Metmyoglobin is the

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

form of the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

-carrying hemeprotein

myoglobin Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglob ...

. Metmyoglobin is the cause of the characteristic brown colouration of meat that occurs as it ages. In living muscle, the concentration of metmyoglobin is vanishingly small, due to the presence of the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

metmyoglobin reductase, which, in the presence of the cofactor NADH and the

coenzyme A cofactor is a non- protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that as ...

cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of ...

b4 converts the Fe³⁺ in the heme prosthetic group of metmyoglobin back to the Fe²⁺ of normal myoglobin. In meat, which is dead muscle, the normal processes of removing metmyoglobin are prevented from effecting this repair, or alternatively the rate of metmyoglobin formation exceeds their capacity, so that there is a net accumulation of metmyoglobin as the meat ages. Metmyoglobin reduction helps limit the oxidation of myoglobin and the oxidation of myoglobin is specific to each species. In other words, metmyoglobin gains electrons in order to limit myoglobin from losing electrons. Metmyoglobin after being oxidized by myoglobin shows the undesirable brown color which can be seen in many types of meat. Metmyoglobin is more susceptible to oxidation when being compared to oxymyoglobin. The metmyoglobin reducing activity varies across species and was studied particularly in beef, porcine, bison, deer, emu, equine, goats and sheep.

Currently there is not a standard technique in measuring the metmyoglobin in all species. But many techniques are used including reflectance spectrophotometry and absorbance spectrophotometry are used.

References

External links

* Hemoproteins {{protein-stub