Second Assignment: The Structure of ACTH, and its precurser Proopiomelanocortin!

Adrenocorticotropic hormone (ACTH) is a 39 amino acid long peptide hormone that is actually the product of post-translational processing. It's precursor is a prohormone known as POMC or Proopiomelanocortin. Proopiomelanocortin is 241 amino acids long after the dignal peptide has been removed, and can be cleaved into a variety of different hormone messengers (Tanaka, 2003). ACTH is cleaved from POMC in the rough endoplasmic reticulum and the golgi apparatus by enzymes known as prohormone convertases. PC1 cleaves the ACTH from an already cleaved ABI product during post-translational processing (Tanaka, 2003). Since ACTH is cleaved from another peptide, it is not directly coded for by a gene. Therefore, in this post we will be looking at the amino acid sequence of POMC as coded for by the DNA of several species of frogs and toads.

 

Figure 1:  This image illustrates the action of PC1 cleaving ACTH from the  prohormone POMC. Taken from (Tanaka, 2003). Along with the 2D structure of ACTH. Taken from http://pubchem.ncbi.nlm.nih.gov/image/imagefly.cgi?cid=16132265&width=300&height=300.

 ACTH is only 39 amino acids long, and as such, has a secondary structure of a single alpha helix (Brunton et al. 2013). Amino acids 1-10 are responsible for binding and signalling the MC2R receptor in the Zonae Fasciculata of the adrenal medulla (Gao & Wong 1998). This area contains many serine acids which help to create hydrogen bonds with the receptor. Once bound, the receptor changes conformation and activates a G-protein linked secondary messenger pathway eventually leading to the production and release of glucocorticoids. 

Figure 2: This figure shows the alignment for the Marshfrog (Accession number M62770.1), African Clawed Frog ( Accession number: NM_001087369.1) and the oriental fire bellied toad (Accession number AY692246.1), as well as the approximate position of ACTH in the POMC prohormone.

KEY: " * " (Asterisk) indicates the position of a single fully conserved residue, " : " (Colon) indicates conservation of groups with strongly similar properties. " . " (Period) indicates conservation of groups with weakly similar properties.


As we can see from the above alignment, the section of POMC corresponding to ACTH is highly conserved, showing that the sequence is functionally very important. We can see that specifically, the serine residues at the beginning of the peptide are conserved, due to their function in binding of the MC2R receptor.

 Figure 3: This figure shows the Percent Identity Matrix for the POMC alignments of the Marsh Frog, African-clawed Frog, and the Oriental fire-bellied toad.

  From the Percent Identity Matrix, we can see that ~ 70-75% of amino acid sequence is conserved between each genus of toad and frog. All of these species are fairly analagous, and even though they are extremely geographically seperated, their hormonal structure is still similar.

References:
 
1) Shigeyasu Tanaka. (2003). Comparative Aspects of Intracellular Proteolytic processing of Peptide Hormone Precursers: Studies of Proopiomelanocortin Processing. Zoological Science. 20. P. 1183-1198
2) http://pubchem.ncbi.nlm.nih.gov/image/imagefly.cgi?cid=16132265&width=300&height=300
3) Brunton, Lawrence L., Lazo, John S., & Keith L. Parker. (2013). Goodman and Gilman's The Pharmological Basis for Therpeutics. 12th Ed. McGraw and Hill. New York. 
4)Gao, Xinfeng., & Tuck C. Wong. (1998). Studied of the Binding and Structure of Adrenocorticotropin Peptides in Membrane Mimics by NMR Spectroscopy and Pulsed-Field Gradient Diffusion. BioPhysical Journal. 74(11). P. 1871-1888