  We investigate the unification constraints in the minimal sypersymmetric
grand unified theories based on SU(5) gauge symmetry. The most general
constraints on the spectrum of minimal supersymmetric SU(5) and flipped SU(5)
are shown. The upper bound on the mass of the colored Higgs mediating proton
decay is discussed in detail in the context of the minimal supersymmetric
SU(5). In the case of the minimal SUSY SU(5) we show that if we stick to the
strongest bound on the colored triplet mass coming from dimension five proton
decay contributions there is no hope to test this model at future nucleon decay
experiments through the dimension six operators. We find a lower bound on the
partial proton decay lifetime for all relevant channels in the context of
flipped SUSY SU(5). We conclude that flipped SUSY SU(5) might be in trouble if
proton decay is found at the next generation of experiments with a lifetime
below 10^{37} years.
