Amino Acids and Proteins

Proteins are biopolymers made by linking 20 types of α-amino acids through peptide bonds. They are central to all life processes, including enzymes, antibodies, muscles, hair, and nails.

Basic Knowledge

α-Amino acids: A structure in which -NH₂, -COOH, -H, and a side chain -R are attached to the central carbon (α-C). The 20 types of standard amino acids with different R groups make up natural proteins. Except for glycine (R=H), they have an asymmetric carbon and only the L form is used in living organisms.
Classification:
・Neutral: glycine (G), alanine (A), valine (V), leucine (L), isoleucine (I), serine (S), threonine (T), etc.
・Acidic (a side-chain -COOH): aspartic acid (D), glutamic acid (E)
・Basic (a side-chain -NH₂): lysine (K), arginine (R), histidine (H)
・Sulfur-containing: cysteine (C), methionine (M)
・Aromatic: phenylalanine (F), tyrosine (Y), tryptophan (W)
Zwitterions: In aqueous solution, they simultaneously have -NH₃⁺ and -COO^-. The pH at which they are electrically neutral is the isoelectric point (pI), and they can be separated by electrophoresis.
Peptide bond: Water is removed from the -COOH group of one amino acid and the -NH₂ group of another to form an -CO-NH- bond (corresponding to an amide bond). Multiple bonds form a peptide, and long chains form a protein.
Detection reactions:
・Ninhydrin reaction: amino acids/peptides become blue-violet (detection of α-amino groups).
・Biuret reaction: tripeptides or longer peptides + CuSO₄ (basic solution) produce reddish purple (detection of -CO-NH- bonds).
・Xanthoproteic reaction: aromatic amino acids turn yellow with concentrated nitric acid (orange in alkali); nitration of the aromatic ring occurs.

Further Study (Principles and Applications)

Higher-order structures of proteins:
・Primary structure: amino acid sequence, determined by DNA in genes
・Secondary structure: α-helix and β-sheet, stabilized by hydrogen bonds
・Tertiary structure: three-dimensional structure of one entire polypeptide, stabilized by hydrophobic interactions, S-S bonds (disulfides between cysteines), hydrogen bonds, and ionic bonds
・Quaternary structure: association of multiple subunits (hemoglobin = tetramer of 2α + 2β chains)
Denaturation: Heat, acids, bases, organic solvents, and heavy metals disrupt higher-order structures (coagulation of egg white, hair permanents, etc.).
Essential amino acids: The 9 amino acids that humans cannot synthesize in the body (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and histidine). They must be obtained from food.
Enzymes: Protein catalysts that lower activation energy and accelerate reaction rates by 10⁶〜10¹⁷ times. They have substrate specificity, an optimum temperature (about 37℃), and an optimum pH depending on the site (stomach pepsin pH 2; intestinal trypsin pH 8). Examples include amylase (starch digestion), pepsin (protein digestion), and lipase (fat digestion).

Points of Caution (Common Confusions and Frequent Errors)

① The biuret reaction requires a tripeptide or longer peptide (at least two peptide bonds); a dipeptide is negative. ② The xanthoproteic reaction is positive only for amino acids with aromatic side chains (Phe, Tyr, and Trp). ③ The isoelectric point of neutral amino acids is around pH 5.5〜6.3; acidic amino acids have a low pI (Asp pI=2.77), while basic amino acids have a high pI (Lys pI=9.74). ④ An S-S bond is a disulfide bond formed by two cysteine molecules; it is broken and re-formed during a hair permanent.

Practice

1. Explain why α-amino acids become zwitterions in aqueous solution, using structural formulas.
2. Write the general structure of a dipeptide using glycine and alanine as examples.
3. What is protein denaturation? Name five factors that cause it.