## Materials and Methods
### Activity Assays of UK114 Deiminase Activity  
The deaminase activity of UK114 was determined by measuring the decrease of velocity of semicarbazone formation from various \(\alpha\)-imino acids generated in situ by l-amino acid oxidase (LAAO) from the corresponding l-amino acid (Figure 3a). Reaction conditions were similar to those that were described by [6] in order to allow for direct data comparison. Briefly, assay mixtures contained the l-amino acid (5 mM), neutralized semicarbazide (10 mM), catalase (10 \(\upmu\)g/mL), snake venom LAAO (5-800 \(\upmu\)g/mL, Sigma-Aldrich, Merck KGaA, Darmstadt, Germany), and varying amounts of UK114 in 50 mM sodium pyrophosphate buffer, pH 8.7. The reaction was started by either addition of the l-amino acid or LAAO and was monitored spectrophotometrically in a HP 8452A diode array spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) by collecting spectra every 5-20 s, depending on the experiments. In the experiments done with different amino acids, the concentration of l-amino acid oxidase (5-800 \(\upmu\)g/mL) was selected in order to obtain a similar initial velocity of semicarbazone formation in the absence of UK114 of 22-25 \(\upmu\)M/min as monitored at 25 \({}^{\circ}\)C at 248 nm (\(\upvarepsilon_{248}\), 10.3 mM\({}^{-1}\) cm\({}^{-1}\)). Again, the conditions were chosen to reproduce those described in [6] for better comparison of the results. Reactions (0.15-1 mL) were set up in quartz cuvettes (1 cm light path). LAAO activity was also measured by monitoring the initial velocity of hydrogen peroxide production in a coupled assay with horse radish peroxidase (HRP) and o-dianisidine, as previously described for DAAO assays [61]. The assays contained the given amino acid (5 mM or varying concentrations for determination of the steady-state kinetic parameters \(V_{\text{max}}\) and \(K_{\text{m}}\)), o-dianisidine (0.4 mM), HRP (10 \(\upmu\)g/mL) and varying concentrations of LAAO (5-800 \(\upmu\)g/mL). The increase of absorbance at 436 nm was monitored. An extinction coefficient of 11.6 mM\({}^{-1}\) cm\({}^{-1}\) was applied. Under these conditions, by comparing the initial velocity of hydrogen peroxide formation with that of semicarbazone formation, 50-60 percent of the imino acid appears to be trapped as the semicarbazone (Figure S4 and Table S2). This result is in agreement with the finding that the semicarbazide concentration yielding 50% of the expected semicarbazone was 11.6 mM at pH 8.7, where the imino acid stability is high (\(k_{\text{hydrolysis}}\) 1.8 min\({}^{-1}\) vs. 6 min\({}^{-1}\) at pH 7.7), as reported in the early work by [PERSON] and [PERSON] [11], on which this assay is based. However, semicarbazide concentration was kept constant at 10 mM in order to allow direct comparison with data from other laboratories and to avoid inhibition of LAAO that is observed at higher concentrations. For the same reason, l-Leu concentration was kept constant at 5 mM, although it appears to inhibit LAAO (Figure S4). Control experiments showed that UK114 at concentrations up to 20 \(\upmu\)M did not interfere with the initial velocity of hydrogen peroxide production from l-Leu by LAAO. The possibility to generate 2-imino acids from d-amino acids and pig kidney d-amino acid oxidase was tested by setting up activity assays for the measurement of the initial velocity of oxidation of selected d-amino acids (d-Leu, d-Ala, d-Ser, d-Thr, and d-Lys) as H\({}_{2}\)O\({}_{2}\) production in the HRP-coupled assay and as semicarbazone formation. Assay conditions were identical to those described for d-amino acids and LAAO. DAAO was purified from pig kidneys, as previously described [62]. Semicarbazone formation was observed using d-Ala, d-Leu, but not d-Ser, d-Thr, or d-Lys, indicating that either semicarbazide cannot react with the imino acid that is formed from the latter three amino acids or (more likely) their hydrolysis takes place prior to release from DAAO active site [54].