**3. Molecular structure of β-defensin-2**

The HBD2 is a small cationic peptide with a positive net charge (+6). It has 41 amino acid residues; the complete gene is approximately 4 kb. It consists of six cysteines in positions 1–5, 2–4 and 3–6, joined by three disulfide bonds. Its secondary structure consists of an N-terminal region linked to an alpha helix, three β-strands arranged in an antiparallel sheet and a C-terminal region [35–37] (**Figure 1**). Its structure has an amphipathic nature, with hydrophilic and hydrophobic amino acids on the surface of protein; it is stabilized by the disulfide bonds, which protect it from degradation by proteases [38, 39]. The alpha helix is also stabilized by the disulfide bonds (I and V) and by the first beta sheet that has a domain (Gly-X-CysIV), which is responsible for the native structure and the correct folding of the peptide [40, 41]. The N-terminal region binds to the membrane of microorganisms.

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**Figure 2.**

*peptide [3].*

*Multifunctional Activity of the β-Defensin-2 during Respiratory Infections*

The specific conformation of N-terminal region of β-defensins may be important

*Secondary structure of HBD-2 that consists of an N-terminal region linked to an alpha helix, three β-strands* 

The C-terminal region consists mainly of cationic amino acids (lysine and arginine) that are distributed asymmetrically with their positive charges and are

The gene codify for HBD2 is located on chromosome 8p23. The gene is approximately 4 kb and composed of a 5'and 3' non-translatable region, two exons separated by an intron. The first exon has 81 bp and codes for the signal peptide and the second exon has 238 bp and codes for a short anion segment called propeptide and the mature peptide [3, 25, 42]. The 5′ region of HBD2 has specific sites that bind to the transcription factor kB (NF-kB). In addition, there are sites that bind to other transcription factors such as C/EBP, and NF-IL-6, which are important for their

The HBD2 have a polymorphic nature, and the number of copies of the gene varies in each individual [45]. It has been reported that there are 2–12 copies per diploid genome, and the number of copies is related to the level of expression. It has been suggested that these variations may have consequences on the function of immune system. Some infectious and inflammatory diseases are related to the

*Genetic structure of HBD2. It encompasses untranslated 5 'and 3' regions and two exons separated by an intron. The first exon codes for the signal peptide and the second exon codes for the propeptide and the mature* 

*DOI: http://dx.doi.org/10.5772/intechopen.80611*

for the biological properties of these proteins. [37].

*arranged in an antiparallel sheet and a C-terminal region [41].*

number of copies of the HBD2 gene [44, 46] (**Figure 2**).

important in antimicrobial activity [3, 38, 40].

**4. Genetic structure of β-defensin-2**

expression [42–44].

**Figure 1.**

*Multifunctional Activity of the β-Defensin-2 during Respiratory Infections DOI: http://dx.doi.org/10.5772/intechopen.80611*

**Figure 1.**

*Immune Response Activation and Immunomodulation*

**2. General characteristics and classification**

the positions of the cysteines of C1-C5, C2-C4 and C3-C6 [26].

bacteria, fungi and viruses [1, 24].

system [1, 28].

the infectious process [1, 28].

**3. Molecular structure of β-defensin-2**

respiratory viruses (influenza, respiratory syncytial virus, rhinovirus) [21–23]. The mechanisms of viral inactivation vary and include not only direct binding of the virus to the peptide but also indirect methods of inactivation via intracellular modulation of the viral replication, modulation of signaling pathways necessary for antiviral effects, and recruitment of immune cells that contribute to antiviral activity [2, 22]. This revision chapter focuses on the structural and general characteristics of HBD2, multifunctional activities and expression in respiratory diseases. We present some studies concerning the effect of respiratory viruses and their relationship with HBD2, mechanisms of action and their relevance as therapeutic agents.

Defensins are a family of antimicrobial peptides that form part of the innate and adaptive immune system and constitute the first line of host defense against microorganisms. It has shown the broad antimicrobial activity spectrum against

The defensins are small cationic peptides of 28–42 amino acids, characterized by a β-sheet structure linked by tree disulfide bonds, which are formed by six cysteine residues [2, 25]. Based on the distribution of their cysteines and disulfide bonds, defensins are classified into two groups in humans: α- and β-defensins [1]. α-defensins have 29–35 amino acids and the positions of the cysteines are C1-C6, C2-C4 and C3-C5, while the β-defensins are composed of 38–42 amino acids with

The α-defensins are expressed mainly in neutrophils and called human neutrophil peptides with four types (HNP1–4). There are other α-defensins (HD5 and HD6), known as enteric defensins, that are expressed in Paneth cells in the small intestine [27]. The expression of HBD6 has also been confirmed in the female genitourinary

β-defensins are mainly expressed in epithelial cells throughout the body, including mucous membranes and skin [29–32]. Four types (HBD1–4) have been identified in humans, but several analyses indicate that there may be approximately 31 types, of which HBD5 and 6 are expressed in epididymis with their importance in defense against infections. The other defensins are known only for their antimicrobial activity [33, 34]. The β-defensin-1 is expressed constitutively, while the other three (HBD2–4) are expressed by the effect of proinflammatory cytokines or during

The HBD2 is a small cationic peptide with a positive net charge (+6). It has 41 amino acid residues; the complete gene is approximately 4 kb. It consists of six cysteines in positions 1–5, 2–4 and 3–6, joined by three disulfide bonds. Its secondary structure consists of an N-terminal region linked to an alpha helix, three β-strands arranged in an antiparallel sheet and a C-terminal region [35–37] (**Figure 1**). Its structure has an amphipathic nature, with hydrophilic and hydrophobic amino acids on the surface of protein; it is stabilized by the disulfide bonds, which protect it from degradation by proteases [38, 39]. The alpha helix is also stabilized by the disulfide bonds (I and V) and by the first beta sheet that has a domain (Gly-X-CysIV), which is responsible for the native structure and the correct folding of the peptide [40, 41]. The N-terminal region binds to the membrane of microorganisms.

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*Secondary structure of HBD-2 that consists of an N-terminal region linked to an alpha helix, three β-strands arranged in an antiparallel sheet and a C-terminal region [41].*

The specific conformation of N-terminal region of β-defensins may be important for the biological properties of these proteins. [37].

The C-terminal region consists mainly of cationic amino acids (lysine and arginine) that are distributed asymmetrically with their positive charges and are important in antimicrobial activity [3, 38, 40].
