Recent Topics in Antioxidant Research

**503**

(OH•

**Chapter 24**

**Abstract**

Environments

narrow main channel, bottleneck size

tion of harmful ROS, such as OH•

conduct their metabolisms.

proteins, and lipids, from strongly harmful OH•

**1. Introduction**

Evolutionary Strategies of

Highly Functional Catalases

for Adaptation to High H2O2

Enzymatic evolutionary strategies for adaptation to a high H2O2 environment have been evaluated using catalases with high catalytic efficiency isolated from two H2O2-tolerant bacteria, *Exiguobacterium oxidotolerans* and *Psychrobacter piscatori*. The entrance size of the narrow main channel in catalase has been estimated by determining the formation rate of the intermediate state of peracetic acid (b), which is a larger substrate than H2O2 versus that of catalase activity with H2O2 (a) (calculated as b/a). The ratio of b/a in *E*. *oxidotolerans* catalase (EKTA) is much higher than that of *P*. *piscatori* catalase (PKTA). To elucidate the structural differences between the catalases, the amino acids present in the main channel have been compared between the two catalases and other catalases in the database. The combination of amino acid residues, which contribute high catalytic efficiency in the narrow main channel of EKTA were different from those in PKTA. In this review, we discuss strategic differences in the elimination of high concentration of H2O2 owing to differences in the phylogenetic positions of catalases. In addition, we describe the relationships between the environmental distributions of genera involved in H2O2-resistant bacteria and their catalase functions based on the main channel structure of catalase.

**Keywords:** H2O2-tolerant bacteria, *Exiguobacterium*, *Psychrobacter*, *Vibrio*, catalase,

Oxygen is important for metabolism, acting as a terminal electron acceptor in aerobic bacteria, and these bacteria produce intracellular reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide (O2•−), and hydroxyl radical

) as by-products of oxygen metabolism [1–4]. H2O2 is not a strongly harmful substance; however, the presence of H2O2 in bacterial cells may lead to the genera-

of catalase is critical for the protection of cellular components, such as DNA, RNA,

intracellular catalases is important for the metabolism of aerobic microorganisms to

, via the Fenton reaction. Therefore, the presence

[5–7]. Moreover, the production of

*Isao Yumoto, Yoshiko Hanaoka and Isao Hara*

## **Chapter 24**
