Position-specific weight matrix tables show relatively few substitutions for cysteine and tryptophan compared with other proteins. Which of the following are possible explanations for this? Please select all that apply.

Question

Quizplus · Accepted Answer

The fact that there have been relatively few substitutions for cysteine and tryptophan suggests that changes at these amino acids are subject to high levels of negative evolutionary pressure. Both cysteine and tryptophan have unusual structures that may enhance their importance in protein structures. This would increase the likelihood of their playing an essential role and increase the chance that their substitution would be deleterious and subject to negative selection pressure. Cysteine can form disulfide bonds with other cysteine residues: these are important structural components of mature proteins. Tryptophan has a ring structure that introduces bends into proteins.
In addition, the codons for cysteine and tryptophan are similar to the UGA stop codon. Mutations that that resulted in the stop codon are highly likely to be deleterious and would be subject to strong negative selection. Tryptophan is relatively large but cysteine is relatively small. It is important to remember that different evolutionary pressures will be acting on different individual amino acids in different positions, so while several of the options are possible, the options are not true for every case. For example, in some positions, it may be possible for a cysteine not involved in disulfide bonds to be replaced by alanine without a deleterious effect, and this may be evolutionarily accepted.

Position-Specific Weight Matrix Tables Show Relatively Few Substitutions for Cysteine