The effect of polymer conformational transition on thermoreversible gelation with multiple cross-link junctions in natural and synthetic polymers is studied on the basis of the recent theory of associating polymers. The effective number of associative groups carried by each chain in such solutions is not; a fixed number but varies with conformation change induced by the temperature and other environmental conditions. Transition from intra- to intermolecular bonding due to denaturation of some proteins on heating leads to high-temperature gelation, while coil-to-helix transition (or coil-to-globule transition), followed by aggregation of helices (globules), leads to low-temperature gelation. We calculate the sol/gel transition concentration as a function of temperature and find that, in both cases, it is not a monotonic function but takes a minimum value at an intermediate temperature. At low temperatures gelation is prevented due to the lack of active groups or due to the strict restriction in helix sequence selection. Typical phase diagrams in which sol/gel transition coexists with phase separation are derived.