**3. Antioxidative enzymes**

#### **3.1 Catalases**

Initially, cyanobacteria were partitioned into two forms, those that have and those that need ascorbate peroxidase [42]. This depended on the perception that the principal bunch searches H2O2 with a peroxidase utilizing a photo-reductant as an electron benefactor. Cyanobacteria have three types of catalases (**Figure 2**) which differ significantly in terms of their structure and amino acid sequence. Bernroitner [43] examined the presence of these three catalases in 44 cyanobacterial genomes and executed a phylogenetic exploration of the enzymatic activities. The findings show that while monofunctional heme-containing catalase (KatE) is the most common type of catalase found in bacteria, archaea, and eukarya, it is extremely rare in cyanobacteria. Only one complete KatE gene was found in *Nostoc punctiforme* PCC73102, whereas pseudogenes (incomplete or fusion genes) were found in *Nostoc sp.* PCC7120, *Cyanothece sp.* ATCC51142, *and Synechococcus elongatus* [43].

KatG bifunctional catalase/peroxidase has both catalases and peroxidase activity. Unlike KatE, it was found in a variety of known cyanobacterial genomes. Cyanobacterial KatGs are known to form a well-segregated clade in the evolutionary representation, implying that KatG evolved in cyanobacterial evolution [43]. Mn catalase (MnCat) is a di-manganese catalase that does not contain heme. Except for *Gloeobacter violaceus* PCC7421, all species have MnCat. It is thought to be found only in diazotrophic cyanobacteria, except for *Gloeobacter violaceus* PCC7421 [43] (**Figure 2**).

**Figure 2.** *Types of catalase enzyme.*

#### **3.2 Superoxide dismutase**

SODs are common metalloenzymes and are classified into four types based on FeSOD, CuZnSOD, MnSOD, and NiSOD. All have metal redox-active centers that, respectively, include Fe (III), Cu (II), Zn (II), Mn (III), and Ni (II/III) at the active site [44]. Cyanobacteria have all four kinds of SOD, and many cyanobacterial species include more than one type of SOD [45]. It should be emphasized that some actinobacteria and archaea have a single gene that, depending on the environment, can either make FeSOD or MnSOD [46]. Cambialistic SOD refers to Fe/MnSOD that exhibits similar activity in Fe- and Mn-bound forms (Sheng et al. 2014). There are currently no known cabalistic Fe/MnSODs. While FeSOD and NiSOD or FeSOD and MnSOD are present in various other single-celled strains, the marine species of *Prochlorococcus* has only one NiSOD [44]. In contrast, strains that are heterocystous, heterotrichous, and flagellated exclusively have iron and manganese forms. Despite having comparable structural characteristics, FeSOD and MnSOD can be identified from one another by structural traits due to the existence of a transmembrane domain, residues mainly for some metals that differ between the two representations, and highly conserved residues found only in the manganese form [47]. Many investigations have found SODs to be involved in protective processes in cyanobacteria.

#### **3.3 Peroxidases**

Ascorbate peroxidase is essential for the detoxification of H2O2 in plants [48]. These enzymes convert H2O2 to monodehydroascorbate and water using ascorbate as the electron source. Ascorbate and dehydroascorbate are produced spontaneously by monodehydroascorbate. Dehydroascorbate reductase converts dehydroascorbate to ascorbate by using glutamine. NADPH-glutamine reductase then regenerates oxidized glutamine. This highlights the importance of the ascorbate-glutamine cycle in plant oxidative stress response. *Nostoc muscorum* PCC 7119 and *Synechococcus* PCC 6311 have both been found to contain ascorbate peroxidase-like activities, and dehydroascorbate reductase and glutamine reductase were both engaged in the regeneration of ascorbate and glutamine, respectively, in *Synechococcus* PCC 7942.

Peroxiredoxins (Prx-s), also known as alkyl-hydro peroxidases, are another widespread group of thiol-explicit cell reinforcement proteins that utilize thioredoxin and other thiol-containing decreasing specialists as electron givers to diminish H2O2, alkyl hydroperoxides, and peroxynitrite [49]. It is believed that peroxiredoxins are crucial for decreasing endogenously produced ROS.
