The series of molecular signals initiated by an extracellular ligand binding to a receptor on the surface of the target cell, where the receptor possesses catalytic activity or is closely associated with an enzyme such as a protein kinase, and ending with the regulation of a downstream cellular process, e.g. transcription. [ GOC:signaling GOC:mah http://openisbn.com/search.php?q=0815316194&isbn=1/ ]