Serum transferrin (abbreviated: Tf) is a monomeric protein whose main function in the human body is the transport of iron(III). Each transferrin molecule can bind up to two iron(III) ions with stepwise binding constants K

_{1} and K

_{2} at biological conditions except that the temperature is 25 °C corresponding to the react ions:

Fe

^{III} + Tf

(Fe

^{III})Tf K

_{1} = 4.7·10

^{20}Fe

^{III} + (FeIII)Tf

(Fe

^{III})

_{2}Tf K

_{2} = 2.4·10

^{19}In the diferric protein, (Fe

^{III})

_{2}Tf, the two iron(III) ions are bound at two similar, but non-identical sites, and the two possible monoferric protein products, (Fe

^{III})Tf, can be denoted {Fe

^{III} · Tf} and {Tf · Fe

^{III}}. Their relative abundance at equilibrium is given by the constant

K = [{Tf · Fe

^{III} }] [{Fe

^{III} · Tf }]

^{-1} = 5.9.

**The questions are:**1)Calculate the values of the two constants K1' = [{Fe

^{III} . Tf}] [FeIII]

_{-1} [Tf]

_{-1} and K1" =

[{Tf . Fe

^{III} }] [Fe

^{III}]

^{-1} [Tf]

^{-1}, respectively, corresponding to the formation of each monoferric form of transferrin.

This is not so hard, K1'=6.8·10

^{19} and K1"=4·10

^{20}.

2)Calculate the values of the two constants K2' = [(Fe

^{III})

_{2}Tf] [Fe

^{III}]

^{-1} [{Fe

^{III} . Tf }]

^{-1} and

K2" = [(Fe

^{III})

_{2}Tf] [Fe

^{III}]

^{-1} [{Tf . Fe

^{III}}]

^{-1} respectively, corresponding to the formation of diferric transferrin from each of the monoferric forms.

Here I am confused. They used the following expression:

K1'K2'=K1"K2"=K1K2

By what logic did they get this?