Also known as AANAT, arylalkylamine-N-acetyltransferase is a catalyst that "plays a major role in the regulation of the melatonin circadian rhythm in vertebrates" (Ferry, et al., abstract).  The reaction which AANAT catalyzes is that which occurs between acetyl-CoA and serotonin resulting in N-acetylserotonin (Ferry, et al., abstract). N-acetylserotonin is the precurser to melatonin.
For a realated review click here.

The actual structure of this enzyme can be seen in the  Rasmol image of sheep (Ovis aries)Serotonin N-Acetyltransferase (MMDB Id: 12412 PDB Id: 1B6B ).
For more information on this image click here.
 

Unfourtunatly, Genbank only provides sequence information for three of the five genome organims (Homo sapiens, Drosophila melanogaster, Mus musculus , C. elegans, and S. cervisiae). The Genbank results for the amino acid  and cDNA sequences of these three organisms plus eight others are available through the links below.
 
 

For a list of AANAT and serotonin related abstracts click here.
For MacDNAsis results click here. (For only the largest ORF for the Human cDNA, click here.)

Ferry, G, et al.  2000. Substrate Specificity and Inhibition Studies of Human Serotonin N-Acetyltransferace. JBC Online.