Photothermal Conversion and Temperature Elevation by a Minimal Heme Protein

Heat plays a crucial role in biological systems, functioning both as an energy source and as a signaling stimulus. However, precise intracellular heating tools remain limited. Small, biocompatible molecular heaters capable of localized photothermal conversion are therefore essential for advancing cellular thermobiology. Here, we investigated the photothermal conversion ability of cytochrome b₅ from Hadesarchaea archaeon YNP_N21 (HaCytb5), a minimal heme protein. Temperature changes in aqueous solution under 532 nm laser irradiation were monitored using Raman spectroscopy. HaCytb5 exhibits exceptional thermal stability attributed in part to a covalent heme-polypeptide linkage. Upon irradiation, it induced a temperature increase up to 7.0 K, sufficient to activate thermosensitive proteins. Importantly, His-tagging did not significantly alter the thermal stability or heating ability of HaCytb5, indicating that organelle-localization signals can be incorporated without sacrificing its molecular heater function. Furthermore, thermal diffusion modeling reasonably aligns with experimental data, confirming efficient photothermal heating. These findings highlight HaCytb5 as a potent, minimally perturbative thermogenic agent, offering a promising nanoscale tool for localized intracellular heating and potential investigation of temperature heterogeneity within living cells.