Why are carboxylic acid anhydrides and halides not found in biological systems?

Answer 1

Halides are found in biological systems - most organisms have tons of dissolved chloride ion, and mammals at least have quite a bit of dissolved iodide.

Are you asking why biological systems don't have HALOGENS? ?Halides are ions (like chloride, aka Cl-) and halogens are the molecular forms (like chlorine, aka Cl2). ?Halogens and acid anhydrides are toxic to biological systems because they are very reactive, and would halogenate and form adducts on proteins, DNA, etc. ?They are also very reactive and would not exist long in the aqueous environment of a biological system.

But fun fact: an enzyme in your thyroid gland (i think called thyroid perioxidase) makes transient amounts of iodine (I2) from iodide (I-) in the synthesis of thyroid hormone. ?But these are very small amounts localized to the thyroid gland, and only transiently.

Answer 2

Carboxylic acid anhydrides and halides are not commonly found in biological systems because they are highly reactive and can cause damage to biological molecules. Cells have evolved mechanisms to use more stable functional groups, like carboxylic acids and esters, for biological processes. Anhydrides and halides are typically used as synthetic intermediates in chemical reactions rather than in biological systems.