Many of the group II hormones bind to receptors that couple to effectors through a guanine-nucleotide binding protein (G-protein) intermediary. These receptors typically have seven α-helical hydrophobic plasma membrane-spanning domains, here illustrated by the seven interconnected cylinders extending through the lipid bilayer in Figure 1. Receptors of this class, which signal through G-proteins, are known as G-protein–coupled receptors (GPCRs). To date, hundreds of GPCR genes have been identified, and represent the largest family of cell surface receptors in humans. Not surprisingly, a wide variety of responses are mediated by the GPCRs.

Fig1. Components of the hormone receptor–G-protein effector system. Receptors that couple to effectors through G-proteins, the G-protein–coupled receptors (GPCRs), typically have seven α-helical membrane-spanning domains (here shown as long cylinders). In the absence of hormone (left), the heterotrimeric G-protein complex (α,β,γ) is in an inactive guanosine diphosphate (GDP)-bound form and is probably not associated with the receptor. This complex is anchored to the plasma membrane through prenylated groups on the βγ subunits (wavy lines) and perhaps by myristoylated groups on α subunits (not shown). On binding of hormone (H) to the receptor, there are conformational changes within the receptor (as indicated by the tilted membrane-spanning domains) and association of the G-protein complex with the rearranged receptor—this then activates the G-protein complex. This activation results from the exchange of GDP with guanosine triphosphate (GTP) on the α subunit, after which α and βγ dissociate. The α subunit binds to and activates the effector (E). E can be adenylyl cyclase, Ca2+, Na+, or Cl− channels (αs ), or it could be a K+ channel (αi ), phospholipase Cβ (αq ), or cGMP phosphodiesterase (αt ); see Table 42–3. The βγ subunit can also have direct actions on E. (Reproduced with permission from Becker KL: Principles and Practice of Endocrinology and Metabolism, 3rd ed. Philadelphia, PA: Lippincott; 2001.)

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سماحة السيد الصافي يدعو إلى توحيد الجهود في تحقيق التراث لضمان الرصانة والدقة وخدمة الساحة العلمية

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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

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مواضيع ذات صلة

Cholesterol Side Chain Cleavage Enzyme (p450scc)

Steroidogenic Acute Regulatory Protein (StAR)

Cytochrome P450 Steroid Hydroxylases

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