M to O

Contents

Macrophages, Magnetic circular dichroism, Magnetic resonance imaging, Magnetic susceptibility, Magnetotactic, MCD, Menkes' disease, Met, Metabolism, Metalloenzyme, Metallo-immunoassay, Metallothionein, Metastable, Methane mono-oxygenase, Methanogens, Michaelis-Menten kinetics, Micronutrient, Mitochondria, Mixed valency, Moco, Model, Molybdenum cofactor , Molybdopterin, Mono-oxygenase, Mössbauer effect, Motif, MRI, Multicopper oxidases, Multienzyme, Multiheme, Mu()-symbol, Mutagenesis, Mutation, Myocrysin, Myoglobin, NAD⁺, NADH, NADP⁺, NADPH, nif, Nitrate reductase, Nitrite reductase, Nitrogenase, Nitrogen fixation, NMR, N-terminal amino-acid residue, Nuclearity, Nuclear magnetic resonance spectroscopy, Nucleation, Nucleic acids, Nucleobases, Nucleosides, Nucleotides, Octahedron, ODMR, OEC, Oligonucleotide, Operon, Optically detected magnetic resonance (ODMR), Ovotransferrin, Oxidase, Oxidation number, Oxidative addition, Oxidoreductase, Oxygen-evolving complex.

Blood cells which are able to ingest a wide variety of particulate materials. They are a type of phagocyte.

Magnetic circular dichroism (MCD)

A measurement of circular dichroism of a material which is induced by a magnetic field applied parallel to the direction of the measuring light beam.Materials which are achiral still exhibit MCD (the Faraday effect), since the magnetic field leads to the lifting of the degeneracy of electronic orbital and spin states and to the mixing of electronic states. MCD is frequently used in combination with absorption and CD studies to effect electronic assignments. The three contributions to the MCD spectrum are the A-term , due to Zeeman splitting of the ground and/or excited degenerate states, the B-term, due to field-induced mixing of states, and the C-term, due to a change in the population of molecules over the Zeeman sublevels of a paramagnetic ground state. The C-term is observed only for molecules with ground-state paramagnetism, and becomes intense at low temperatures; its variation with field and temperature can be analyzed to provid magnetic parameters of the ground state, such as spin, g-factor, and zero-field splitting. Variable-temperature MCD is particularly effective in identifying and assigning electronic transitions originating from paramagnetic chromophores.

Magnetic resonance imaging (MRI)

The visualization of the distribution of nuclear spins (usually water) in a body by using a magnetic field gradient (NMR imaging). A similar technique, but less widely used, is to visualize the distribution of paramagnetic centres(EPR imaging).

Magnetic susceptibility

For paramagnetic materials the magnetic susceptibility may be measured experimentally and used to give information on the molecular magnetic dipole moment, and hence on the electronic structure of the molecules in the material. The paramagnetic contribution to the molar magnetic susceptibility of a material, , is related to the molecular magnetic dipole moment m by the Curie relation : = constant m²/T.

Magnetotactic

Able to orient in a magnetic field.

MCD

See magnetic circular dichroism.

Menkes' disease

A sex-linked inherited disorder, causing defective gastrointestinal absorption of copper and resulting in copper deficiency early in infancy.

Met

A qualifying prefix indicating the oxidized form of the parent protein, e.g. methemoglobin.

Metabolism

The entire physical and chemical processes involved in the maintenance and reproduction of life in which nutrients are broken down to generate energy and to give simpler molecules (catabolism) which by themselves may be used to form more complex molecules (anabolism).

Metalloenzyme

An enzyme that, in the active state, contains one or more metal ions which are essential for its biological function.

Metallo-immunoassay

A technique in which antigen-antibody recognition is used, with attachment of a metal ion or metal complex to the antibody. The specific absorption or (radioactive) emission of the metal is then used as a probe for the location of the recognition sites. See also imaging, radionuclide.

Metallothionein

A small, cysteine-rich protein that binds heavy metal ions, such as zinc, cadmium and copper in the form of clusters.

Metastable

See stable.

Methane mono-oxygenase

A metalloenzyme that converts methane and dioxygen to methanol using NADH as co-substrate. Two types are known, one containing a dinuclear oxo-bridged iron center, the other is a copper protein.

