A new spin-1/2 quasi-one-dimensional antiferromagnet KCu-MoO4(OH) is prepared by the hydrothermal method. The crystal structures of KCuMoO4(OH) and the already-known Na-analogue NaCuMoO4(OH) are iotypic, comprising chains of Cu2+-known in edge-sharing CuO4(OH)(2) octahedra Despite the structural similarity, their magnetic properties are quite different: because of the different arrangements of d(x2-y2) orbitals carrying spills. For NaCuMoO4(OH), d(x2-y2) orbitals are linked by superexchange couplings via two bridging oxide ions, which gives a ferromagnetic nearest-neighbor interaction J(1) of -51 K and an antiferromagnetic next-nearest-neighbor interaction J(2) of 315 k in the chain. In contrast, a staggered d(x2-y2) orbital arrangement in KCuMoO4(OH) results in superexchange couplings via only one bridging oxide ion, which makes J(1), antiferromagnetic as as 238 K and 12 negligible: This comparison between the two isotypic compounds,demonstrates an Important role of orbital arrangements in determining the magnetic properties of cuprates.