  We provide detailed estimates of the proton lifetime in the context of simple
supersymmetric SU(5) grand unified models with an extra compact spatial
dimension, described by the orbifold S/(Z_2 x Z_2') and by a large
compactification scale Mc. We focus on a class of models where the grand
unified symmetry is broken by the compactification mechanism and where baryon
violation proceeds mainly through gauge vector boson exchange so that the
proton lifetime scales as the fourth power of Mc. We carefully compute Mc from
a next-to-leading analysis of gauge coupling unification and we find that Mc
can only be predicted up to an overall factor one hundred. The simplest model,
where the dominant decay mode is (pi^0 e+) and has no flavour suppression, is
strongly constrained by existing data, but not totally ruled out. We also
analyze models where some of the matter fields are localized in the extra space
and proton decay is flavour suppressed. In models associated to anarchy in the
neutrino sector the preferred decay channel is (K+ {\bar \nu}) and the lifetime
can be within the reach of the next generation of experiments.
