What is the protein sequence taken as input in the Path-A prediction system
Not necessarily, they can be enzymes, but they include a lot more (the whole proteome).

It takes a FASTA format file containing a set of query protein sequences from a single organism (a partial or complete proteome) and identifies those sequences that are likely to participate in any of its supported metabolic pathways
Path-A predicts the pathways supported by arbitrary sets of proteins, using validated prediction techniques based on sequence alignment and machine learning.