Source comparison
MT-I vs MT-II — The Structural Divergence
The structural difference between these two peptides functions like the difference between a single-purpose key and a master key. MT-I’s linear shape fits one lock well — the MC1R receptor on skin cells that drives pigmentation. MT-II’s cyclic shape fits multi
This comparison does not assign a generated winner or score.
- The structural difference between these two peptides functions like the difference between a single-purpose key and a master key. MT-I’s linear shape fits one lock well — the MC1R receptor on skin cells that drives pigmentation. MT-II’s cyclic shape fits multiple locks across the body, including receptors in the brain that control appetite and sexual arousal. This is the origin of every downstream divergence: pharmacology, safety, regulation, and clinical utility.
- MT-I preserves the full 13-residue alpha-MSH sequence (Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2). Its linear architecture retains approximately 10-fold selectivity for MC1R over other melanocortin receptors — modest pharmacologically, but sufficient to concentrate effects on cutaneous melanogenesis and photoprotection.⁶ The linear peptide does not meaningfully cross the blood-brain barrier, which accounts for the absence of central nervous system effects in clinical use.⁹
- MT-II condenses the pharmacophore into a 7-residue ring (Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2). The lactam bridge constrains the peptide backbone, increasing proteolytic stability but also eliminating receptor discrimination. MT-II activates MC1R, MC3R, MC4R, and MC5R with sub-nanomolar to low-nanomolar affinity.⁸ Its cyclic structure confers sufficient lipophilicity to cross the blood-brain barrier, enabling direct activation of hypothalamic MC4R — the receptor subtype mediating appetite regulation and sexual arousal.¹⁰
- Neither peptide activates MC2R, the ACTH-specific adrenal receptor. This means neither compound triggers cortisol synthesis — an important safety boundary.⁶⁸