Methanogens

Strictly anaerobic archaea, able to use avariety of substrates (e.g., dihydrogen, formate, methanol, methylamine, carbon monoxide or acetate) as electron donors for the reduction of carbon dioxide to methane.

Michaelis-Menten kinetics

The dependence of the initial rate of conversion of a substrate (S), of the product (P) by an enzyme or other catalyst (E). The simplest mechanism :

E + S ES E + P

yields, under initial steady state conditions, and [P] = 0, the Michaelis-Menten equation,

where is the rate of conversion (Ms^-1), V = k₂[E] is the maximum rate at [S] = for a particular enzyme/catalyst concentration, k₂ is the turnover number (s^-1), and K_m = (k_-1 + k₂)/k₁ is the Michaelis constant under the conditions used. In the case of an impure enzyme or catalyst, [E] is given as gl^-1 instead of M. This equation leads to a hyperbolic dependence of upon [S], which is frequently observed in practice even when [S] is not in great excess over [E].

Micronutrient

A compound essential for cellular growth, being present in concentrations less than about 1 mM in the growth medium (see also trace elements).

Mitochondria

Cytoplasmic organelles of most eukaryotic cells, they are surrounded by a double membrane and produce adenosine 5'-triphosphate as useful energy for the cell by oxidative phosphorylation. The proteins for the ATP-generating electron transport of the respiration chain are located in the inner mitochondrial membrane. Mitochondria contain many enzymes of the citric acid cycle and for fatty acid -oxidation. They also contain DNA which encodes some of their proteins, the remainder being encoded by nuclear DNA.

Mixed valency

This is one of several names, such as `mixed oxidation state' or `non-integral oxidation state' used to describe coordination compounds and clusters in which a metal is present in more than one level of oxidation. The importance in biology is due to the often complete delocalization of the valence electrons over the cluster, allowing efficient electron-transfer processes. See also oxidation number.

Moco

See molybdenum cofactor.

Model

A synthetic coordination entity that closely approaches the properties of a metal ion in a protein and yields useful information concerning biological structure and function. Given the fact that the term is also loosely used to describe various types of molecular structures, constructed, for example, in the computer, the term biomimetic is more appropriate.

Molybdenum cofactor (Moco)

The molybdenum complex of the molybdopterin prosthetic group (ligand). In the molybdenum cofactor the minimal coordination of the Mo atom is thought to be provided by the chelating dithiolenato group of the molybdopterin and either two oxo or one oxo and one sulfido ligands.

Molybdopterin

The prosthetic group associated with the Mo atom of the molybdenum cofactor found in all molybdenum-containing enzymes except nitrogenase. Many of the enzymes catalyze two-electron redox reactions that involve the net exchange of an oxygen atom between substrate and water. The molybdopterin prosthetic group contains a pterin ring bound to a dithiolene functional group on the 6-alkyl side chain. In bacterial enzymes a nucleotide is attached to the phosphate group.

Mono-oxygenase

An enzyme that catalyzes the insertion of one atom of oxygen, derived from O₂, into an aromatic or aliphatic compound. The reaction is coupled to the oxidation of a co-substrate such as NAD(P)H or 2-oxoglutarate.

Mössbauer effect

Resonance absorption of gamma radiation by specific nuclei arranged in a crystal lattice in such a way that the recoil momentum is shared by many atoms. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is ⁵⁷Fe. The parameters derived from the Mössbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron.

Motif

A pattern of amino acids in a protein sequence which has a specific function, e.g. metal binding. See also consensus sequence.

MRI

See magnetic resonance imaging.

Multicopper oxidases

A group of enzymes that oxidize organic substrates and reduce dioxygen to water. These contain a combination of copper ions with different spectral features, called type 1 centers, type 2 centers, and type3 centers, where the type 2 and type 3 sites are clustered together as a trinuclear unit. Well-known examples are : laccase, ascorbate oxidase and ceruloplasmin.

Multienzyme

A protein possessing more than one catalytic function contributed by distinct parts of a polypeptide chain (domains), or by distinct subunits, or both.

Multiheme

Refers to a protein containing two or more heme groups.

Mu()-symbol

See bridging ligand.

