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...

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.