Catalyzes both 1,3- and 1,5-keto-enol tautomerization of the diacid 2-hydroxymuconate (2-hydroxy-2,4-hexadienedioate) to produce 2-oxo-4-hexenedioate. This reaction is highly stereoselective and produces a mixture of stereoisomers, where the (3S)-isomer of 2-oxo-4-hexenedioate predominates. Also catalyzes the tautomerization of 2-hydroxymuconate to 2-oxo-3-hexenedioate, however this reaction is slower and occurs after the tautomerization of 2-hydroxymuconate to 2-oxo-4-hexenedioate. Using 2-hydroxy-2,4-pentadienoate, phenylenolpyruvate, (p-hydroxyphenyl)-enolpyruvate and 2-hydroxy-2,4-heptadiene-1,7-dioate, YwhB is a highly efficient 1,3-keto-enol tautomerase, but clearly not a 1,5-keto-enol tautomerase. Tautomerization of the two monoacids 2-hydroxy-2,4-pentadienoate and phenylenolpyruvate produces a mixture of stereoisomers, where the (3R)-isomers predominate. (2Z,4E)-2-hydroxyhexa-2,4-dienedioate = (3E)-2-oxohex-3-enedioate kcat is 3 sec(-1) for 2-hydroxy-2,4-heptadiene-1,7-dioate, 4.1 sec(-1) for (p-hydroxyphenyl)-enolpyruvate, 10 sec(-1) for 2-hydroxymuconate, 26 sec(-1) for 2-hydroxymuconate, 112 sec(-1) for 2-hydroxy-2,4-pentadienoate, 125 sec(-1) for phenylenolpyruvate. Homohexamer. The mono- and diacids apparently bind in different orientations in the active site of YwhB, but the highly stereoselective nature of the YwhB reaction using a diacid suggests that the biological substrate for YwhB may be a diacid. Belongs to the 4-oxalocrotonate tautomerase family.