Catalyzes the conversion of D-ribulose 5-phosphate to formate and 3,4-dihydroxy-2-butanone 4-phosphate (Probable). Has also an unrelated function in expression of mitochondrial respiration (PubMed:12595523). D-ribulose 5-phosphate = (2S)-2-hydroxy-3-oxobutyl phosphate + formate + H(+) Binds 2 divalent metal cations per subunit. Magnesium or manganese. Cofactor biosynthesis; riboflavin biosynthesis; 2-hydroxy-3-oxobutyl phosphate from D-ribulose 5-phosphate: step 1/1. Homodimer. S-glutathionylation of Cys-56 is reversible and dependent on the cytoplasmic isoform of glutaredoxin-2. Present with 5460 molecules/cell in log phase SD medium. Belongs to the DHBP synthase family.