Mutagenesis

The introduction of permanent heritable changes, i.e. mutations into the DNA of an organism. In case of site-directed mutagenesis the substitution or modification of a single amino acid at a defined location in a protein is performed by changing one or more base pairs in the DNA using recombinant DNA technology.

Mutation

A heritable change in the nucleotide sequence of genomic DNA (or RNA in RNA viruses), or in the number of genes or chromosomes in a cell, which may occur spontaneously or be brought about by chemical mutagens or by radiation (induced mutation).

Myocrysin

See gold drugs.

Myoglobin

A monomeric dioxygen-binding hemeprotein of muscle tissue, structurally similar to a subunit of hemoglobin.

NAD⁺

Oxidized form of nicotinamide adenine dinucleotide. Note that despite the plus sign in the symbol, the coenzyme is anionic under normal physiological conditions.

NADH

Reduced form of nicotinamide adenine dinucleotide.

NADP⁺

Oxidized form of nicotinamide adenine dinucleotide phosphate. Note that despite the plus sign in the symbol, the coenzyme is anionic under normal physiological conditions.

NADPH

Reduced form of nicotinamide adenine dinucleotide phosphate.

nif

A set of about 20 genes required for the assembly of the nitrogenase enzyme complex.

Nitrate reductase

A metalloenzyme, containing molybdenum, that reduces nitrate to nitrite.

Nitrite reductase

A metalloenzyme that reduces nitrite. Dissimilatory nitrite reductases contain copper and reduce nitrite to nitrogen monoxide. Assimilatory nitrite reductases contain siroheme and iron-sulfur clusters and reduce nitrite to ammonia.

Nitrogenase

An enzyme complex (EC 1.18.6.1) from bacteria that catalyzes the reduction of dinitrogen to ammonia: N₂ + 8e^- +10H⁺ 2 ⁺NH₄ + H₂ with the simultaneous hydrolysis of at least 16 ATP molecules. The electron donor is reduced ferredoxin or flavodoxin. Dihydrogen is always a co-product of the reaction. Ethyne (acetylene) can also be reduced to ethene (ethylene) and in some cases ethane. All nitrogenases are iron-sulfur proteins. Three different types which differ in the type of cofactor present, have been identified: molybdenum-nitrogenase (the most common, which contains the iron-molybdenum cofactor), vanadium-nitrogenase, and iron-only nitrogenase. See also FeMo-cofactor.

Nitrogen fixation

The assimilation of dinitrogen by microbial reduction to ammonia and conversion into organonitrogen compounds such as amino acids. Only a limited number of microorganisms are able to fix nitrogen. See also nitrogenase.

NMR

See nuclear magnetic resonance spectroscopy.

N-terminal amino-acid residue

See amino-acid residue.

Nuclearity

The number of central atoms joined in a single coordination entity by bridging ligands or metal-metal bonds is indicated by dinuclear, trinuclear, tetranuclear, polynuclear, etc.

Nuclear magnetic resonance (NMR) spectroscopy

NMR spectroscopy makes it possible to discriminate nuclei, typically protons, in different chemical environments. The electron distribution gives rise to a chemical shift of the resonance frequency. The chemical shift, , of a nucleus is expressed in parts per million (ppm) by its frequency, _n, relative to a standard, _ref, and defined as = 10⁶ (_n - _ref)/_o, where _o is the operating frequency of the spectrometer. It is an indication of the chemical state of the group containing the nucleus. More information is derived from the spin-spin couplings between nuclei, which give rise to multiplet patterns. Greater detail may be derived from two- or three-dimensional techniques. These use pulses of radiation at different nuclear frequencies, after which the response of the spin system is recorded as a free-induction decay (FID). Multi-dimensional techniques, such as COSY and NOESY, make it possible to deduce the structure of a relatively complex molecule such as a small protein (molecular weight up to 25 000). In proteins containing paramagnetic centres, nuclear hyperfine interactions can give rise to relatively large shifts of resonant frequencies known as contact and pseudo-contact (dipolar) shifts, and considerable increases in the nuclear spin relaxation rates. From this type of measurement, structural information can be obtained about the paramagnetic site.

