A series of 5-acyl sulfonamides derived from pyridine-2,5-dicarboxylic acid (15) has been prepared and several members of this series have been shown to be more potent, in vitro, as inhibitors of prolyl 4-hydroxylase than 15. Several chain-extended pyridinedicarboxylic acids have also been prepared and shown to be potent inhibitors of prolyl 4-hydroxylase. The structure-activity in both these series is discussed. The results indicate that the 5-carboxylic acid binding site, in the enzyme, can accept a carboxylic acid or an acyl sulfonamide equally well. This indicates a much greater degree of freedom in this distal carboxylic acid binding site than is predicted by the current theoretical model of the active site.