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Modifying Mg~(2+) Binding and Exchange with the N-Terminal of Calmodulin

机译：用钙调蛋白的N末端修饰Mg〜（2+）的结合和交换

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摘要

To follow Mg~(2+) binding to the N-tem1inal of calmodulin (CaM), we substituted Phe in position 19, which immediately precedes the first Ca_2+/Mg~(2+) binding loop, with Trp, thus making F_(19)WCaM (W-Z). W-Z has four acidic residues in chelating positions, two of which form a native Z-acid pair. We then generated seven additional N-tem1inal CaM mutants to examine the role of chelating acidic residues in Mg~(2+) binding and exchange with the first EF-hand of CaM. A CaM mutant with acidic residues in all of the chelating positions exhibited Mg~(2+) affinity similar to that of W-Z. Only CaM mutants that had a Z-acid pair were able to bind Mg~(2+) with physiologically relevant affinities. Removal of the Z-acid pair from the first EF-hand produced a dramatic 58-fold decrease in its Mg~(2+) affinity. Additionally, removal of the Z-acid pair led to a 1.8-fold increase in the rate of Mg~(2+) dissociation. Addition of an x- or Y -acid pair could not restore the high Mg~(2+) binding lost with removal of the Z-acid pair. Therefore, the Z-acid pair in the first EF-hand of CaM supports high Mg~(2+) binding primarily by increasing the rate of Mg~(2+) association.
机译：为了跟踪Mg〜（2+）与钙调蛋白（CaM）的N末端的结合，我们用Trp取代了位置19处的Phe，该位置紧接在第一个Ca_2 + / Mg〜（2+）结合环之前，从而使F_（ 19）WCaM（WZ）。 W-Z在螯合位置具有四个酸性残基，其中两个形成天然Z-酸对。然后，我们产生了七个额外的N末端CaM突变体，以检查螯合酸性残基在Mg〜（2+）结合中的作用以及与CaM的第一个EF手交换。在所有螯合位置均带有酸性残基的CaM突变体表现出与W-Z相似的Mg〜（2+）亲和力。只有具有Z-酸对的CaM突变体才能以生理相关亲和力结合Mg〜（2+）。从第一只EF手去除Z-酸对后，其Mg〜（2+）亲和力急剧下降58倍。另外，去除Z-酸对导致Mg〜（2+）解离速率增加1.8倍。 X或Y酸对的加入不能恢复由于去除Z酸对而失去的高Mg〜（2+）结合。因此，CaM的第一个EF手中的Z酸对主要通过增加Mg〜（2+）缔合的速率来支持高Mg〜（2+）结合。

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