<prompt> Which one of the following transmitter is not a protein glycopeptidepolypeptide?SerotoninDynorphinSubstance-PMet-enkephalin  <response>: A Serotonin Serotonin 5-hydroxytryptamine, or 5-HT is a monoamine neurotransmitter synthesized in serotonergic neurons in CNS enterochromaffin cells in the gastrointestinal tract of animals including humans. Dynorphin is a class of peptides produced by many different populations of neurons, which has some opiate-like activity. It is thus classed as an endogenous opioid peptide. Dynorphin functions primarily as a kappa opioid receptor agonist, meaning that it acts mainly at kappa opioid receptors. Substance Pis a neuropeptide: a short-chain polypeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family. Enkephalin is a pentapeptide ending with either leucine leu or methionine met. Both are products of the proenkephalin gene. Met-enkephalin is Tyr-Gly-Gly-Phe-Met. Leu-enkephalin has Tyr-Gly-Gly-Phe-Leu. Enkephalins play many roles in regulating pain. 