Complex sentence: Strictly speaking, EC numbers do not specify enzymes, but enzyme-catalyzed reactions.Different enzymes (for instance from different organisms) catalyze the same reaction, then they receive the same EC number.
Split into simpler sentences: Strictly speaking, EC numbers do not specify enzymes, but enzyme-catalyzed reactions. If different enzymes (for instance from different organisms) catalyze the same reaction, then they receive the same EC number.