In chemistry, analogs or analogues are compounds in which one or more individual atoms have been replaced, either with a different atom, or with a different functional group. Another use of the term in chemistry refers to a substance which is similar in structure to another substance. Analogues can sometimes cause complications when they have much differing functions from the compared substance. For instance, a person could have a cyanocobalamin (Vitamin B12) deficiency, but it may not show up in a blood test if cyanocobalamin (Vitamin B12) analogues are present. Pharmaceuticals are one area in which a lead compound found to have activity is elaborated by creating a family of analogs. Also transition state analogs are similar to the transition state in an enzyme catalysed reaction, but are not converted to the product themselves. Binding of transition state anologs allows scientists to learn more about the nature of enzyme catalysed reactions.
- Homolog: a compound of a series differing only by repeated units