The systematic (IUPAC) name for the amino acid threonine is (2S,3R)-2-amino-3-hydroxybutanoic acid. The systematic name indicates that threonine has two stereocenters, at positions 2 and 3, with S and R configurations, respectively. What is ths the Fischer projection of threonine. How many other possible stereoisomers of threonine are there?

Here's how I would draw the Fischer projection of threonine.

Step 1. Draw the structural formula of threonine.

Step 2. Draw the longest continuous chain of carbon atoms vertically, with #"COOH"# at the top. Draw horizontal lines to make crosses at #"C-2"# and #"C-3"#.

Step 3. Attach atoms to the bonds.

Arbitrarily put #"NH"_2# on the right of #"C-2"# and #"OH"# on the right of #"C-3"#. Put #"H"# atoms on the left of #"C-2"# and #"C-3"#.

I have a 25 % chance of being right. If I am wrong, I will simply reverse the locations of the #"H"# atoms.

Step 4. Determine the configuration at #"C-2"#.

The order of priority of the functional groups is #"NH"_2 = 1#; #"COOH" = 2#; #"C-3 = 3"#; #H = 4.#

The sequence #"NH"_2 → "COOH" → "C-3"# is clockwise (R).

But the #"H"# atom is pointing forward, so we must reverse the assignment to (R).

We want (S), so we must must interchange #"H"# and #"NH"_2#.

Step 4. Determine the configuration of #"C-3"#.

The order of priority of the functional groups is #"OH = 1"#; #"C-2 = 2"#; #"CH"_3 = 3#; #"H" = 4#.

The sequence #"OH → C-2 → CH"_3# is counterclockwise (S).

But the #"H"# atom is pointing forward, so we must reverse the assignment to (R). That's correct!

The Fischer projection of threonine is therefore

The number of possible stereoisomers is #2^n#, where #n# is the number of stereocentres.

Since threonine has 2 stereocentres, there are #2^n = 4# stereoisomers.

So there are three other stereoisomers of threonine.