Isomerase that catalyzes the interconversion of fructose-6-P and mannose-6-P and has a critical role in the supply of D-mannose derivatives required for many eukaryotic glycosylation reactions. {ECO:0000269|PubMed:8307007, ECO:0000305|PubMed:9525984, ECO:0000305|PubMed:9585601}. This is the function of MPI (mannose phosphate isomerase, ENSG00000178802).