Catalytic subunit of the tagatose-1,6-bisphosphate aldolase KbaYZ, which catalyzes the reversible aldol condensation of dihydroxyacetone phosphate (DHAP or glycerone-phosphate) with glyceraldehyde 3-phosphate (G3P) to produce tagatose 1,6-bisphosphate (TBP). Requires KbaZ subunit for full activity and stability. D-tagatofuranose 1,6-bisphosphate = D-glyceraldehyde 3-phosphate + dihydroxyacetone phosphate Binds 1 zinc ion per subunit. The catalytic efficiency measured with tagatose-1,6-bisphosphate as substrate is 340-fold higher than that with fructose-1,6-bisphosphate. Carbohydrate metabolism; D-tagatose 6-phosphate degradation; D-glyceraldehyde 3-phosphate and glycerone phosphate from D-tagatose 6-phosphate: step 2/2. Homotetramer. Forms a complex with KbaZ. In contrast to E.coli strains C and EC3132, K-12 strains cannot grow on N-acetylgalactosamine and D-galactosamine, because they carry a deletion and thus lack active PTS systems specific for these compounds. Therefore, KbaYZ in K-12 strains is not involved in the degradation of these compounds. Belongs to the class II fructose-bisphosphate aldolase family. TagBP aldolase KbaY subfamily.