Triose phosphate isomerase.

Enol formation can be either acid or base catalyzed. In the first step of acid-catalyzed enolization, the carbonyl oxygen acts as a Brønsted base and abstracts a proton from hydronium in the solvent environment. The arrival of a proton increases the already strong pull oxygen exerts on the bonding electrons it shares with carbon in the carbonyl group, so that oxygen pulls the π bonding pair it shares with carbon completely into its own orbit where they become a nonbonding pair while carbon simultaneously compensates by forming a new π bond with the α carbon, which loses a proton in the process.

Alternatively to acid catalysis, enolization might also be promoted by basic conditions. A proton bonded to the α carbon of an aldehyde or ketone is substantially more acidic (K_a around 10^-16 to 10^-20) than is typical with C-H bonds. This is because, subsequent to deprotonation, an enolate anion is formed in which delocalization of negative charge occurs by resonance with the carbonyl oxygen (decrease in internal energy, enthalpy, and free energy compared to typical carbanions, explaining the elevated K_a). Base catalyzed enol formation begins with abstraction of a proton by hydroxide ion from the α carbon of the aldehyde or ketone, leading to the enolate anion. Proton transfer from water to the enolate ion oxygen forms the neutral enol.

Keto-enol tautomerism is extremely, hugely, enormously important in biochemistry, which is why AAMC is extremely fond of including elements of this chemistry on the MCAT. Enolate anions in biochemistry are a primary source of nucleophilic carbon for the formation of carbon-carbon bonds. For example, in the pyruvate carboxylase mechanism, the enolate of pyruvate attacks carboxy-biotin. Variations of adol addition mechanism, also involving a nucleophilic enolate, occurs in aldolase, citrate synthase, HMG-CoA synthase, and transaldolase. In triose phosphate isomerase, keto-enol tautomerism is the vehicle to move between glyceraldehye-3-phosphate and dihydroxyacetone phosphate. In that mechanism, you see simultaneous acid and base catalysis, a testament to the flexibility of enzymes.