  We list scalar leptoquarks that mediate proton decay via renormalizable
couplings to the Standard Model fermions. We employ a general basis of baryon
number violating operators to parameterize contributions of each leptoquark
towards proton decay. This then sets the stage for investigation of bounds on
the leptoquark couplings to fermions with respect to the most current Super
Kamiokande results on proton stability. We quantify if, and when, it is
necessary to have leptoquark masses close to a scale of grand unification in
the realistic SU(5) and flipped SU(5) frameworks. The most and the least
conservative lower bounds on the leptoquark masses are then presented. We
furthermore single out a leptoquark without phenomenologically dangerous
tree-level exchanges that might explain discrepancy of the forward-backward
asymmetries in $t \bar t$ production observed at Tevatron, if relatively light.
The same state could also play significant role in explaining muon anomalous
magnetic moment. We identify contributions of this leptoquark to dimension-six
operators, mediated through a box diagram, and tree-level dimension-nine
operators, that would destabilize proton if sizable leptoquark and diquark
couplings were to be simultaneously present.
