Oxygenase - Oxygenase

Japanese: オキシゲナーゼ - おきしげなーぜ(英語表記)oxygenase
Oxygenase - Oxygenase

A type of oxidoreductase (a general term for enzymes that catalyze oxidation-reduction reactions), a general term for enzymes that catalyze the reaction in which oxygen atoms (O) from molecular oxygen (O 2 ) are directly incorporated into organic substrates. Also called oxygenase. A group of enzymes that belong to "EC1. Oxidoreductases" according to the classification of enzymes established by the Enzyme Commission of the International Union of Biochemistry (now the International Union of Biochemistry and Molecular Biology) (see the Enzyme Number section). Oxidoreductases are enzymes that catalyze oxidation-reduction reactions in living organisms, and are called oxidases, oxygenases, dehydrogenases, reductases, etc. depending on the type of reaction.

An oxidation-reduction reaction can be said to be a reaction in which electrons are transferred from one substance to another. The substance that has lost electrons is said to be oxidized, and the substance that has received electrons is said to be reduced. In general, when a substance
(a) When oxygen is received
(b) When hydrogen is stolen
(c) When an electron is lost, the substance is said to be oxidized, but the reactions (a) and (b) can also be explained as changes in (c). When an organic compound undergoes reactions (a) or (b), the oxidation number of carbon (a number that indicates the transfer of electrons. If an electron is lost, it is expressed as +1, and if an electron is gained, it is expressed as -1) does not change, so it is difficult to see that it is (c). In this case, the electronegativity (the tendency of each bonded atom to attract electrons) of the carbon and the atom bonded to the carbon is compared. The electron pair of the covalent bond between atoms with different electronegativities is assigned to the atom with the greater electronegativity. The electronegativity of oxygen, carbon, and hydrogen is oxygen > carbon > hydrogen. In other words, when oxygen is received (a), the carbon bonded to this oxygen has had its electrons lost to the oxygen. Also, when hydrogen is lost (b), the carbon bonded to that hydrogen has had its electrons lost, since it had been attracting the hydrogen's electrons up until that point. Living organisms use enzymes as catalysts to oxidize and reduce various organic and inorganic substances in order to synthesize necessary substances, metabolize unnecessary or harmful substances, and obtain the energy necessary for vital activities.

Among oxidoreductases, enzymes that oxidize substrates (substances that undergo enzymatic reactions) using oxygen as an electron acceptor are collectively called oxidases. Oxygen is mainly converted into water or hydrogen peroxide, or is incorporated into organic compounds. Enzymes that catalyze the latter are often distinguished as oxygenases. In other words, oxygenase is a general term for enzymes that catalyze the reaction in which the oxygen atom (O) of molecular oxygen (O 2 ) is directly incorporated into organic substrates.

Oxygenases are widely distributed in humans, animals, plants, and microorganisms, and play important roles in the metabolism of amino acids, lipids, hormones, and drugs and toxins. They are classified into the following two groups:

(1) A group of enzymes (monooxygenases) that catalyze the reaction of adding one oxygen atom to molecular oxygen ( O2 ): S + O2 + AH2 → SO + A + H2O
(2) A group of enzymes (dioxygenases) that catalyze the reaction of adding two oxygen atoms: S + O 2 → SO 2
Here, S represents the substrate and AH2 represents the hydrogen donor. Monooxygenase includes, for example, a group of enzymes called cytochrome P-450. This enzyme is present in the granular fraction (microsomes and mitochondria) of the adrenal gland, liver, kidney, intestine, lung, testis, etc., and is an enzyme that adds one atom of oxygen to fat-soluble substances. It is involved in the synthesis and decomposition of eicosanoids (a group of physiologically active substances generated from polyunsaturated fatty acids with 20 carbon atoms such as arachidonic acid), ω (omega) oxidation of fatty acids, activation of vitamin D, and metabolism of foreign chemical substances such as drugs. Tryptophan 2,3-dioxygenase (also known as tryptophan pyrrolase) and tryptophan-5-monooxygenase also play an active role in the metabolism of tryptophan (a type of amino acid). Osamu Hayashi (1920-2015) used a stable isotope of oxygen ( 18 O) to prove that the oxygen incorporated into the substrate by oxygenases comes from oxygen in the air.

