Carboxylases incorporate CO₂, or its hydrated form HCO₃⁻, into organic substrates. Assimilatory carboxylases combine CO₂ to a diversity of carbon sources allowing for growth on intransigent, poorly activated organic compounds such as acetone. Acetone carboxylase catalyzes the adenosine triphosphate-dependent carboxylation of acetone to acetoacetate. Acetone carboxylase undergoes a large conformational shift to allow phosphorylated intermediates, carboxyphosphate and phosphenolacetone to interact with the Mn bound active site. The coordinated Mn orients the intermediates for the carboncarbon bond formation producing acetoacetate. Biochemical, molecular, structural, and spectroscopic studies have shown how acetone carboxylase functions and gives clues on how this unique biotin-independent carboxylation mechanism couples ATP hydrolysis to bicarbonate-dependent carboxylation. This article highlights the body of biochemical, spectroscopic, and structural work leading to insights into the catalytic mechanism of acetone carboxylase.