Beyond its role as a key building block of proteins, the role of threonine in human metabolism is uncertain. While it behaves similar to serine in that its hydrogen is easy to remove, and thus it acts as a hydrogen donor in enzymes, it does not have a reputation as being involved in catalytic functions in enzymes, such as serine has in trypsin and chymotrypsin. Both threonine and serine are very hydrophilic, therefore the outer regions of soluble proteins tend to be rich with them.
Nonetheless, threonine is an essential constituent of proteins. For proteins to function correctly, the amino acid constituents must be lined up in a particular order. Otherwise, the protein may not fold into the precise three-dimensional shape needed to function and would not have the exact properties needed. This shows the complex coordination in nature.
An element of human responsibility is also reflected, for one's diet must contain sufficient threonine to produce the proteins. While threonine is so common in proteins that a deficiency is unlikely, nonetheless it points out the need to demonstrate self-discipline. One must, to have a good diet, interact with a variety of plants and animals to receive the correct nutrition. Unlike some organisms that can survive on one source, the nature of humans requires interaction with and dependency upon a great deal of other organisms in nature.