Porcine myoglobin (>95%)

Myoglobin (MB), originally found in mammalian rhabdomyosin and referred to as "intracellular hemoglobin" before being renamed monochromatic myoglobin, plays a critical role in oxygen transport and storage in muscle cells. The monomeric MB protein in mice and humans in its mature state consists of a single polypeptide chain of 153 amino acids and is 17kDa in size. Myoglobin belongs to the globin superfamily along with hemoglobin and performs a variety of functions that are related to the ability of their pseudohemoglobin moieties to bind diatomic gas ligands. The MB folded layer consists of a series of eight alpha helices tightly wrapped around the heme group, the central iron ion of the heme prosthetic group has six coordination sites. Four sites bind to the nitrogen atom of the porphyrin ring, and the fifth site binds to the proximal histidine residue of globin. Gaseous ligands such as oxygen (O₂), nitric oxide (NO·), and carbon monoxide (CO) reversibly bind to the sixth coordination site of ferrous heme iron (Fe²⁺), and the distal histidine (His 64) of globin facilitates gas binding through hydrogen bonding.

MB is normally present as a heme-binding globulin in the cytoplasm of cardiac and skeletal muscle cells. However, MB expression is not limited to myocytes, but also occurs as a protein in the liver, brain and gills of hypoxia-tolerant carp. The expression pattern of MB in humans differs from that of carp and is strongest in the heart. MB RNA levels in human myocardium were 333 and 25 times higher than in healthy colon and mammary tissue, respectively. In carp and zebrafish, MB protein levels in the liver, gills and brain are less than 1% of heart levels. Thus, MB is expressed in different cell lines, but how the protein functions depend on its level of abundance and/or site of expression.

By fine-tuning the intracellular concentration of NO·, MB may contribute to the regulation of various physiological processes, including mitochondrial activity. Under hypoxic conditions, NO· production from nitrite by deoxy-MB inhibits complex IV and mitochondrial respiration. This inhibition is particularly pronounced in hypoxia because NO· competes with O₂ for binding to cytochrome C oxidase. By regulating ROS and NO·, MB may play a key role in various physiological processes in mammalian cells and tissues.

Figure 1. Typical gas binding-related functions of MB in muscle cells
(Source: Aboouf MA, et al. 2023)

Porcine MB