We have investigated the binding of two porphyrins, meso-tetrakis (p-sulfonatophenyl) porphyrin (TSPP) and protoporphyrin IX (PPIX), to tubulin alpha,beta-heterodimers. TSPP had been shown to directly target microtubules in cells. A comparative study between TSPP and PPIX was carried out because the latter is used in clinical applications and is hydrophobic, in comparison with the water soluble TSPP. The results presented in this manuscript show that both porphyrins bind tubulin with nearly identical stoichiometry but with different affinity (1.76 x 10(5) M-1 for PPIX; 1.1 x 10(6) M-1 for TSPP). The combination of spectroscopic data and molecular simulations suggests that both porphyrins bind as monomers and that their binding site is in proximity of one (or more) Trp residues but do not overlap with the binding site of other well characterized ligands. Molecular simulations also show that the sites that yield the lower energy minima place the porphyrins near the surface of the protein. In the case of TSPP, binding is favored by replacing the ion-dipole interaction of monodispersed TSPP in water with ion-ion interactions provided by two basic residues (His and Lys) at the location of the binding site. Although preliminary, the data show that porphyrin binding could be used to explain some of the effects that photosensitizers may directly produce on protein targets.