Questions and Answers
Which group do non-polar, aliphatic R groups belong to?
All amino acids are polar in nature.
False
The one-letter symbol for tryptophan is _____
W
What are the three types of polar amino acids?
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Match the following amino acids with their corresponding R-group classification:
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What is the isoelectric point (pI)?
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Proteins behave as zwitterions at their isoelectric point.
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What are the ionizable groups typically found in amino acids?
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The amino acid __________ has an isoelectric point of 2.77.
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Match the following amino acids with their isoelectric points (pI):
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Study Notes
Classification of Amino Acids
- Amino acids can be classified into five groups according to R-group properties: non-polar aliphatic, non-polar aromatic, and polar groups.
- Non-polar, aliphatic R groups are neutral, hydrophobic, and typically found in the interior of proteins.
- Non-polar, aromatic R groups have neutral, non-polar side chains that engage in hydrophobic interactions.
- Polar amino acids can be categorized as:
- Polar-neutral: solvent-friendly with neutral side chains.
- Polar-acidic: possess a carboxyl group in their side chains.
- Polar-basic: contain an amino group in their side chains.
Amino Acid Nomenclature
- Common names and shorthand notations (three-letter and one-letter symbols) are used for amino acids.
- Three-letter abbreviations normally start with a capitalized first letter, followed by two lowercase letters.
- When multiple amino acids share the same initial letter, the most abundant one is represented by that letter.
- Amino acids are most accurately represented in zwitterion form (having both positive and negative charges).
Isoelectric Point (pI)
- The isoelectric point (pI) is defined as the pH at which a compound has no net charge.
- Each amino acid has a specific pI based on its side chain properties, affecting its solubility and behavior in solution.
Ionizable Groups in Amino Acids
- Most amino acids possess two ionizable groups (carboxyl and amino groups) and can exhibit specific titration curves.
- Amino acids with ionizable side chains feature curves with three distinct leveling-offs due to the additional dissociable groups.
Peptides and Proteins
- Proteins exist as zwitterions and have an isoelectric point (pI) at which they carry no net charge.
- Hemoglobin: pI of 6.8, as it has balanced acidic and basic side chains.
- Serum albumin: pI of 4.9, characterized by more acidic side chains and is predominant in plasma.
Peptide Formation
- Peptides are constructed via covalent peptide bonds formed between the carboxyl group of one amino acid and the amino group of another, releasing water.
- Emil Fischer first elucidated the peptide bond concept in 1902.
Writing and Nomenclature of Peptides
- Peptides are denoted from N-terminal to C-terminal, with convention emphasizing the -NH3+ start and -COO- end.
- IUPAC nomenclature requires replacing the suffix of amino acid names (except for tryptophan) with -yl for all but the C-terminal amino acid, which retains its full name.
Examples of Peptide Names
- For the peptide Ala-Phe-Ser:
- N-terminal: Alanine
- Middle: Phenylalanine
- C-terminal: Serine
- IUPAC name: Alanylphenylalanylserine.
Practice Exercise
- Understanding the number of peptide bonds and classifying them is essential for analyzing peptide structures.
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Description
Test your knowledge on the classification of amino acids based on their R-groups. This quiz focuses on the properties, particularly polarity, of different amino acid groups. Understand how these classifications impact their interactions in biological systems.
