Langur monkeys and ruminants (such as cows) are the only known mammals in which lysozyme functions in the stomach instead of in tears and saliva. The cow/langur enzyme also has five amino acid changes that are not present in other mammals. What is most likely to be true about lysozyme from langurs and cows compared to lysozyme from other mammals?

Question

A.  These five amino acid changes alter the specificity of the enzyme, such that the cow/langur lysozyme attacks viruses instead of bacteria.
B.  These five amino acid changes somehow make the cow/langur lysozyme resistant to pepsin digestion.
C.  The five amino acids that langurs and cows have in common in their lysozyme were present in the common ancestor of all mammals.
D.  Salivary and tear lysozyme functions best at a higher H+ ion concentration than the cow/langur enzyme.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content
· What do you know about the convergent evolution of lysozyme? What other information is related to the question?

Choose Answer
· Given what you now know, what information is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  These five amino acid changes somehow make the cow/langur lysozyme resistant to pepsin digestion.

Clarify Question
· What is the key concept addressed by the question?
        o The question asks about the convergent evolution of lysozyme.
· What type of thinking is required?
        o You are being asked to analyze statements to explain the convergent evolution of lysozyme.

Gather Content
· What do you know about the convergent evolution of lysozyme? What other information is related to the question?
        o A remarkable case of convergent evolution at the molecular level is exhibited by ruminants and the langur monkey, which subsists primarily on leaves. In most mammals, lysozymes are enzymes found in saliva and tears, which attack invading bacteria. However, in both ruminants and langurs, lysozymes have been modified to take on a new role, digesting bacteria in the stomach. In both cases, five identical amino acid changes have evolved. The result is that the lysozyme molecules of ruminants and langurs are more similar to each other than they are to lysozymes in more closely related species.

Choose Answer
· Given what you now know, what information is most likely to produce the correct answer?
        o The stomach contains an acidic environment and the digestive enzyme pepsin, while tears and saliva do not. The tear and saliva lysozyme would be predicted to function better at lower H+ than the cow/langur enzyme. There is no evidence that specificity of lysozyme switched to viruses instead of bacteria. If the common ancestor to cows and langurs had these mutations in common then you would predict to see it in other species.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to analyze statements to explain the convergent evolution of lysozyme. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you think that an acidic environment had lower H+ ion concentrations? Did you think that the common ancestor to cows and langurs had the mutations observed?