[Tokuhisa Sachiko]

"Lloyd L. Ingraham et al., translated by Mitsuyoshi Matsuo, "Biochemistry of Oxygen: Mechanism of Diatomic Oxygen Reactions" (1991, Academic Press Center)

[Reference] | Amino acids | Arachidonic acid | Stable isotopes | Oxidase | Reduction | Reductase | Substrate | Enzyme number | Oxidation | Oxidoreductase | Oxidation number | Catalyst | Dehydrogenase | Electronegativity | Tryptophan | Microsome | Mitochondria

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

酸化還元酵素(酸化還元反応を触媒する酵素の総称)の一種で、分子状酸素(O2)の酸素原子(O)が直接有機基質に取り込まれる反応を触媒する酵素の総称。酸素添加酵素ともいう。国際生化学連合(現在は国際生化学・分子生物学連合)の酵素委員会が制定した酵素の分類による「EC1.酸化還元酵素」に属す酵素群(酵素番号の項目参照)。酸化還元酵素は生体内における酸化還元反応を触媒する酵素であり、反応の様式により、酸化酵素(オキシダーゼ)、酸素添加酵素(オキシゲナーゼ)、脱水素酵素(デヒドロゲナーゼ)、還元酵素(レダクターゼ)などとよばれる。

 酸化還元反応とはある物質から他の物質へ電子が渡される反応であるといってもよい。電子を奪われた物質は酸化された、電子を受け取った物質は還元されたという。一般に、ある物質が
(a)酸素を受け取ったとき
(b)水素を奪われたとき
(c)電子を奪われたとき
その物質は酸化されたというが、(a)および(b)の反応も(c)の変化として説明できる。有機化合物が(a)あるいは(b)の反応を受けたときには、炭素の酸化数(電子の授受を表す数。電子一つ奪われたら+1、受け取ったら-1と表す)に変化がないので(c)であることがわかりにくい。この場合は、炭素と炭素に結合している原子の電気陰性度(結合している原子それぞれの電子を引き付ける傾向)を比較する。電気陰性度に違いがある原子間の共有結合の電子対は、電気陰性度の大きい方の原子に割り当てる。酸素、炭素、水素の電気陰性度は酸素>炭素>水素である。すなわち、(a)酸素を受け取った場合は、この酸素と結合した炭素は電子を酸素のほうへ奪われたことになる。また、(b)水素を奪われた場合は、その水素と結合していた炭素はそれまで水素の電子を引き付けていたので、電子を奪われたことになる。生体は酵素を触媒として、いろいろな有機物および無機物を酸化還元することによって、必要な物質を合成し、不必要または有害な物質を代謝し、あるいは生命活動に必要なエネルギーを獲得している。

 酸化還元酵素のうち、酸素を電子受容体として基質(酵素反応を受ける物質)を酸化する酵素を酸化酵素と総称する。酸素は主として水や過酸化水素となったり、有機化合物中に取り込まれたりする。後者を触媒する酵素はオキシゲナーゼとして区別することが多い。すなわち、オキシゲナーゼは分子状酸素(O2)の酸素原子(O)が直接有機基質に取り込まれる反応を触媒する酵素の総称である。

 オキシゲナーゼは人間をはじめ、動物、植物、微生物に広く分布し、アミノ酸や脂質、ホルモンや薬物毒物の代謝に重要な役割を果たしている。次の2群に分類される。

(1)分子状酸素(O2)の酸素1原子を添加する反応を触媒する酵素群(モノオキシゲナーゼ)。S+O2+AH2→SO+A+H2O
(2)酸素2原子を添加する反応を触媒する酵素群(ジオキシゲナーゼ)。S+O2→SO2
 ここで、Sは基質、AH2は水素供与体を表す。モノオキシゲナーゼには、たとえばシトクロームP-450とよばれている酵素群がある。これは副腎、肝臓、腎臓、腸、肺、精巣などの顆粒(かりゅう)画分(ミクロゾームやミトコンドリア)に存在し、脂溶性の物質に1原子酸素を添加する酵素で、エイコサノイド(アラキドン酸などの炭素数20の多価不飽和脂肪酸から生成する一群の生理活性物質)の合成・分解反応、脂肪酸のω(オメガ)酸化、ビタミンDの活性化反応、薬物など外来化学物質の代謝に関与している。また、トリプトファン(アミノ酸の一種)の代謝においても、トリプトファン2,3-ジオキシゲナーゼ(別名トリプトファンピロラーゼ)やトリプトファン-5-モノオキシゲナーゼが働いている。早石修(はやいしおさむ)(1920―2015)は酸素の安定同位体(18O)を用いて、酸素添加酵素により基質に取り込まれる酸素は空気中の酸素に由来することを確証した。

[徳久幸子]

『Lloyd L. Ingraham他著、松尾光芳訳『酸素の生化学――二原子酸素反応の機構』(1991・学会出版センター)』

[参照項目] | アミノ酸 | アラキドン酸 | 安定同位体 | オキシダーゼ | 還元 | 還元酵素 | 基質 | 酵素番号 | 酸化 | 酸化還元酵素 | 酸化数 | 触媒 | デヒドロゲナーゼ | 電気陰性度 | トリプトファン | ミクロゾーム | ミトコンドリア

出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

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