Assignment 2 Leptin Structure

Leptin is synthesized as a 167 amino acid polypeptide chain but secreted into blood following cleavage of 21 residue signal peptide at the N terminus (Coccia et al., 2010). Circulating leptin is now a 146 residue protein. The protein structure consists of four alpha helices connected by two long crossover links and one short loop.  Cysteine residues at the 94 and 146 position form a disulphide bridge and kink in the D-helix which is essential for proper folding and receptor binding (Coccia et al., 2010).

Figure 1. 3D structure of leptin. The alpha helices are letters A-D, Long loops are A-B and C-D and short loop is between B-C. Disulphide bridge in yellow. (Coccia et al., 2010).

Figure 2: Leptin protein sequence alignment of species,  Macaca mulatta (AAC50730.1)  Homo sapiens (AAH69452.1), Mus musculus (ADM72802.1),Xenopus laevis (NP_001089183), and Ictalurus punctatus (AAZ66785.1). Prepared using ClustalW software program. * = identical amino acid, .  = weak similarity, and : = strong similarity.

Table 1.  Alignment scores for leptin protein sequences for Rhesus Monkey, Human, Mouse, African claw frog, and Catfish

SeqA	Length (aa)	SeqB	Length(aa)	Score
Catfish	122	Mouse	167	24.0
Catfish	122	Monkey	167	22.0
Catfish	122	Frog	169	28.0
Catfish	122	Human	168	22.0
Mouse	167	Monkey	167	80.0
Mouse	167	Frog	169	34.0
Mouse	167	Human	167	82.0
Monkey	167	Frog	169	34.0
Monkey	167	Human	167	89.0
Frog	169	Human	167	35.0

The scores indicate how similar the protein sequences are. As you can see the monkey and human sequence is most similar (89%). The catfish and frog sequences are different from the mammalian species. Even the fish and amphibian are quite different from each other as well.

Although the primary protein sequence is not very similar between humans (mammals) and catfish (fish) the tertiary structure is highly conserved among mammalian and non mammalian species. The similarity can be observed in the figure below.

Figure 3. Comparison of 3D structure of human and atlantic salmon leptin.  (Ronnestad et al., 2010).

Coccia, E., Ceccarelli, M., and Graziano, G. (2010). 9. On the 3D structure of leptins from different organisms and of the leptin-leptin receptor complex. Leptin in Non-mammalian Vertebrates. 173-192.

Ronnestad, I., Nilsen, T., Murashita, K., Angotzi, A., Moen, A., Stegansson, S., Kling, P., Bjornsson, B., Kurokawa, T. (2010). Leptin and leptin receptor genes in Atlantic salmon: Cloning, phylogeny, tissue distribution and expression correlated to long-term feeding status. Generay and Comparative Endocrinology. 168(1). 55-70.