**1. Introduction**

Leucine dehydrogenase (LeuDH; EC 1.4.1.9), a NAD<sup>+</sup> dependent oxidoreductase, which catalyzes reversible L-leucine and other branched chain L-amino acids deamination reaction to the formation of the corresponding α-keto acid [1]. The enzyme was first identified in *Bacillus cereus* [2], and then was found in some microorganisms *Bacillus licheniformis* [3], *Bacillus sphaericus* [4], *Citrobacter freundii* [5], and *Laceyella sacchari* [6]. Moreover, crystal structures of the LeuDH from *Sporosarcina psychrophila* [7] and *Bacillus sphaericus* have been described [8].

LeuDH is used as a biocatalyst to format amino acids for using in the pharmaceutical industry by catalyzing the corresponding α-keto acids [9]. However, some of α-keto acids are unstable and degraded during prolonged incubation at moderate temperatures, such as 37 ◦C [10]. Importantly, cold-adapted enzymes that exhibit high levels of activity at room temperature (20–25 ◦C) should be

useful for converting such unstable α-keto acids. What is more, cold-adapted enzymes have better conversion rates, the specificity of substrate and product, fewer by-products, which are required in the modern industry [11]. Although many LeuDHs have already been characterized, only a few cold-adapted LeuDH have been reported, such as LeuDH from *Alcanivorax dieselolei* [12] and *Sporosarcina psychrophila* [7].

Antarctic sea-ice, due to its specific geographical location and climate, is considered as an extreme environment on the earth. To develop the ability to withstand the extreme environment, sea-ice microorganisms have evolved several adaptive strategies and would be the new and promising microbial sources of cold-adapted enzymes. In our previous studies, some cold-adapted enzymes were isolated from Antarctic sea-ice bacteria and had become interesting for industrial applications [13,14]. It is well-known that L-*tert*-leucine and its derivatives are useful as pharmaceutical active ingredients and chiral auxiliaries, while LeuDH is a key enzyme for the enzymatic production of L-*tert*-leucine. Here, we briefly describe the homology modeling, expression, and characterization of cold-adapted LeuDH from Antarctic sea-ice bacterium. This LeuDH had unique properties make it good candidate for future medical and pharmaceutical industry applications.
