Catalyzes the reversible isomerization between hydroxypyruvate and 2-hydroxy-3-oxopropanoate (also termed tartronate semialdehyde). Does not catalyze the isomerization of D-fructose to D-glucose or that of D-xylulose to D-xylose. Also does not catalyze racemization of serine, alanine, glycerate or lactate. 3-hydroxypyruvate = 2-hydroxy-3-oxopropanoate Not stimulated by addition of pyridoxal 5'-phosphate (0.1 mM), FAD, NAD(+), NADP(+) or ATP (1 mM each). EDTA (10 mM) and metal ions (1 mM) such as Ca(2+), Co(2+), Mg(2+), Ni(2+), Zn(2+) do not affect the enzyme activity. Optimum pH is 6.8-7.2. Homodimer. By glyoxylate. Does not affect growth on glycolate or glyoxylate. Belongs to the hyi family.