Cysteine is an example of an amino acid that has a side chain ($$\text{R}$$ group) of $$\text{CH}_2\text{SH}$$. Structurally, a central carbon atom is bonded to a carboxyl group ($$\text{COO}^-$$), a hydrogen atom ($$\text{H}$$), an amino group ($$\text{NH}_3$$), and the $$\text{CH}_2\text{SH}$$ side chain.

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A side chain, or residue (represented as $$\text{R}$$ in structural formulas), is the variable fourth group bonded to the central $$\alpha$$ carbon of an amino acid. These groups are composed primarily of carbon, hydrogen, oxygen, nitrogen, and sulfur in the form of hydrocarbons, acids, amides, alcohols, and amines. Their unique chemical properties allow them to form hydrogen, ionic, and disulfide bonds, along with polar/nonpolar interactions that are essential for folding proteins into secondary, tertiary, and quaternary structures.

Side Chain

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

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Alanine is an example of an amino acid where the side chain ($$\text{R}$$ group) is a methyl group ($$\text{CH}_3$$). Its structure consists of a central carbon atom attached to a carboxyl group ($$\text{COO}^-$$), a hydrogen atom ($$\text{H}$$), an amino group ($$\text{NH}_3$$), and the methyl side chain ($$\text{CH}_3$$).

Alanine

Serine is an example of an amino acid with a side chain ($$\text{R}$$ group) of $$\text{CH}_2\text{OH}$$. Its chemical structure is composed of a central carbon atom bonded to a carboxyl group ($$\text{COO}^-$$), a hydrogen atom ($$\text{H}$$), an amino group ($$\text{NH}_3$$), and the $$\text{CH}_2\text{OH}$$ side chain.

Serine

Lysine is an example of an amino acid where the side chain ($$\text{R}$$ group) is represented by $$\text{(CH}_2\text{)}_4\text{NH}_3^+$$. Structurally, it features a central carbon atom attached to a carboxyl group ($$\text{COO}^-$$), a hydrogen atom ($$\text{H}$$), an amino group ($$\text{NH}_3$$), and the $$\text{(CH}_2\text{)}_4\text{NH}_3^+$$ side chain.

Lysine

Aspartate is an example of an amino acid containing a side chain ($$\text{R}$$ group) of $$\text{CH}_2\text{COO}$$. Its structure consists of a central carbon atom linked to a carboxyl group ($$\text{COO}^-$$), a hydrogen atom ($$\text{H}$$), an amino group ($$\text{NH}_3$$), and the $$\text{CH}_2\text{COO}$$ side chain.

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