Cys/Met

[Newtonian21]

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I am in the last chapter (Nitrogen compounds) of BR the Organic Chemistry book. On page 245-246, they mentioned Methionine, and Cysteine to be semi-hydrophobic, and hydrophilic amino acids. However, per this website Amino Acids - Sulfur-Containing, these two amino acids are considered to be hydrophobic, and Methionine is considered one of the most hydrophobic amino acids there is. Could anyone throw more lights on this? Per BR, Cys is hydrophilic due to its protic side chain, is found on the exterior of a folded protein. Which of these sources are rights? Thanks!

 

[CAFFEINE!]

For methionine, you need to consider the fact that sulfur is minimally more electronegative than carbon. Furthermore, a sulfur atom is roughly the same size as a methylene group, so the side chain of methionine reacts the same way as though it were a butyl group.

For cysteine, things are slightly different since sulfur is the terminal atom in the side chain. The pKa of the sulfhydryl hydrogen is ~8.3ish which makes it slightly basic, giving it the ability to form disulfide linkages with other cysteine residues. Because the sulfur can lose its hydrogen while forming a disulfide linkage, some people consider cysteine to be a polar amino acid, but you can easily argue that it's more of a non-polar amino acid. Overall, you need to go into the MCAT with the understanding that cysteine is just one of ~those amino acids~ that isn't exclusively polar, or exclusively non-polar (it's somewhere in-between).

 

[BTC0821]

I am in the last chapter (Nitrogen compounds) of BR the Organic Chemistry book. On page 245-246, they mentioned Methionine, and Cysteine to be semi-hydrophobic, and hydrophilic amino acids. However, per this website Amino Acids - Sulfur-Containing, these two amino acids are considered to be hydrophobic, and Methionine is considered one of the most hydrophobic amino acids there is. Could anyone throw more lights on this? Per BR, Cys is hydrophilic due to its protic side chain, is found on the exterior of a folded protein. Which of these sources are rights? Thanks!

Cys and Ser are quite similar and I tend to think of them as such. Cys just being the thiol version of Ser's alcohol R group. Met is really its own thing in that it behaves similarly to how an ether would (if thinking of their oxygen counterparts). I could very well be wrong but I wouldn't imagine M to be one of the most hydrophobic amino acids considering it would be compared to the aromatics such as Phe.

 

[aldol16]

Cys and Ser are quite similar and I tend to think of them as such. Cys just being the thiol version of Ser's alcohol R group. Met is really its own thing in that it behaves similarly to how an ether would (if thinking of their oxygen counterparts). I could very well be wrong but I wouldn't imagine M to be one of the most hydrophobic amino acids considering it would be compared to the aromatics such as Phe.

Indeed quite similar: Amino Acids Reference Chart.