ACHEFISH
Acetylcholine is involved in the transfer of messages across a variety of
synapses in vertebrates and invertebrates. The diverse physiological effects
attributable to this molecule arise from the presence of specific receptors
in the postsynaptic cell membranes. Several classes of receptor are known, 
most of which translate the binding of acetylcholine into the opening of 
channels for the passage of ions, such as sodium and potassium.

Acetylcholinesterase is an enzyme that catalyses the hydrolysis of 
acetylcholine to choline and acetate:

     Acetylcholine + H(2)O -> Choline + Acetate 	     

The enzyme also acts on a variety of acetic esters and catalyses trans-
acetylations. It is found in, or attached to, cellular or basement membranes
of presynaptic cholinergic neurons and postsynaptic cholinoceptive cells.
To prevent continuous firing of nerve impulses, acetylcholinesterase has a
high K(cat) (~14000/s), to ensure that acetylcholine is broken down quickly.

Cholinesterases constitute a family of enzymes that fall into two main
types, depending on their substrate preference: enzymes that preferentially
hydrolyse acetyl esters are termed acetylcholinesterase (AChE) (EC 3.1.1.7); 
and those that prefer other types of ester, such as butyrylcholine are 
termed butyrylcholinesterase (BChE) (EC 3.1.1.8). 

The 3D structure of acetylcholinesterase from the electric ray has been
determined. The fold belongs to the alpha/beta class, with a 3-layer
alpha-beta-alpha sandwich architecture.

ACHEFISH is a 4-element fingerprint that provides a signature for fish
acetylcholinesterases. The fingerprint was derived from an initial alignment
of 4 sequences: the motifs were drawn from short conserved regions within
the central portion of the alignment - motif 1 spans the N-terminus of 
helix 12 and part of the preceding loop; motif 2 includes the N-terminal
end of helix 14 and part of its preceding loop; motif 3 encompasses the 
N-terminus of helix 18 and part of the preceding loop; and motif 4 encodes
most of helix 19. Two iterations on OWL30.2 were required to reach
convergence, at which point a true set comprising 6 sequences was identified.

An update on SPTR37_9f identified a true set of 3 sequences.
