Refer to the following paragraph and the figure to answer the following question

Question

Refer to the following paragraph and the figure to answer the following question
http://session.masteringbiology.com/problemAsset/1233849/2/3-1.jpg
Since structure correlates so well with function, biochemists are constantly looking for new ways to probe the complex structure of proteins in order to understand what they do and how they do it. One of the most powerful techniques in existence today is X-ray crystallography. The main difficulty with this technique is getting the protein to crystallize. Once crystallized, the protein is bombarded with X-rays to create a pattern that can be analyzed mathematically to determine the three-dimensional structure of the protein. This analysis has been performed by Krzysztof Palczewski on the protein rhodopsin, which is a light-sensitive protein found in species ranging from ancient bacteria (archaea) to humans. The structure (schematically shown above, where each letter represents an amino acid) is characterized by a single polypeptide chain with several ?-helical segments that loop back and forth across the cell membrane. Another notable feature is the disulfide bond (-S-S-) that can be seen at the bottom of the third transmembrane segment. [Figure adapted from K. Palczewski et al., Science 289 (2000): 739.]
If you were reading off the sequence of amino acids in the figure to a biologist friend, what should the first three letters be?
M-N-G
A-P-A
It doesn't matter, since the protein has no polarity or directionality.

Explanation / Answer

First three letters should be M-N-G. Proteins have polarity and directionality. All the proteins should read from N-terminus to C-terminus. N-terminus is the one where first amino acid has amide (NH2) group free where as C- trminus has corboxyl group, (COOH) free. Always one should read from N-terminus to the C-terminus. In the diagaram it shows that N-terminus is towards extracellular side and C-terminus is towards Cytoplasmic side.

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