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A hemoprotein is a protein whose molecular structure contains heme as an integral iron–porphyrin cofactor. Heme consists of a protoporphyrin IX macrocycle that coordinates a central iron ion; interactions between the iron, porphyrin, axial ligand residues, and surrounding amino acids establish the chemical and spectroscopic properties of the holoprotein ^[1][2]. The protein framework forms a heme-binding pocket that positions the cofactor, controls its coordination state and redox environment, and regulates access of small ligands or substrates ^[2]. Hemoproteins encompass globins, including hemoglobin, whose globin subunits each contain a heme group coordinated by a proximal histidine residue. In tetrameric hemoglobin, heme-containing subunits form an allosterically coupled assembly in which ligand binding is linked to conformational transitions and cooperative oxygen binding ^[3]. The term therefore covers the heme cofactor, its binding site, and the protein architecture that collectively specify heme-dependent molecular activity ^[1][2][3].