the remaining hydroxyl group from the water molecule.

Thus the ester is effectively reversed to its component alcohol and carboxylic acid.

Hydrolysing peptide link of amino acids

In other hydrolysis reactions such as hydrolysing the peptide links of amino acids only one of the products, the carboxylic acid product, has a hydroxide group derived from the water. The amine product gains the remaining hydrogen ion.

Hydrolysis can be considered as the opposite of condensation, in which two fragments are joined for each water molecule produced. As hydrolysis is a reversible reaction, condensation and hydrolysis can take place at the same time the position of equilibrium determining the amount of each product.

### Irreversibility of hydrolysis under physiological conditions

Under physiological conditions (i.e. in dilute aqueous solution), a hydrolytic cleavage reaction, where the concentration of a metabolic precursor is low (on the order of 10-3 to 10-6 molar), is essentially thermodynamically irreversible. To give an example:

A + H2O → X + Y
$K_d = \frac{\left[X\right] \left[Y\right]} {\left[H_2O\right] \left[A\right]}$

Assuming that x is the final concentration of products, and that C is the initial concentration of A, and W = [H2O] = 55.5 molar, then x can be calculated with the equation:

$\frac{x \times x}{W\left(C - x\right)} = K_d$

let Kd×W = k

then $x = \frac {-k + \sqrt {k^2 + 4kC} } {2}$

For a value of C = 0.001 molar, and k = 1 molar, x/C > 0.999. Less than 0.1% of the original reactant would be present once the reaction is complete.

This theme of physiological irreversibility of hydrolysis is used consistently in metabolic pathways, since many biological processes are driven by the cleavage of anhydrous pyrophosphate bonds.