What is meant by the Pka value of a side chain in an amino acid?

[sameeralord]

I know how to get the answer for this question using hesselback equation but I don't understand the question. I have some questions about the questiuon.

1. The PKa value of the sulphydryl (-SH) group of cysteine is 8.33. Calculate the fraction of anion to free sulfhydryl group at PH 7.O.

So is this Pka value referring to

SH + H20 <---> H+ + S- reaction

Why is this reaction significant. How does getting the PH of the equlibrium constant of this reaction provide any meaningful value.

Also in this amino acid there is only one SH group(meaning two atoms) so how can an equilibrium reaction occur.

My question is related to PKa values. I don't understand how getting the PH of the equilibrium constant can be meaningful and that two atom problem.

Thank you so much

 

[chemisttree]

Since you already know that your sulfhydryl side chain can deprotonate, you are halfway there. Sulfhydryl deprotonates just as an ammonium ion might or a carboxylic functionality might. The characteristic pH at which exactly half of them are deprotonated is a useful way to characterize them (pKa) just as it is useful to describe the strength of an acid using pKa. The fraction of these side chain groups that are charged often determines the tertiary structure and solubility of proteins or peptides that contain them. So it is very meaningful to know what fraction of the ionizable side chains are present in a charged state and what fraction is present in an uncharged state. Their pKa is useful in that determination.

If your question relates to how to employ the Henderson-Hasselbalch equation to answer your question, you need to show us a bit more work.

Start by writing down the equation and using what you know in that equation...

 

[quicknote]

for your questions and for seeking clarification on the topic. The Pka value of a side chain in an amino acid refers to the pH at which half of the side chain will be in its protonated form (H+) and half will be in its deprotonated form (A-). This is important because it tells us about the acidic or basic properties of the side chain and how it will behave in different pH environments.

In the case of the sulphydryl (-SH) group in cysteine, the Pka value of 8.33 tells us that at a pH of 8.33, half of the -SH groups will be in their protonated form (H-SH) and half will be in their deprotonated form (S-). This is known as the isoelectric point (pI) of the amino acid, where the overall charge on the molecule is neutral.

The equilibrium reaction you mentioned, SH + H2O ↔ H+ + S-, is a simplified representation of the protonation/deprotonation of the -SH group. In reality, the equilibrium involves the transfer of a proton from the -SH group to a water molecule, resulting in the formation of a hydronium ion (H3O+) and a thiolate ion (RS-). This reaction is important because it determines the pH at which the side chain will be mostly protonated or deprotonated.

As for the question about the two atoms, the equilibrium reaction occurs at the molecular level, where individual molecules of cysteine are interacting with water molecules. So even though there is only one -SH group in the amino acid, there are many molecules of cysteine involved in the equilibrium reaction.

In summary, the Pka value of a side chain in an amino acid provides important information about its acidic or basic properties and how it will behave in different pH environments. It is determined through equilibrium reactions and helps us understand the behavior of individual molecules at the molecular level.