Structural Biochemistry, Enzyme, Prosthetic Group

Prosthetic group

A prosthetic group is a tightly bound, specific non-polypeptide unit required for the biological function of some proteins. The prosthetic group may be organic (such as a vitamin, sugar, or lipid) or inorganic (such as a metal ion), but is not composed of amino acids. Prosthetic groups are bound tightly to proteins and may even be attached through a covalent bond, as opposed to cosubstrates, which are loosely bound. In enzymes, prosthetic groups are often involved in the active site, playing an important role in the functions of enzymes.

Vitamins are another common prosthetic group. This is one of the reasons why vitamins are required in the human diet. Inorganic prosthetic groups, however, are usually transition metal ions such as iron. The Heme group in hemoglobin is a prosthetic group located in the porphyrin, which is a tetramer of cyclic carbon groups. It contains an organic component called a protoporphyrin made up of four pyrrole rings and an iron atom in the ferrous state (Fe2+). The red color of blood and muscles is attributed to the Heme groups. The difference between a prosthetic group and a cofactor depends on how tightly or loosely bound to the enzyme they are. If tightly connected, the cofactor is referred to as a prosthetic group.

Heme group

Heme b (Fe-protoporphyrin IX) (Image: Yikrazuul, Public domain)

A heme group is a prosthetic group consisting of a protoporphyrin ring and a central iron (Fe) atom. A protoporphyrin ring is made up of four pyrrole rings linked by methine bridges. Four methyl, two vinyl, and two propionate side chains are attached.

Heme of hemoglobin protein is a prosthetic group of heterocyclic ring of porphyrin of an iron atom; the biological function of the group is for delivering oxygen to body tissues, such that bonding of ligand of gas molecules to the iron atom of the protein group changes the structure of the protein by amino acid group of histidine residue around the heme molecule.

The iron lies in the center is an organic component called protoporphyrin, which is bound to four pyrrole nitrogen atom linked by a methine bridge that forms a tetrapyrrole ring. The iron can either be in the ferrous (Fe2+) or the ferric (Fe3+) oxidation state. However, it is only able to bind to oxygen when in the ferrous state. The iron can form two additional bonds in fifth and in sixth coordination which on both side of heme plane. The fifth coordination sites is linked to a distal histidine while the sixth coordination site can, not always, bind to oxygen. Upon binding, the Heme group will actually shrink in size and descend further into the plane of the porphyrin ring. Along with it, the distal histidine will follow, and this histidine is attached to the alpha-Beta interface thus resulting in local to complete conformational change.

A conjugate protein combined with its specific prosthetic group is termed a holoprotein, while a protein in its absence is called an apoprotein. Prosthetic groups have varying functions, such as oxidizing-reducing reactions (redox), methylation reactions, oxygenation reactions, and so forth.

The heme group gives muscles and blood their distinctive red color.

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