Key takeaway
Urease is the smell machine — a bacterial enzyme that converts urine's urea into ammonia gas at high speed.
Behind the sharp ammonia smell of old urine is a single enzyme: urease. Produced by bacteria colonizing the deposit, urease is the molecular machine that converts urine’s urea into ammonia gas — and it is remarkably fast. Understanding urease explains the timing, the intensity, and the persistence of urine odor.
What urease does
Urease catalyzes one reaction: splitting urea (CO(NH₂)₂) into ammonia (NH₃) and carbon dioxide (CO₂). One urea molecule yields two ammonia molecules. The reaction is what turns mildly-odorous urea into the sharp gas that dominates aged urine smell.
Urease is also notable for speed — it is one of the most efficient enzymes known, catalyzing the reaction at extremely high rates.
Urease is a chemical sawmill — it takes in urea and cuts out ammonia gas, fast.
Where it comes from
Urease is not in the urine — it is produced by bacteria that colonize the deposit afterward. Many common environmental and skin bacteria produce urease as part of their nitrogen metabolism. The bacteria arrive in the deposited urine and begin producing the enzyme.
This is why fresh urine does not smell like ammonia yet — the enzyme has not been produced yet.
Why it is so fast
Urease has a nickel-containing active site evolved for this specific reaction — it is extraordinarily efficient. Once a bacterial colony is established and producing urease, urea conversion is rapid. This is why the ammonia smell ramps up so quickly in the first day or two.
The pH cascade it drives
The ammonia urease produces raises the deposit’s pH — the residue becomes alkaline. That alkaline shift changes the bacterial community (alkaline-tolerant species take over), releases more ammonia as gas, and changes the odor profile. Urease does not just make ammonia — it remodels the whole deposit’s chemistry.
Why it matters for treatment
Urease explains the timeline — the delay before sharp odor, the peak, the fade as urea depletes. It also explains the treatment targets: remove the urea substrate (clean the residue) or the bacteria producing the enzyme (sanitize or enzymatic treatment), and the ammonia production stops.
Ventilating the ammonia does not help — the urease is still running.
Urease beyond urine
Urease matters elsewhere too — it is the same enzyme in litter box ammonia production, in any urine-contaminated material, and in medical contexts (some pathogenic bacteria use it). Wherever urea meets urease-producing bacteria, ammonia follows.
Where PawFresh fits
PawFresh’s enzymatic treatment removes the organic residue — the substrate and the bacterial habitat — that sustains the urease reaction.
Sources & further reading
- Urease biochemistry and bacterial ureolysis research general-science
Frequently asked questions
What makes the ammonia in urine smell?
Urease — a bacterial enzyme that splits urea into ammonia. It is produced by bacteria colonizing the deposit after deposition.
Why doesn't fresh urine smell like ammonia?
The urease has not been produced yet — bacteria need time to colonize and produce the enzyme before ammonia forms.
Why does the sharp smell fade then return?
Urea depletes (fade), but humidity re-wets the residue and restarts bacterial urease production (return).
Can you stop urease?
Remove the substrate (urea residue) or the bacteria producing it — enzymatic treatment digests the residue the colony depends on.