Citrate, binding

If citrate binds incorrectly, the OH and H are in the wrong orientation for the reaction... 

The second water ligand leaves the Fe i coordination sphere, leaving it five-coordinate. We have now arrived at a crucial step in the reaction mechanism. The cA-aconitate has to flip from its isocitrate binding mode to its citrate binding mode, i.e., a 180° rotation is required to interconvert Ca-H and Cp-R. It is hypothesized (cf. [15]) that this flip is brought about by displacement of the cA-aconitate by another molecule of cA-aconitate. This implies release of the cA-aconitate into solution and formation, if only transient, of 6. 

A cA-aconitate molecule from solution binds in the citrate binding mode. All subsequent steps, i.e., the formation of 8,9,10, and the final release of the citrate, are simply the reverse of the corresponding steps in the left-hand part of Figure 3, except for the inverted positions of the Ca and Cp groups. As a consequence, there are no irreversible steps in the mechanism. 

In contrast to the patient with CKD, the calcium balance is usually not an important issue for the ARF patient due to the limited duration of the iUness. One exception to this is in patients receiving continuous RRT that is anticoagulated with citrate. Citrate binds to serum calcium and without calcium, blood cannot form a clot. Citrate is typically infused as blood leaves the body. A calcium infusion must be administered either at the end of the extracorporeal circuit or centrally to provide sufficient unbound calcium to the patient. Insufficient calcium infusions wiU result in hypocalcemia, arrhythmias, and even death, so frequent monitoring of unbound serum calcium concentrations is essential. 

Patients undergoing OLT receive a substantial amount of crystalloid and blood products during the operative procedure. This often results in an edematous state in the postoperative period, especially in patients who had ascites preoperatively. The large citrate load from administered blood products has been implicated in causing hypocalcemia (due to citrate binding of ionized calcium) and metabolic alkalosis (due to conversion of citrate to bicarbonate) in the postoperative period. Low serum concentrations of magnesium are common in the postoperative period. Reduced intake from restricted diets and increased urinary excretion secondary to cyclosporine therapy contribute to hypomagnesemia. 

For animal fluids, amino add and protein binding appear to dominate for the trace elements. However, for Ca " and Mg citrate binding may well occur. A computer simulation exercise on blood plasma (further details in Section 22.3) suggests approximately 5% of these metal ions are in the form of dtrate complexes. The same exerdse indicates that any Fe" present would be totally bound in anionic citrate species. Interestingly the greater zinc nutritional value of human milk as opposed to bovine milk is related to zinc being present as labile citrate complexes in the former but as a tightly bound protein complex in the latter. ... 

Figure 4 Citrate binding to the active site [4Fe-4S] cluster of aconitase, as deduced from ENDOR...

The protomer consists of four polypeptide chains one of which contains the covalently linked biotinyl prosthetic group [180,206], Each protomer has one tight citrate-binding site (Ad = 3 fiM) and one tight acetyl-CoA binding site (Ad = 4 fiM) [180]. 

Human serum transferrin is an iron(m) tr ispoTt protein which gives up Fe + ions to bone marrow and placental tissues in preference to other cells such as liver cells the protein can bind two iron atoms per molecule but an anion such as HCOa" must be present for binding to occur. Vanadyl ion has been used (as an e.s.r. spin label) to show that the two metal-binding sites are non-equivalent and the same conclusion has been drawn from studies on the dissociation of labelled protein-bound iron. On the other hand, Harris and Aisen have obtained evidence from radiotracer experiments that the two sites are kinetically equivalent. The kinetics of iron(in) (in the form of ferric citrate) binding to the analogous apoprotein from hen s egg (apo-ovotransferrin) have been reported and the following minimum mechanistic scheme has been proposed ... 

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