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BIO NMDCAT Proteins
nmdcat.online June 29, 2026

The fundamental structural feature common to all standard amino acids found in proteins is the presence of

A. An amino group and a carboxyl group attached to the same α-carbon atom
✓
B. A sulfhydryl group and a hydroxyl group on the β-carbon
C. A purine ring and a phosphate group
D. An aromatic ring and a guanidinium group

📝 Explanation

All 20 standard amino acids (except proline, which is an imino acid) are α-amino acids. They contain a central α-carbon to which an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R-group are attached.

📖 Additional Information

  • An amino group and a carboxyl group attached to the same α-carbon atom
  • A sulfhydryl group and a hydroxyl group on the β-carbon
  • A purine ring and a phosphate group
  • An aromatic ring and a guanidinium group

All 20 standard amino acids (except proline, which is an imino acid) are α-amino acids. They contain a central α-carbon to which an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R-group are attached.

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