*4.1. Antioxidant Activity*

An imbalance in the creation and neutralization of free radicals causes oxidative stress, which leads to a variety of degenerative illnesses [229]. Several free radicals, particularly reactive oxygen species (ROS), were created in living organisms as a result of metabolic activity, and hence have an impact on health (Figure 7). ROS were formed in form of hydrogen peroxide (H2O2), superoxide radical (O2 <sup>−</sup>), hydroxyl radical (·OH), or nitric oxide (NO). Oxidative stress causes unconscious or prominent enzyme activation, as well as oxidative damage for cellular systems [230]. ROS attack or damage important macromolecules including lipids membrane, proteins, or DNA, resulting in a variety of conditions include inflammatory or neurodegenerative diseases, diabetes mellitus, cancer, or severe tissue injuries [231,232] (Figure 7).

**Figure 7.** Damage caused via reactive oxygen species (ROS). Adapted from ref. [233] obtained from mdpi journals.

Antioxidants may have a favorable impact on human health because they may protect the body from damage caused via reactive oxygen species (ROS) [234]. To determine the antioxidant activity of marine derived bioactive peptides, researchers used electron spin resonance spectroscopy as well as intracellular free-radical scavenging assays.

ROS can produce several detrimental biological events, such as DNA oxidative lesions, membrane peroxidation, structural changes in proteins and functional carbohydrate, and so on. All of these structural and functional changes have direct clinical effects, speed up the aging process while also causing pathological phenomena, such as increased capillary permeability and impaired blood cell function [235]. All of these antioxidant systems behave differently depending on their structure and characteristics, whether hydrophilic or lipophilic, and where they are located (intracellular or extracellular, in cell or organelles membrane, in the cytoplasm, etc.). All of the above processes work in concert to establish a network that protects live cells from the damaging impacts of reactive oxygen species (ROS).

Figure 8 represents reactive oxygen species and neutralization with several biomolecules [236]. Hydrophobic amino acids in peptide chain contribute to their possible antioxidant effect [237]. Seaweeds also include nutraceutical and medicinal chemicals such phenols that have antioxidant activity. Polyphenols generated by seaweeds received special attention because their pharmacological action and broad range of health-promoting advantages, as polyphenols play a vital role in a variety of seaweed biological activities. Seaweed phenolic compounds are metabolites with hydroxylated aromatic rings that are chemically defined as molecules. In this context, Al-Amoudi et al. [25] stated that sulfated polysaccharides from three marine algae (Phaeophyta *Sargassum crassifolia* (S), Chlorophyta *Ulva lactuca* (U) and Rhodophyta *Digenea simplex* (D) exert antioxidant activity.

**Figure 8.** Reactive oxygen species and neutralization by several biomolecules.
