LIMBS binding site

These agents bind to Cl binding site of Na -K -2C1" cotransporter glycoprotein and inhibit its transport function in ascending limb of loop of Henle. 

Figure 11. Proposed reaction cycles of the Na+-K+-ATPase (A) and the SR Ca2+-AT-Pase (B) involving transitions between different conformational states of the enzymes (see text for further explanation). The cytoplasmic side of the membrane is upward and the extracytoplasmic side downward. Brackets indicate that all the cation binding sites reside in an occluded state. A tentative H+-countertransport limb is shown for the SR Ca2+-ATPase (most likely n = 2). s indicates a relatively slow reaction step. ATP boxes indicate steps accelerated by ATP not being hydrolyzed. Mg2+ serving as a cofactor in phosphorylation and dephosphorylation is not shown.

Although the cellular mechanism of action has not been fully elucidated, furosemide (frusemide) appears to bind competitively to the chloride-binding site of the Na, K" ,2Cl -cotransporter on the luminal surface of the epithelial cells in the thick ascending limb of the loop of Henie, thereby inhibiting the reabsorption of sodium, potassium and chloride ions (Hinchcliff Muir 1991,... 

In each case the sulfated metabolite undergoes active tubular. secretion by the OATS in proximal tubular cells and. hence, attains high levels in luminal fluid. The negatively charged sulfate moiety pruhably hinds to the Cr-binding. site on (he luminal membrane-bound INa /IK /2CI co-tran-sport. system of (hick ascending limb and macula densa cells. 

Figure 18-13 Various seemingly unrelated agents that block Na reabsorption in the thick ascending limb of Henle s loop, The precise mechanism of action of muzolimine is unknown. All of the other agents are inactive as such and must be biotransformed to active metabolites before their diuretic activity can be expressed. In each case, like furosemide, bumetanide, torsemide, and ethacrynic acid, the active metabolites have an anionic moiety that may permit binding to the Cl -binding site on the 1Na /1K /2CI cotransport system in thick ascending limb cells.

However, they are secreted efficiently by the organic acid transport system in the proximal tubule and thereby gain access to their binding sites on the N -K -2CL symport in the luminal membrane of the thick ascending limb. Probenecid shifts the plasma concentration-response curve to furosemide to the right by competitively inhibiting furosemide secretion by the organic acid transport system. 

Fig. 7. The linear cluster of metal binding sites in the j3 I-like domain. (A) The liganded structure in Mn with RGD (Xiong et aL, 2002) in gold. (B) The unliganded Ov/Js structure in Ca (Xiong etal, 2001). The structures were superimposed by using the I-like domain, so that equivalent posidons in (A) and (B) are verdcally aligned. The orientation is with the LIMBS, MIDAS, and ADMIDAS from left to right.

The drug exerts its inhibition of Na (and CP) reabsorption in early distal tubule and the ascending limb of loop of Henle. It is also demonstrated to show its action primarily to inhibit Na reabsorption both at the cortical diluting site and in the proximal convoluted tubule. Its long duration of action ranging between 12 to 24 hours is appreciably attributed to protein binding as well as enterohepatie recycling. 

Fig. 7. Water at protein-DNA interfaces. Four protein-DNA complexes are shown with the DNA placed in front of the molecular surface of the protein, colored according to the electrostatic potential (red, negative blue, positive). The complexes are with the lambda repressor dimer (limb) three zinc fingers from the Zif268 transcription factor (laay) the human TATA box-binding protein (Icdw) and the dimeric E2 domain of papilloma virus (2bop). Red spheres represent interface water molecules. Figure taken from Nadassy et al. (1999) and drawn with GRASP (see web sites).

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