Enolase (phosphopyruvate hydratase) is a metalloenzyme. It is responsible for the catalysis of the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP), which is the ninth step of glycolysis. Enolase is part of the lyases family, and more specifically belongs to the hydro-lyases. These cleave carbon-oxygen bonds. The reaction that the enzyme causes can be reversed, although this depends on the environmental concentrations of substrates. Enolase can be found in all tissues and organisms that carry out glycolysis or fermentation.
Enolase has three subunits in humans: α, β, and γ. Each of these is encoded by a separate gene that can combine into five isoenzymes (αα, αβ, αγ, ββ, and γγ). Three are more common in human cells, αα or non-neuronal enolase, γγ or neuron-specific enolase and ββ or muscle-specific enolase. These are also known as enolase 1, 2 and 3. They are found in a variety of tissues in the body. For example, enolase 1 is found in the liver, brain, kidney and spleen, but is also present in all normal human cells at various levels. Enolase 2 is found in high levels in neurons and neural tissues. Enolase 3 is mostly found in high levels in muscle tissue.
Enolase is part of the enolase superfamily. It has a compact, globular structure and is a highly conserved enzyme. Its five active-site residues are important for activity. The overall mechanism for converting 2-PG to PEP is possibly an E1cB elimination reaction which involves a carbanion intermediate. When the substrate, 2-phosphoglycerate, binds to α-enolase, its carboxyl group coordinates with two magnesium ion cofactors in the active site. The acidity of the alpha hydrogen is increased while the negative charge on the deprotonated oxygen is stabilized. Conformational changes also occur in the enzyme to aid catalysis.
One of the ways that enolases have been used in medicine is through diagnostics. Concentration levels of enolase have been sampled to try and diagnose some health conditions and determine how severe they are. A higher concentration of enolase in cerebrospinal fluid can indicate low-grade astrocytoma, showing a stronger correlation compared to concentrations of other enzymes. Tumours could grow at a faster rate in patients who have a higher level of CSF enolase. People who have had a recent myocardial infarction or cerebrovascular accident have shown higher levels of enolase too. Levels of CSF neuron-specific enolase, serum NSE, and creatine kinase (type BB) could help with determining the prognosis of cardiac arrest patients. NSE levels could also be linked to cerebrovascular accidents. The rare condition Hashimoto's encephalopathy has also been associated with autoantibodies to alpha-enolase.
Small-molecule inhibitors of enolase have been synthesized as chemical probes of the catalytic mechanism of the enzyme. There have been studies looking into how they might be used as treatments for cancer and infectious diseases. Phosphonoacetohydroxamate has been trialed as an anti-trypanosome drug and as an anti-cancer agent for glioblastoma. Methylglyoxal is another inhibitor of human enolase, while ENOblock was once thought to be but was found to interfere with the enolase in vitro enzymatic assay.