Lipids as Signals, Cofactors, and Pigments:- Phosphatidylinositols and Sphingosine Derivatives Act as Intracellular Signals

Phosphatidylinositol and its phosphorylated derivatives act at several levels to regulate cell structure and metabolism (Fig. 10–17). Phosphatidylinositol 4,5-bisphosphate (Fig. 10–8) in the cytoplasmic (inner) face of plasma membranes serves as a specific binding site for certain cytoskeletal proteins and for some soluble proteins involved in membrane fusion during exocytosis. It also serves as a reservoir of messenger molecules that are released inside the cell in response to extracellular signals interacting with specific receptors on the outer surface of the plasma membrane. The signals act through a series of steps (Fig. 10–17) that begins with enzymatic removal of a phospholipid head group and ends with activation of an enzyme (protein kinase C). For example, when the hormone vasopressin binds to plasma mem brane receptors on the epithelial cells of the renal collecting duct, a specific phospholipase C is activated. Phospholipase C hydrolyzes the bond between glycerol and phosphate in phosphatidylinositol 4,5-bisphosphate, releasing two products: inositol 1,4,5-trisphosphate (IP₃), which is water-soluble, and diacylglycerol, which remains associated with the plasma membrane. IP₃ triggers release of Ca₂ from the endoplasmic reticulum, and the combination of diacylglycerol and elevated cytosolic Ca₂ activates the enzyme protein kinase C.

FIGURE 10–17 Phosphatidylinositols in cellular regulation. Phosphatidylinositol 4,5-bisphosphate in the plasma membrane is hydrolyzed by a specific phospholipase C in response to hormonal signals. Both products of hydrolysis act as intracellular messengers.

This enzyme catalyzes the transfer of a phosphoryl group from ATP to a specific residue in one or more target proteins, thereby altering their activity and consequently the cell’s metabolism. Inositol phospholipids also serve as points of nucleation for certain supramolecular complexes involved in signaling or in exocytosis. Proteins that contain certain structural motifs, called PH and PX domains (for pleckstrin homology and Phox homology, respectively), bind phosphatidylinositols in the membrane with high specificity and affinity, initiating the formation of multienzyme complexes at the membrane’s cytosolic sur face. A number of proteins bind specifically to phosphatidylinositol 3,4,5-trisphosphate, and the formation of this phospholipid in response to extracellular signals brings the proteins together at the surface of the plasma membrane (see Fig. 12–8). Membrane sphingolipids also can serve as sources of intracellular messengers. Both ceramide and sphingomyelin (Fig. 10–12) are potent regulators of protein kinases, and ceramide or its derivatives are known to be involved in the regulation of cell division.

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شعبة الخطابة الحسينية النسوية تطلق دورة (تراث الطفّ)

معهدُ تراث الأنبياء يعلنُ أسماء الفائزين بمسابقة (مشكاة الأنوار) العلميّة الأولى

الهيأة العليا لإحياء التراث وجمعية العميد تناقشان تحضيرات المؤتمر التمهيدي الثالث لألفية حوزة النجف الأشرف

جامعة العميد تعلن توفر وظائف شاغرة

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الآخبار الصحية

ما هي أفضل العصائر لصحة القلب وتقوية المناعة والنوم العميق؟

الأطعمة الغنية بالبروتينات النباتية تقلل خطر الأمراض المزمنة

وجع الأوتار قد يخفي خطرا صحيا مرتبطا بالقلب

أزمة "السفينة الموبوءة"..الأرجنتين تحقق في مصدر تفشي "هانتا"

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باحثون يحددون تأثير القهوة على الميكروبيوم والمزاج

روسيا.. ابتكار تقنية لمعالجة سبائك التيتانيوم لحماية الطائرات والسيارات

بكين وواشنطن تدرسان إطلاق محادثات رسمية حول الذكاء الاصطناعي

لأول مرة في الولايات المتحدة.. تشغيل مفاعل مصغر نووي يغذي الذكاء الاصطناعي بالطاقة

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مواضيع ذات صلة

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling

Lipids as Signals, Cofactors, and Pigments:- Steroid Hormones Carry Messages between Tissues

Structural Lipids in Membranes:- Sterols Have Four Fused Carbon Rings

Structural Lipids in Membranes:- Sphingolipids at Cell Surfaces Are Sites of Biological Recognition

Structural Lipids in Membranes:- Sphingolipids Are Derivatives of Sphingosine

Structural Lipids in Membranes:- Chloroplasts Contain Galactolipids and Sulfolipids

Structural Lipids in Membranes:- Some Phospholipids Have Ether-Linked Fatty Acids

Structural Lipids in Membranes:- Glycerophospholipids Are Derivatives of Phosphatidic Acid

Storage Lipids:- Many Foods Contain Triacylglycerols

Storage Lipids:- Triacylglycerols Provide Stored Energy and Insulation

Storage Lipids:- Triacylglycerols Are Fatty Acid Esters of Glycerol

Storage Lipids:- Triacylglycerols Are Fatty Acid Esters of Glycerol