A prosthetic group is the non-amino acid component that is part of the structure of the heteroproteins or conjugated proteins, being tightly linked to the apoprotein. Not to be confused with the cofactor that binds to the enzyme apoenzyme (either a holoprotein or heteroprotein) by non-covalent binding a non-protein (non-

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

) This is a component of a conjugated protein that is required for the protein's biological activity. The prosthetic group may be organic (such as a vitamin, sugar, RNA, phosphate or lipid) or

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

(such as a

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...

ion). Prosthetic groups are bound tightly to proteins and may even be attached through a

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

. They often play an important role in enzyme catalysis. A protein without its prosthetic group is called an apoprotein, while a protein combined with its prosthetic group is called a

holoprotein A holoprotein or conjugated protein is an apoprotein combined with its prosthetic group. Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. Cofact ...

. A non-covalently bound prosthetic group cannot generally be removed from the holoprotein without denaturating the protein. Thus, the term "prosthetic group" is a very general one and its main emphasis is on the tight character of its binding to the apoprotein. It defines a ''structural'' property, in contrast to the term "coenzyme" that defines a ''functional'' property. Prosthetic groups are a subset of

cofactors Cofactor may also refer to: * Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed * A domain parameter in elliptic curve cryptography, defined as the ratio between the order ...

. Loosely bound metal ions and coenzymes are still cofactors, but are generally not called prosthetic groups.Campbell MK and Farrell SO (2009) Biochemistry, 6th edition, Thomson Brooks/Cole, Belmont, California In enzymes, prosthetic groups are involved in the catalytic mechanism and required for activity. Other prosthetic groups have structural properties. This is the case for the sugar and lipid moieties in glycoproteins and

lipoproteins A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, s ...

or RNA in ribosomes. They can be very large, representing the major part of the protein in proteoglycans for instance. The

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consist ...

group in hemoglobin is a prosthetic group. Further examples of organic prosthetic groups are vitamin derivatives: thiamine pyrophosphate, pyridoxal-phosphate and

biotin Biotin (or vitamin B7) is one of the B vitamins. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. The name ''biotin'', bor ...

. Since prosthetic groups are often vitamins or made from vitamins, this is one of the reasons why vitamins are required in the human diet. Inorganic prosthetic groups are usually

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

ions such as iron (in

groups, for example in cytochrome c oxidase and hemoglobin), zinc (for example in

carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) () form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active sit ...

), copper (for example in

complex IV The enzyme cytochrome c oxidase or Complex IV, (was , now reclassified as a translocasEC 7.1.1.9 is a large transmembrane protein complex found in bacteria, archaea, and mitochondria of eukaryotes. It is the last enzyme in the respiratory elec ...

of the respiratory chain) and molybdenum (for example in

nitrate reductase Nitrate reductases are molybdoenzymes that reduce nitrate (NO) to nitrite (NO). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils. Types Euka ...

List of prosthetic groups

The table below contains a list of some of the most common prosthetic groups.

References

External links

Cofactors PowerPoint lecture
{{Enzyme cofactors Cofactors de:Prosthetische Gruppe