Question

Which of the following is the Fischer projection of L-threonine (also known as (2S, 3R)- 2-amino-3hydroxybutanoic acid)?

(A)- 

(B)- 

(C)- 

(D)- 

Answer 1

Hint: The Fischer projection restricts the molecule into a two-dimensional plane, such that the absolute configuration of the molecule remains unchanged. Also, depending on the number of stereocenters in the molecule, the total number of stereoisomers present can be determined, and their Fischer projection can be made.

Complete answer:
The Threonine is an amino acid, consisting of an amino group, a carboxyl group, and a hydroxyl group present in it. Due to the presence of the -OH group, it is polar and hydrophilic in nature. But overall, it is uncharged. Thus, having a chemical formula $C{{H}_{3}}-CH(OH)-CH(N{{H}_{2}})-COOH$. It is seen that it has two stereocenters, that is, the centre carbon atoms, both having different sets of atoms attached to it. so, there are four possible stereoisomers of the Threonine molecule.
The most naturally occurring form is the L-Threonine molecule, which is used in the protein synthesis. It has the IUPAC name as (2S, 3R)- 2-amino-3hydroxybutanoic acid, having a mirror image as the D-threonine.
                                             

In the L-threonine and its mirror image, that is, enantiomer at the two stereocenters the hydrogen atoms are on opposite sides in the Fischer projection.
It is obtained by placing the molecule on the plane of the paper, such that the configuration (2,3S) of the stereocenters can be obtained by rotating the molecule following the Fischer rule. And as we assign the priority to the groups attached to the carbon, and on the basis of the direction of motion, clockwise or anti-clockwise, the R and S configuration is achieved.

Therefore, the Fischer projection of the L-threonine is option (B).

Note:
The other two stereoisomers that are rarely found in nature are the diastereomers of L-Threonine molecules. That is, the L-allo threonine with the configuration as (2S,3S) and its mirror image. It has both the hydrogen atoms on the same side.