Studies of 25-hydroxyvitamin D
3
-24-hydroxylase (CYP24A1) have demonstrated that it is a bifunctional enzyme capable of the 24-hydroxylation of 1α,25-(OH)
2
D
3
, leading to the excretory form, calcitroic acid, and 23-hydroxylation, culminating in 1α,25-(OH)
2
D
3
-26,23-lactone. The degree to which CYP24A1 performs either 23- or 24-hydroxylation is species-dependent. In this paper, we show that the human enzyme that predominantly 24-hydroxylates its substrate differs from the opossum enzyme that 23-hydroxylates it at only a limited number of amino acid residues. Mutagenesis of the human form at a single substrate-binding residue (A326G) dramatically changes the regioselectivity of the enzyme from a 24-hydroxylase to a 23-hydroxylase, whereas other modifications have no effect. Ala-326 is located in the I-helix, close to the terminus of the docked 25-hydroxylated side chain in a CYP24A1 homology model, a result that we interpret indicates that substitution of a glycine at 326 provides extra space for the side chain of the substrate to move deeper into the pocket and place it in a optimal stereochemical position for 23-hydroxylation. We discuss the physiological ramifications of these results for species possessing the A326G substitution, as well as implications for optimal vitamin D analog design.
对25-羟基维生素D3-24-羟化酶(CYP24A1)的研究表明,它是一种具有双重功能的酶,能够对1α,25-(OH)2D3进行24-羟基化,形成排泄形式的钙三酸,以及23-羟基化,最终形成1α,25-(OH)2D3-26,23-内酯。CYP24A1执行23-或24-羟基化的程度因物种而异。在本文中,我们展示了主要对其底物进行24-羟基化的人类酶与仅在有限数量的氨基酸残基上对其进行23-羟基化的负鼠酶有所不同。通过对人类形式的单个底物结合残基(A326G)进行突变,可以将酶的区域选择性从24-羟基化酶显著地改变为23-羟基化酶,而其他修饰则没有影响。Ala-326位于I-螺旋中,靠近CYP24A1同源模型中停靠的25-羟基化侧链的末端,我们解释这个结果表明在326处替换甘氨酸提供了额外的空间,使底物的侧链更深地进入袋中,并将其放置在最佳立体化学位置进行23-羟基化。我们讨论了这些结果对具有A326G突变的物种的生理影响,以及对最佳维生素D类似物设计的影响。