Both myoglobin and hemoglobin are iron-containing heme proteins. Myoglobin is an oxygen-storing protein found in muscle tissue containing a single heme group with Fe\(^{2+}\). Hemoglobin is the oxygen-carrying protein in red blood cells with four heme groups, each containing Fe\(^{2+}\) (oxy) or Fe\(^{3+}\) (met-form). Both contain iron in their porphyrin (heme) rings.