Cytochrome P450 enzymes are responsible for the oxidative metabolism of a diverse family of com pounds; these include prostaglandins, fatty acids, bio genic amines, plant metabolites, and steroids, as well as chemical carcinogens, mutagens, many drugs, and other environmental pollutants. The following para graphs present a brief description of the important steroid related P450 enzymes and their electron transport chain partners.

All of the seemingly diverse steroid hydroxylase reactions are mediated by a family of homologous oxidative enzymes (~57 human enzymes) collectively known as the cytochrome P450 hydroxylases. Each individual P450 enzyme is composed of about 500 amino acids and has a single heme (protoporphyrin ring with a single chelated Fe2+ group). The cytochrome moiety is structurally analogous to the hemoprotein cytochromes of the electron transport chain present in mitochondria that are dedicated to the production of ATP. All contain some kind of covalently bound protoporphyrin ring coordinately bound to one atom of iron, which can be reversibly oxidized and reduced. As a class, most of these P450 enzymes are subject to inhibition by the presence of car bon monoxide. The CO coordinates to the Fe2+, thus blocking the essential cyclical oxidation and reduction that are associated with electron transfer. This inhibition can then be selectively reversed by light with a wave length of 450 nm. Cytochrome P450 steroid enzymes are known to be present in the liver, adrenal cortex, ovary, testis, kidney, placenta, lungs, intestinal mucosa, and selected regions of the brain.

In each P450 hydroxylase enzyme, the general oxidation–reduction reaction catalyzed is as shown in Figure 1. In principle, the oxygen atom of the hydroxyl group shown as “final hydroxylated product” could be derived from either H2O or molecular oxygen (O2). But as indicated in the equation, it is known that the hydroxyl oxygen is derived exclusively from molecular oxygen in all steroid cytochrome P450-containing hydroxylases. As a class these enzymes are designated as mixed-function oxidases. The term “mixed function” indicates that one atom of the diatomic substrate oxygen ends up in the steroid (as part of the hydroxyl) and the other ends up as part of water. This result has been confirmed by utilization of ¹⁸O2 and the subsequent appearance of ¹⁸O both in the steroid and in H2¹⁸O.

Fig1. General reaction of a cytochrome P450-catalyzed steroid hydroxylase. There is a family of related steroid hydroxylase members that utilize NADPH and O2 as substrates and produce as products a hydroxylated steroid and H2O.

Virtually all steroid P450 hydroxylases are mem brane bound and are present in either the mitochondria (type 1) or endoplasmic reticulum (type 2) of the cell (see Figure 2). Also Table 1 tabulates the subcellular localization of the various cytochrome P450 steroid hydroxylases. Depending upon whether the cytochrome P450 hydroxylase is localized in the mitochondria or microsomes, there are two somewhat different electron transport chains that function to transfer a pair of electrons from NADPH to the cytochrome P450 enzyme. These are summarized in Figure 2. In the mitochondrial system there are at least three separate components: (i) a flavoprotein dehydrogenase known as ferredoxin oxidoreductase, which accepts the electrons from NADPH; (ii) a nonheme iron protein, termed ferredoxin (molecular weight 13,000), which accepts the electrons from the flavoprotein and transfers them to (iii) the cytochrome P450 hydroxylase protein. The molecular details of the mitrochondrial electron transport process are summarized in the legend to Figure 3. Since the microsomal electron transport system lacks a ferredoxin component, the flavoprotein dehydrogenase transfers electrons directly to the cytochrome P450 enzyme. The molecular details of the microsomal electron transport process are summarized in the legend to Figure 4.

Fig2. Electron transport chains for sterol hydroxylases. (A) Mitochondrial hydroxylases showing the participation of the three protein components, ferredoxin oxidoreductase, ferredoxin, and a P450 steroid hydroxylase. (B) Endoplasmic reticulum hydroxylases showing the participation of the two protein components, P450 reductase and a P450 steroid hydroxylase. In both the mitochondrial and endoplasmic reticulum, the inhibitory actions of exposure to carbon monoxide, CO, on the P-450 hydroxylase by the binding of the CO to the oxygenated-P-450 heme group, which displaces and releases the O2.

Fig3. Electron transport to inner mitochondrial membrane (IMM) cytochrome P450 hydroxylases. There are three key mitochondrial proteins that participate in the transfer of electrons from NAPDH to the terminal P450 that generates as a final product a hydroxylated steroid. First the flavin adenine dinucleotide group (FAD) of ferredoxin reductase (FeRed) accepts two electrons from NADPH, thus producing the product NADP+. The reduced FAD bound to FeRed then passes the two electrons to an iron-sulfur ferredoxin (Fedx; 12 kDal) protein (see small diamond with two maroon dots). The Fedx then transfers the electrons to the heme of the P450 that is associated with the IMM. In order to facilitate the two electron transfers, first the negative charges of the Fedx(−) guide the Fedx docking with the positive charges of the FeRed(+) and then after movement to the P450 docking with the positive charges (+) of the P450. For the conversion of one cholesterol into one pregnenolone, three pairs of electrons from NAPH must be transported via the FeRed, Fedx to the P450 to carry out the side chain cleavage process. [Modified with permission from W. L. Miller and R. J. Auchus, The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr. Rev. 32, 81–151 (2011).]

