2.A.7.11.1
Possible UDP-galactose:UMP and documented ATP/ADP exchanger (antiporter).  Residues essential or important for activity have been identified (Chan et al. 2010). The apparent affinities of SLC35B1 for ATP/ADP on the internal face were approximately 13 times higher than those on the external side (Schwarzbaum et al. 2022). ATP/ADP exchange by SLC35B1 is not strict; other di- and trinucleotides can act as counter-substrates for ATP, although mononucleotides and nucleotide sugars were not transported. Conserved residues K117 and K120 in TMS 4 and K277 in TMS 9 play critical roles in transport. The fact that SLC35B1 can promote ATP transport in exchange for ADP or UDP suggests a more direct coupling between ATP import requirements and the need for eliminating ADP and UDP, which are generated as side products of reactions taking place in the ER-lumen (Schwarzbaum et al. 2022). SLC35B1 contributes to the uptake of UDPGA into the endoplasmic reticulum for glucuronidation catalyzed by UDP-glucuronosyltransferases (Ondo et al. 2020).