*3.1. Structure*

Humanin is a secretory 24-amino acid peptide with the amino acid sequence *H-Met-Ala-Pro-Arg-Gly-Phe-Ser-Cys-Leu-Leu-Leu-Leu-Thr-Ser-Glu-Ile-Asp-Leu-Pro-Val-Lys-Arg-Arg-Ala-OH (H-MAPRGFSCL LLLTSEIDLPVKRRA-OH)* and a molecular weight of 2687.26 Da (Figure 1). Following its discovery, the structure and biological functions of Humanin have been very well characterized. Using a combination of oligonucleotide synthesis, dimerization experiments, and bioinformatics, Yamagishi et al. conducted a comprehensive investigation that led to the identification of the amino acids essential for Humanin secretion and for its neuroprotective function [22]. This study revealed that Humanin is a signal peptide and the core domain of Humanin is formed by Leu9-Leu10-Leu11 residues with Leu10 being the key player. Leu9-Leu10-Leu11 and Pro19-Val20 are absolutely essential for the extracellular secretion of full-length Humanin. The neuroprotective action of Humanin requires Pro19, Ser14, Thr13, Leu12, Leu9, Cys8, Ser7, and Pro3. Ser7 and Leu9 residues are required for the self-dimerization of Humanin, which is necessary for its neuroprotective function. Providing critical insights into the amino acid requirements of Humanin would be useful in synthesis of peptides tailored to perform a specific biological function and for targeted improvement of particular cellular functions.

**Figure 1.** Humanin and Small Human-Like Peptides (SHLPs) Open Reading Frames (ORFs) in the human mitochondrial DNA.
