Pathways of DPD and AI-2 formation. The schematic integrates pathways described for production of 4,5-dihydroxy-2,3-pentanedione (DPD) from D-ribulose-5-phosphate  and S-ribosylhomocysteine (SRH)  with the subsequent formation of AI-2 molecules (yellow box) detected by V. harveyi and S. enterica serovar Typhimurium . Ribulose-5-phosphate is formed enzymatically from other sugar phosphates and its reactive open-chain carbonyl form in aqueous solution facilitates DPD generation. DPD cyclisation leads to several products forming via 2,4-dihydroxy-2-methylhydrofuran-3-one intermediates, including two distinct autoinducer molecules, S-THMF-borate ((2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate; detected by V. harveyi) and R-THMF ((2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran; detected by S. enterica serovar Typhimurium) as well as MHF (4-hydroxy-5-methyl-3(2H)-furanone). Intermediates or side products shown are: S-THMF, (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran; R-DHMF, (2R,4S)-2,4-dihydroxy-2-methyldihydrofuran-3-one; S-DHMF: (2S,4S)-2,4-dihydroxy-2-methyldihydrofuran-3-one; Hcy, homocysteine.