Fig4. Electron transport to microsomal forms of cytochrome P450 hydroxylases. There are only two key microsomal proteins that participate to transfer electrons from NAPDH to the terminal P450 that generates a hydroxylated steroid. The P450-oxidoreductase (POR), which receives a pair of electrons from NADPH, has two bound cofactors that facilitate the transfer of electrons; they are the flavin adenine dinucleotide (FAD) and the flavin mononucleotide (FMN), which work sequentially together to transfer electrons to the second microsomal protein, the P450 oxidoreductase. Receipt of electrons by the POR’s bound FAD results in an enzyme conformational change that facilitates the FAD passing its electrons via a redox reaction to the FMN. Then after another conformational change, the FMN domain of the POR protein interacts with the P450 protein’s heme group to facilitate (in a redox reaction) the transfer of electrons that will be needed to attach a hydroxyl group on a steroid substrate. The interaction of P450 with the POR is facilitated by the negatively charged acidic residues on the surface of the FMN domain of POR with the positively charged basic residues on the P450 redox-partner. The binding site for the substrate steroid is located on the opposite side of the heme ring (shown as a rectangle) from that interacting with the POR’s reduced FMN. [Modified with permission from W. L. Miller and R. Jl Auchus,The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr. Rev. 32, 81–151 (2011).]

Table1. ^a Key Human Enzymes Concerned with the Production of Steroid Hormones

The most important component of both the mitochondrial and microsomal electron transport chain is the cytochrome P450 protein, in that it defines the substrate specificity of the enzyme. Each P450 hydroxylase has a substrate-binding domain that is comparable in its ability to define substrate specificity to that of the ligand-binding domains of steroid receptors and plasma transport proteins for steroid hormones. Thus, the three-dimensional structure of the substrate-binding domain of a P450 hydroxylase determines which of the some 22–27 car bons of the substrate will acquire a new hydroxyl group. Comparison of the subcellular localization of the various hydroxylase enzymes with the sequence of steroid movement through a metabolic pathway indicates the important role of cellular compartmentalization. Thus in the conversion of cholesterol into cortisol in the adrenal cortex, the steroid must move sequentially from the mitochondria (side chain cleavage) to the microsomes (17α- and 21-hydroxylation) and then back to the mitochondria (11β-hydroxylation).

اخر الاخبار

اخبار العتبة العباسية المقدسة

سماحة السيد الصافي يؤكد أهمية التدريب المستمر وتطوير قدرات الملاكات الطبية للارتقاء بالخدمات الصحية

العتبة العباسية المقدسة تقيم مجلسًا عزائيًا لإحياء ذكرى شهادة الإمام الكاظم (عليه السلام)

العتبة العباسية المقدسة تنشر لافتات السواد بذكرى شهادة الإمام الكاظم (عليه السلام)

العتبة العباسية المقدسة تشارك في مراسم تبديل رايتي قبتي الإمامين الكاظم والجواد (عليهما السلام)

المزيد

الآخبار الصحية

6 فواكة تساعدك على الشعور بتحسّن عند المرض

حفاظا على صحة العين.. هذا أفضل وقت لتناول فيتامين أ

الاستخدام اليومي لسماعات الأذن.. أخطر مما تتصور !

هل توقيت نشاطك اليومي يؤثر على صحة دماغك؟.. العلماء يجيبون

المزيد

الآخبار التكنلوجية

ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

تحديثات وأرباح عبر المشاهدات.. ما الذي تخطط له "لينكد إن"؟

"الكوكايين الوردي".. مخدر جديد يثير قلق السلطات الأميركية

دراسة: تغيّرات في أسلوب القيادة قد تكشف مبكرا عن الخرف !

المزيد

مواضيع ذات صلة

Cholesterol Side Chain Cleavage Enzyme (p450scc)

Steroidogenic Acute Regulatory Protein (StAR)

Major Protein Components of Muscle Fibers

The Major Components of Cartilage are Type II Collagen & Certain Proteoglycans

Bone is Affected by many Metabolic & Genetic Disorders

The Assembly of Membranes is Complex

Transport Vesicle Formation and Function Involves SNAREs & Other Factors

Misfolded Proteins Undergo Endoplasmic Reticulum Associated Degradation

Proteoglycans & Glycosaminoglycans

The ER Functions as The Quality Control Compartment of The Cell

Proteins Follow Several Routes to be Inserted Into or Attached to The Membranes of The Endoplasmic Reticulum

Proteins sorted via the Rough ER branch have N-Terminal signal peptides