Answer and explanation;
-Aba is a suitable replacement because Aba and Cys have approximately the same sized side chain and are similarly hydrophobic.
-However, Aba cannot form disulfide bonds so it will not be a suitable replacement if these are required.
-Alpha-Aminobutyric acid is biosynthesized by transaminating oxobutyrate, a metabolite in isoleucine biosynthesis.
All living creatures are classified into systems and sub-systems based on their similar characteristics. They are divided from bigger groups into smaller groups based on the detail of their similarities i.e. how they look, move, reproduce and how they relate to each other. A practical way of understanding the classification of living organisms is that organisms are linked to other similar organisms via family trees. The classification of all living creatures includes at least four levels: order, families, genus and species.