The conserved residues in the hemoglobins and myoglobins include the proximal and distal to the heme iron.

Histidine

Explanation:

A heme group is a prosthetic group composed of a protoporphyrin ring and an iron (Fe) atom that binds to oxygen (O2) in hemoglobin and myoglobin. Hemoglobin (Hb) is a protein in red blood cells (erythrocytes) that carries oxygen from the lungs to the tissues, which is composed of four polypeptide chains, each with a heme group. Moreover, myoglobin is a monomeric protein located mainly in the muscles, which contains one heme group, being thus similar to hemoglobin monomers. The proximal and distal are evolutionarily conserved amino acid (histidine) residues whose names (proximal and distal) refer to their position with the iron (Fe) atom in the heme group. The proximal histidine connects the heme group with the proteins (either in monomeric myoglobin or in one subunit of Hb), whereas distal histidine contributes to stabilize these proteins while carrying O2.

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