Enolase (2-phospho-D-glycerate hydrolase) is an essential glycolytic enzyme that catalyses the interconversion of 2-phosphoglycerate and phosphoenolpyruvate [PMID:1859865, PMID:1840492]. In vertebrates, there are 3 different, tissue-specific isoenzymes, designated alpha, beta and gamma. Alpha is present in most tissues, beta is localised in muscle tissue, and gamma is found only in nervous tissue. The functional enzyme exists as a dimer of any 2 isoforms. In immature organs and in adult liver, it is usually an alpha homodimer, in adult skeletal muscle, a beta homodimer, and in adult neurons, a gamma homodimer. In developing muscle, it is usually an alpha/beta heterodimer, and in the developing nervous system, an alpha/gamma heterodimer [PMID:3390159]. The tissue specific forms display minor kinetic differences. Tau-crystallin, one of the major lens proteins in some fish, reptiles and birds, has been shown [PMID:3589669] to be evolutionary related to enolase.

Neuron-specific enolase is released in a variety of neurological diseases, such as multiple sclerosis and after seizures or acute stroke. Several tumour cells have also been found positive for neuron-specific enolase. Beta-enolase deficiency is associated with glycogenosis type XIII defect.