This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces the following amino acid within the alpha-helical structure - \nSelect and explain. \nA. Alanine, B. Aspartic acid, C. Tyrosine, D. Glycine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: "Because of its small size, glycine often induces bends in a-helix".\n"Proline, hydroxyproline, glycine, glutamate, aspartate, arginine, lysine, isoleucine, threonine, and serine are\xa0helix-destabilizing structure".\xa0\n"Glycine and proline are the helix-destabilizing amino acids".\n\nBoth glycine and aspartic acid can destabilize a-helix. However, glycine is much more destabilizing than aspartic acid (aspartate).\n\nAmino acids destabilizing the alpha helix\n\nProline is never found in a-helix. The peptide bond of nitrogen of proline lacks a hydrogen atom to contribute hydrogen bond (proline is imino acid:- NH, not amino -NH2). When incorporated in this structure, it causes the chain to bend sharply (as at that level stabilizing hydrogen bond is not available). This disrupts regular helical conformation. Proline can only be stably accommodated within the first turn of an a-helix.\nGlycine also tends to disrupt α-helix. Because of its high conformational flexibility and small size, it often induces bends in α-helices.\nAmino acids with charged R-groups (i.e. charged amino acids):- They interfere with the formation of α-helix because of electrostatic (ionic) attraction between the charged groups. These amino acids are:-\n\n\nPositively charged (basic):- Histidine, arginine, lysine\nNegatively charged (acidic) :- Aspartate (aspartic acid), glutamate (glutamic acid)\n\n\nAmino acid with bulky R group due to branch at β-carbon:- They destabilize α-helix because side chain of these amino acids projects out of the plane of the main a-helix chain. These amino acids are isoleucine and valine.\nAmino acids whose side chain can form hydrogen bond:- These amino acids disrupt α-helix by acting as a competitor of the main hydrogen donor and acceptors. These amino acids are serine, asparagine,\xa0and threonine.\nYou should make it clear in your mind that destabilizing property of amino acid does not mean that they cannot occur in α-helix. They have a different propensity for α-helix.\nFor example, proline can never occur in α-helix as it cannot form a hydrogen bond, the major stabilizing force in α-helix. Glycine is also not commonly found as it is too small and induces bends in a-helix. Other amino acids destabilize α-helix when present in large numbers.\n\nPropensity of α-helix (higher the number less the chance to be found in α-helix)\n\nAlanine (0.0) > Leucine (0.21) = arginine (0.21)> Methionine (0.24) > Lysine (0.26) > glutamine (0.39) > glutamic acid (0.40) > isoleucine (0.41) > tryptophan (0.49) > serine (0.5) > tyrosine (0.53) > phenylalanine (0.54) > valine = histidine (0.61) > aspargine (0.65) > threonine (0.66) > cysteine (0.68) > aspartic acid (0.69) > glycine (1) > proline (3.16).\nThat means alanine is the most stabilizing unit.\nBeside proline and glycine, any amino acid can occur in α-helix, however with a different propensity for α-helix and also with the different destabilizing property.', 'role': 'assistant'}]