[hpl912]

alkaline phosphatase (AP) has binding sites for Zn2+ which definitely shows activity.
i have tested also with Ca2+, Mg2+, Ni2+ and Co2+ which results as very low activity.

so that means that AP doesn't really need a divalent cation for activity?? such as water?

then why different bivalent metal ions acts different on AP? for example Zn2+ which gives activity and Ni2+ that doesn't result in activity? is it the size of the metal ion that makes the difference?

[Borek]

AP doesn't really need a divalent cation for activity?? such as water?

then why different bivalent metal ions acts different on AP?

You ahev already answered your own question - because they are different.

[Yggdrasil]

Enzyme active sites are highly structured and the chemistry that occurs at the active sites is highly dependent on the identity of the ions there.  So, the AP enzyme requires a specific divalent cation, and other divalent cations cannot substitute.

The AP enzyme catalyzes a hydrolysis reaction, so the enzyme needs some way to activate a water molecule so that it can act as a nucleophile.  As it turns out Zn²⁺ ions act as good lewis acids and can help the enzyme deprotonate water at neutral pH in order to form OH^- ions for the hydrolysis reaction.  Likely the reason why other divalent cations don't work is that the enzyme active site has evolved ligands with a geometry that favorably coordinates zinc, but this geometry is not good for binding other divalent cations.

[hpl912]

AP doesn't really need a divalent cation for activity?? such as water?

then why different bivalent metal ions acts different on AP?

You ahev already answered your own question - because they are different.

okay... this is all I can get of an answer from a senior member with over 7k posts?
I think we all, and you, in a science forum must know that especially in this field by only saying "different" without an explanation of how this "difference" makes it different is not an acceptable answer.
If you do work in some laboratory and your boss asked you about why you've got different results between two different experiments, I hope you wouldn't just answer him back "because they are different, you answered your own question".



@Yggdrasil:
Thanks for the explanation! 
so since not all divalent cations can be substituted for activity, does it means then that the active site is basically specific to size of the ion other than the charge requirement?
about the water hydrolysis case, i don't get it clear on how this affects AP activity.... is it because of the generation of a substrate or deprotonation reaction?
and is this a non-divalent case that activates the activity of AP (I also ran the test with water and it resulted with activity same as when done with Zn2+ but just a bit lower)?
   

[Yggdrasil]

Likely the active site is specific for both the size of zinc ions and their coordination geometry.

AP catalyzes the following reaction

R-OPO₃^2- + OH^- --> R-OH + PO₄^3-

The zinc ion is involved in generating and positioning the OH^- ion for the reaction.

[Borek]

You ahev already answered your own question - because they are different.

okay... this is all I can get of an answer from a senior member with over 7k posts?
I think we all, and you, in a science forum must know that especially in this field by only saying "different" without an explanation of how this "difference" makes it different is not an acceptable answer.

On some level on generality that's the only acceptable answer. In fact Ygg answer - although it looks much more elaborate - is not more precise. Replacing the cation is like replacing wheels on F1 bolid with - say - truck wheels. No wonder it goes slower. Ygg answer points to reasonable chemical characteristic of Zn cation, but still the main reason for the enzyme to loose part of its activity when the cation is replaced is that they have different size and different coordination geometry. In short - they are different.

[hpl912]

ah ok.. thank you guys for the explanations