We present for the first time a detailed study of the properties of molecular gas in metal-rich environments such as early-type galaxies (ETGs). We have explored photon-dominated region chemistry for a wide range of physical conditions likely to be appropriate for these sources. We derive fractional abundances of the 20 most chemically reactive species as a function of the metallicity, as a function of the optical depth and for various volume number gas densities, far-ultraviolet (FUV) radiation fields and cosmic ray ionization rates. We also investigate the response of the chemistry to the changes in α-element enhancement as seen in ETGs. We find that the fractional abundances of CS, H2S, H2CS, H2O, H3O+, HCO+ and H2CN seem invariant to an increase of metallicity whereas C+, CO, C2H, CN, HCN, HNC and OCS appear to be the species most sensitive to this change. The most sensitive species to the change in the fractional abundance of α-elements are C+, C, CN, HCN, HNC, SO, SO2, H2O and CS. Finally, we provide line brightness ratios for the most abundant species, especially in the range observable with Atacama Large Millimeter Array (ALMA). Discussion of favourable line ratios to be used for the estimation of supersolar metallicities and α-elements are also provided.