Tetrose

A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 (aldotetroses) or a ketone functional group in position 2 (ketotetroses).^[1][2]

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  D-Erythrose

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  D-Threose

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  D-Erythrulose

The aldotetroses have two chiral centers ("asymmetric carbon atoms") and so 4 different stereoisomers are possible. There are two naturally occurring stereoisomers, the enantiomers of erythrose and threose having the D configuration but not the L enantiomers. The ketotetroses have one chiral center and, therefore, two possible stereoisomers: erythrulose (L- and D-form). Again, only the D enantiomer is naturally occurring.

References

1. Thisbe K. Lindhorst (2007). Essentials of Carbohydrate Chemistry and Biochemistry (1 ed.). Wiley-VCH. ISBN 3-527-31528-4.
2. John F. Robyt (1997). Essentials of Carbohydrate Chemistry (1 ed.). Springer. ISBN 0-387-94951-8.