Complex sentence: Fragment A inhibits the synthesis of new proteins in the affected cell by catalyzing ADP-ribosylation of elongation factor EF-2--a protein that is essential to the translation step of protein synthesis.
Split into simpler sentences: Fragment A inhibits the synthesis of new proteins in the affected cell. It does this by catalyzing ADP-ribosylation of elongation factor EF-2--a protein that is essential to the translation step of protein synthesis.