Catalyzes the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P). It appears that the physiological function of G-API may be to synthesize the regulatory molecule A5P, which in turn participates in the induction of the gut operon through an unknown mechanism. It is also able of sustaining the biosynthetic pathway of 3-deoxy-D-manno-octulosonate (KDO), a unique 8-carbon sugar component of lipopolysaccharides (LPSs). D-arabinose 5-phosphate = D-ribulose 5-phosphate Optimum pH is 8.25. Homotetramer. Belongs to the SIS family. GutQ/KpsF subfamily. Truncated N-terminus. Truncated N-terminus.