Homoserine is an amino acid which is used by plants and bacteria to make methionine, threonine, and isoleucine (three of the twenty amino acids used to make proteins). Homoserine is similar to the amino acid serine (another of the twenty amino acids used to make proteins), except it has an extra methylene group. It is also formed when cystathionine is metabolized into the amino acid cysteine.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.
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