Suppose that, in a given protein, one lysine is replaced by aspartic acid. Does

Question

Suppose that, in a given protein, one lysine is replaced by aspartic acid. Does this charge occur in the primary structure or in the secondary structure? How might it result in a change in tertiary structure? In quaternary structure? Would this be considered a mutation? In humans mutations can lead to a number of diseases or disorders, please list one and detail/ describe the mutation along with the disease/ disorder. Suppose that, in a given protein, one lysine is replaced by aspartic acid. Does this charge occur in the primary structure or in the secondary structure? How might it result in a change in tertiary structure? In quaternary structure? Would this be considered a mutation? In humans mutations can lead to a number of diseases or disorders, please list one and detail/ describe the mutation along with the disease/ disorder.

Explanation / Answer

Lysine is a positively charged amino acid whereas Aspartic acid is a negatively charged amino acid. The primary structure of a protein is its amino acid sequence. So there won't be any change in the primary structure of the protein if the amino acids are replaced. Lysine is more energitically favorable to form alpha helix than aspartic acid. So the possibility of a change in the secondary structure of the protein is high. Tertiary structure of a protein is due to the interaction between the amino acid side chains in their primary structure. Since Lysine has NH3+ in its side chain and Aspartic acid has COO- in its side chain, the change in the tertiary structure can only be determined with the charge of their neighbouring amino acids. Similarly, the formation of the quarternary structure can either be favored or repelled based on the charge of the other protein subunits.

Replacement of one amino acid is a substitution mutation. It can lead to diseases or disorders. Example of one such disorder is Sickle-cell hemoglobin (HbS). HbS forms as a result of a single amino acid substitution in the beta chain of Hemoglobin(Hb). Replacement of the glutamate residue at position 6 in the beta chain by valine residue is the only chemical difference between normal Hb and HbS. This residue is present on the outer surface of the molecule. The change produces a sticky hydrophobic spot on the surface that results in abnormal quaternary association of hemoglobin. This makes the deoxyHbs less soluble than normal deoxyHb. The formation of insoluble deoxyHbs fibers distorts the RBC into the elongated sickle shape structure which is the characteristic of the disease, Sickle cell Anemia.

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