A lyase that reversibly degrades 2-hydroxyisobutyryl-CoA (2-HIB-CoA) to acetone and formyl-CoA (PubMed:34952003). Probably also cleaves 2-hydroxy-2-methylbutyryl-CoA to butanone and formyl-CoA. Does not act on 2-hydroxy-2-ethylbutyryl-CoA (Probable). A C-terminal lid closes the active site upon substrate binding, and with residues Leu-127 and Ile-492 restricts the size of the active site cavity so it can only use short-chain (C4 and C5) acyl substrates (PubMed:34952003). Part of a pathway that allows cells to grow on 2-methylpropane-1,2-diol or 2-hydroxyisobutyric acid (2-HIBA) as a sole carbon source (PubMed:32351493). 2-hydroxyisobutanoyl-CoA = acetone + formyl-CoA Binds 1 thiamine diphosphate per dimer. Activity is stimulated by thiamine diphosphate. kcat is 1.3 sec(-1). A homotetramer formed by a dimer of dimers; active sites are located in the dimer interface. 29-fold induced by growth on 2-hydroxyisobutyric acid (2-HIBA), but not by growth on acetone (at protein level). The C-terminal lid closes the active site upon substrate binding. This class of enzyme could be engineered to produce biotechnologically interesting condensations of formyl-CoA with aldehydes and ketones. Belongs to the TPP enzyme family.