pIs and proteins - (Aug/01/2008 )
Hello all,
I was reading a paper "Critical regions for the sweetness of brazzein," FEBS Letters 544 (2003) 33-37 and came across an interesting line in the discussion. The line is "Because its (brazzein) pI is so low (5.4) (compared to other sweet proteins), brazzein may have a higher potential than the other sweet proteins for engineering enhanced sweetness through the introduction of positivie charges at critical sites." In my biochemical background, if the pH is greater than the pI, the overall charge is negative. Is this a faulty statement or am i missing something.
Thanks,
Hmm, it's been quite awhile since my biochemistry class but what I remember is the PI of a protein is the pH at which the protein will have a net zero charge. Therefore, if the pH is lower (more acidic) the protein will give up hydrogen atoms and take on a negative charge. The paper is saying (or I interpret it as) with a PI of 5.4, in a normal physiological environment of a pH 7.2-7.4, the protein will be positively charged. The pH is well above it's PI and therefore the protein retains the hydrogen atoms.
But if you look at most titration curves, for glutamate:
it looses its C1 carboxylic acid proton first at pH 2.5 (-), at pH 4, it looses its side chain carboxylic acid proton (-), and then at pH 10 it loses it amino group proton.
So pH <2.5 = total positive charge
Between pH 2.5 and 4 = neutral
between 4 and 10 = one negative
>10 is two negatives.
Is this correct?
it looses its C1 carboxylic acid proton first at pH 2.5 (-), at pH 4, it looses its side chain carboxylic acid proton (-), and then at pH 10 it loses it amino group proton.
So pH <2.5 = total positive charge
Between pH 2.5 and 4 = neutral
between 4 and 10 = one negative
>10 is two negatives.
Is this correct?
Im sorry to say that titration curves go flying out the window when you enter the cellular environment.
but as a general rule if the pI is less than the pH of a protein, it is positive, right?
Say the pI of a protein was 7.0. At pH 7.0, it would have a net charge of 0. At pH 2, everything would be protonated, giving a net positive charge (carboxyls no charge, amines charged); at pH10 the reverse would be true (everything deprotonated (carboxyls unprotonated and negative charge, amines unprotonated and uncharged),giving a net negative charge).
Therefore:
If pH <pI, protein is positively charged
If pH >pI, protein is negatively charged
Remember that this only affects the surface of the protein. Sometimes pI calculations presume the protein is denatured.