Nucleation

The process by which nuclei are formed, usually in solution. The term "nucleus" as used here is defined as the smallest solid phase aggregate of atoms, molecules or ions which is formed during a precipitation and which is capable of spontaneous growth.

Nucleic acids

Macromolecules composed of sequences of nucleotides that perform several functions in living cells, e.g. the storage of genetic information and its transfer from one generation to the next (DNA), and the expression of this information in protein synthesis (mRNA, tRNA), and may act as functional components of subcellular units such as ribosomes (rRNA). RNA contains D-ribose, DNA contains 2-deoxy-D-ribose as the sugar component. Currently, synthetic nucleic acids can be made consisting of hundreds of nucleotides. See also oligonucleotide.

Nucleobases

See nucleosides.

Nucleosides

Compounds in which a purine or pyrimidine base is -N-glycosidically bound to C-1 of either 2-deoxy-D-ribose or of D-ribose, but without any phosphate groups. The common nucleosides in biological systems are adenosine, guanosine, cytidine, and uridine (which contain ribose) and deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine (which contain deoxyribose).

Nucleotides

Nucleosides with one or more phosphate groups esterified mainly to the 3'- or the 5'- position of the sugar moiety. Nucleotides found in cells are adenylic acid, guanylic acid, uridylic acid, cytidylic acid, deoxyadenylic acid, deoxyguanylic acid, deoxycytidylic acid and thymidylic acid. See also adenosine 5'-triphosphate, NAD⁺, NADP⁺.

Octahedron

See coordination.

ODMR

See optically detected magnetic resonance.

OEC

See oxygen-evolving complex.

Oligonucleotide

Macromolecules composed of short sequences of nucleotides that are usually synthetically prepared and used e.g. in site-directed mutagenesis.

Operon

A functional unit consisting of a promoter, an operator and a number of structural genes, found mainly in prokaryotes. An example is the operon nif. The structural genes commonly code for several functionally related enzymes, and although they are transcribed as one (polycistronic) mRNA, each has its separate translation initiation site. In the typical operon, the operator region acts as a controlling element in switching on or off the synthesis of mRNA.

Optically detected magnetic resonance (ODMR)

A double resonance technique in which transitions between spin sublevels are detected by optical means. Usually these are sublevels of a triplet, and the transitions are induced by microwaves.

Ovotransferrin

An iron-binding protein from eggs, structurally similar to the transferrins.

Oxidase

An enzyme which catalyzes the oxidation of substrates by O₂.

Oxidation number

The oxidation number of an element in any chemical entity is the number of charges which would remain on a given atom if the pairs of electrons in each bond to that atom were assigned to the more electronegative member of the bond pair. The oxidation (Stock) number of an element is indicated by a roman numeral placed in parentheses immediately following the name (modified if necessary by an appropriate ending) of the element to which it refers. The oxidation number may be positive, negative or zero. Zero, not a roman numeral, is represented by the usual cipher, 0. The positive sign is never used. An oxidation number is always positive unless the minus sign is explicitly used. Note that it cannot be non-integral (see also mixed valency). Non-integral numbers may seem appropriate in some cases where a charge is spread over more than one atom, but such a use is not encouraged. In such ambiguous cases, the charge number, which designates ionic charge, can be used. A charge (Ewens-Bassett) number is a number in parentheses written without a space immediately after the name of an ion, and whose magnitude is the ionic charge. Thus the number may refer to cations or anions, but never to neutral species. The charge is written in arabic numerals and followed by the sign of the charge. In a coordination entity, the oxidation number of the central atom is defined as the charge it would bear if all the ligands were removed along with the electron pairs that were shared with the central atom. Neutral ligands are formally removed in their closed-shell configurations. Where it is not feasible or reasonable to define an oxidation state for each individual member of a group or cluster, it is again recommended that the overall oxidation level of the group be defined by a formal ionic charge, the net charge on the coordination entity.

Oxidative addition

The insertion of a metal of a coordination entity into a covalent bond involving formally an overall two-electron loss on one metal or a one-electron loss on each of two metals.

Oxidoreductase

An enzyme of EC class 1, which catalyzes an oxidation-reduction reaction.

Oxygen-evolving complex (OEC)

The enzyme which catalyzes the formation of O₂ in photosynthesis. Contains a cluster of probably four manganese ions.

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