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Questions and Answers
What structural feature is common to all amino acids except proline?
At physiologic pH (7.4), what charge does the carboxyl group of an amino acid acquire?
Which amino acids primarily exhibit hydrophobic interactions due to their side chains?
Which of the following amino acids contains a hydroxyl group that can participate in hydrogen bonding?
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Which amino acids can lose a proton at alkaline pH?
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What type of bonding is primarily involved in the linkage of amino acids within proteins?
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How are amino acids classified based on their side chain properties?
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What determines the clustering behavior of nonpolar amino acids in aqueous environments?
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Which amino acids are considered as proton donors due to their acidic side chains?
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At physiologic pH, which amino acids possess fully ionized and positively charged side chains?
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What characterizes the α-carbon of most amino acids?
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Which amino acids are NOT typically found in the L-configuration in proteins?
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What would you find at the side chain of histidine at physiologic pH?
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Which of the following characteristics is true about glycine?
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What are the charges on the side chains of aspartate and glutamate at physiologic pH?
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What is the significant structural attribute of the D and L forms of amino acids?
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Study Notes
Structure of Amino Acids
- Proteins are the most abundant and functionally diverse molecules in living systems
- Proteins are linear polymers of amino acids
- Each amino acid has a carboxyl group, a primary amino group, and a distinctive side chain (R-group)
- Proline is an exception, it has a secondary amino group
- The α-carbon atom is bonded to the carboxyl group, amino group, and the R-group
- At physiologic pH (7.4) the carboxyl group is dissociated, forming the negatively charged carboxylate ion (COO-) and the amino group is protonated (NH3+)
Classification of Amino Acids Based on Side Chain Properties
- Amino acids are classified according to the properties of their side chains
- Polar amino acids have uneven distribution of electrons (e.g., acids and bases)
- Nonpolar amino acids have even distribution of electrons
Nonpolar Amino Acids
- Do not gain or lose protons
- Do not participate in hydrogen or ionic bonds
- Side chains promote hydrophobic interactions
- In aqueous solutions (polar environments), side chains of nonpolar amino acids tend to cluster together in the interior of the protein
- This phenomenon is known as the hydrophobic effect
Uncharged Polar Amino Acids
- Have zero net charge at neutral pH
- Side chains of cysteine and tyrosine can lose a proton at alkaline pH
- Serine, threonine, and tyrosine contain a polar hydroxyl group (OH-) that can participate in hydrogen bond formation
- The side chains of asparagine and glutamine contain a carbonyl group and an amide group that can participate in hydrogen bonds
Acidic Amino Acids
- Aspartic and glutamic acids are proton donors
- At physiologic pH, their side chains are fully ionized and contain a negatively charged carboxylate group
- Aspartic acid is also known as aspartate, glutamic acid is also known as glutamate
Basic Amino Acids
- The side chains of basic amino acids accept protons
- At physiologic pH, the side chains of lysine and arginine are fully ionized and positively charged
- Histidine is weakly basic and largely uncharged at physiologic pH
- When histidine is incorporated into a protein, its side chain can be either positively charged or neutral depending on the environment
Abbreviation and Symbols of Amino Acids
- Each amino acid name has an associated three-letter abbreviation and a one-letter symbol
Optical Properties of Amino Acids
- The α-carbon of an amino acid is attached to 4 different chemical groups and is chiral
- Glycine is an exception because its α-carbon has 2 hydrogen substituents, making it optically inactive
- Amino acids that have an asymmetric center at the α-carbon can exist in 2 forms, designated D and L, that are mirror images of each other
- These forms are termed stereoisomers, optical isomers, or enantiomers
- All amino acids found in proteins are of the L-configuration
- D-amino acids are found in some antibiotics and in plant and bacterial cell walls
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Description
Explore the fundamental structure and classification of amino acids. This quiz covers key concepts such as the unique properties of amino acids, their distinct side chains, and the role of polar and nonpolar amino acids in protein function. Test your knowledge of these essential components of proteins!
