Lyases are a class of enzymes that break down (lyase: ly- = lysis, -ase = enzyme) bonds like $\ce{C-C, C-S}$ and so on (except hydrolysis), and can eliminate molecules like $\ce{H2O}$ or $\ce{CO2}$. They are referred to as synthases, respectively, some authors categorize synthases among lyases.

My question is, (if word "synthesis" still means joining two substances into one more complex by creation of bonds between them), why are synthases placed among lyases if lyases actually break down bonds and synthases are (by their name) supposed to join, create bonds? Respectively, why was their name derived from 'synthesis' if their role actually is to break down?

Lastly I have even seen that JCBN (Joint Commission on Biochemical Nomenclature) dictated that "synthase" may be used for any enzyme catalysing synthesis, and that even without paying attention to whether the enzyme uses NTP like ATP (ligases = synthetases) or not (synthases).

I'd like to let everybody know that I understand the difference between synthases and synthetases, and that this difference isn't my point of question. I also do not controvert the fact that in the literature synthases are referred to as lyases, but somehow I kind of miss logic of naming enzymes responsible for breakdown after 'synthesis' which is actually opposite reaction than analysis. Until today I thought 'synthesis' stands for joining (syn-) and 'analysis' for breakdown into simpler products.

Here I attach in the images the answers from Wikipedia and forums for you to read.

answer from Quora

  • $\begingroup$ Wikipedia says: Lyase catalyses breaking, elimination. And synthase catalyses synthesis process. $\endgroup$
    – Diana51
    Jun 14 '19 at 15:37

Enzymes always catalyze reactions in both directions (all catalysts do). When an enzyme is first discovered and named, it is not always clear which direction is physiologically relevant (and sometimes both are).

There is a nice answer on StackExchange Biology talking about the enzyme pyruvate kinase, which acts as a phosphatase (i.e. in the opposite direction than the name suggests) in glycolysis. In the lab, you can study the reaction in both directions, depending on the concentrations of species present.

For the class of enzymes you are talking about, the same idea applies. A lyase can work in the other direction, as a synthase, and vice versa. If you want to classify enzymes in terms of mechanism and structure, there is no reason to separate the lyases from the synthases. They are expected to be highly similar or identical (if we are just using two names for the same enzyme).


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