Biochemistry Chapter 3: Amino Acids and Peptides

Questions and Answers

What functional groups are present in amino acids?

Amino and hydroxyl groups

Amino and carboxyl groups (correct)

Hydroxyl and carbonyl groups

Carboxyl and sulfhydryl groups

What distinguishes L-amino acids from D-amino acids?

Their configuration relative to D-glyceraldehyde (correct)

The number of peptide bonds formed

The presence of an additional amino group

The type of side chain attached to the alpha carbon

Which ion predominates at pH 7 for amino acids?

Neutrally charged molecule

Carboxylate anion (correct)

Cation

Dication

What is the role of the side chain group (R) in amino acids?

It influences the amino acid's identity Signup and view all the answers

Which of the following best describes a peptide bond?

A bond between the carboxyl group of one amino acid and the amino group of another Signup and view all the answers

Which property allows amino acids to act as both acids and bases?

Existence of both amino and carboxyl groups Signup and view all the answers

What is the significance of the three-dimensional structure of amino acids?

It determines the biological activity of proteins Signup and view all the answers

Which statement is correct regarding the stereochemistry of amino acids?

L- and D-amino acids are optical isomers of each other. Signup and view all the answers

What is formed when an amino acid is in its zwitterion form?

It has equal positive and negative charges. Signup and view all the answers

Which amino acids are exclusively found in connective tissue proteins like collagen?

Hydroxylysine and hydroxyproline Signup and view all the answers

At neutral pH, which form do free amino acids tend to exist in?

Zwitterionic form Signup and view all the answers

What characterizes the titration curve of alanine?

It behaves as a diprotic acid. Signup and view all the answers

Which statement is true regarding amino acids without charged side chains?

They exist in neutral solution as zwitterions. Signup and view all the answers

What happens to an amino acid's structure when it is titrated?

Each functional group reacts with hydrogen ions. Signup and view all the answers

What is true about the cationic form of an amino acid?

It occurs when the amino acid is protonated. Signup and view all the answers

What distinguishes histidine from standard amino acids during titration?

It can behave as a triprotic acid. Signup and view all the answers

What does the isoelectric pH (pI) represent for a molecule?

The pH at which the molecule has no net charge Signup and view all the answers

How is the isoelectric pH (pI) calculated for glycine?

It is the average of the two pK values for its amino and carboxyl groups Signup and view all the answers

What is the calculated pI for aspartic acid?

2.975 Signup and view all the answers

At low pH, which amino acid is most likely to have a net charge of +2?

Arginine Signup and view all the answers

What is the pI calculated for lysine?

9.74 Signup and view all the answers

What is the pI for glutamic acid based on the provided values?

3.22 Signup and view all the answers

Which of the following amino acids is likely to have a net charge of -2 at high pH?

Aspartic acid Signup and view all the answers

Which equation is used to calculate the pI for basic amino acids?

pI = (pK a2 + pK a3) / 2 Signup and view all the answers

Which amino acids are classified with basic side chains?

Arginine and lysine Signup and view all the answers

What is the pKa of the side-chain imidazole group of histidine?

6.0 Signup and view all the answers

What is the ratio of uncharged to charged side chains of histidine at pH 7.0?

10:1 Signup and view all the answers

Which of the following amino acids has a nonpolar side chain?

Alanine Signup and view all the answers

Which amino acid contains a guanidino group in its side chain?

Arginine Signup and view all the answers

Why is serine not classified as having a basic or nonpolar side chain?

It has a polar side chain. Signup and view all the answers

Which amino acids share a structural parent with distinct derivatives?

Tyrosine and thyroxine Signup and view all the answers

Which of the following statements about basic amino acids is accurate?

Basic side chains are positively charged at or near neutral pH. Signup and view all the answers

Which amino acid is unique because it does not have a chiral center?

Glycine Signup and view all the answers

What is the typical configuration of most alpha-amino acids at the alpha-carbon?

L configuration Signup and view all the answers

Which of the following amino acids contains a sulfur atom in its side chain?

Methionine Signup and view all the answers

Which characteristic is NOT associated with the amino acid proline?

Contains an indole ring Signup and view all the answers

In amino acids with nonpolar side chains, which of the following amino acids is categorized as aromatic?

Phenylalanine Signup and view all the answers

Which of the following amino acids is NOT classified as nonpolar (hydrophobic)?

Aspartic acid Signup and view all the answers

Which amino acid is typically represented by the one-letter abbreviation 'W'?

Tryptophan Signup and view all the answers

Which term is used to denote the first carbon in the side chain of amino acids?

β-carbon Signup and view all the answers

Study Notes

Amino Acids Overview

Amino acids consist of an amino group (—NH2), a carboxyl group (—COOH), and a side chain (R) attached to the α-carbon.
Most amino acids in proteins are L-amino acids, which are chiral and have a specific stereochemistry akin to L-glyceraldehyde.
Glycine is the only amino acid without a chiral center.

Structural Properties of Amino Acids

All amino acids except glycine exhibit chirality with at least one chiral center at the α-carbon.
Amino acids are frequently categorized by side chains into nonpolar, polar, acidic, and basic groups.
Common abbreviations for amino acids follow either three-letter or one-letter formats.

Classifications of Amino Acids

Nonpolar amino acids (hydrophobic) include: glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, methionine.
Basic amino acids with positively charged side chains at physiological pH (near 7) include: histidine, lysine, and arginine.
Amino acids with nonpolar side chains include alanine, leucine, and phenylalanine; histidine and lysine are categorized as basic.

Titration and Zwitterions

Amino acids act as both acids and bases, exhibiting zwitterionic forms at neutral pH—carboxyl groups carry a negative charge, while amino groups carry a positive charge.
Neutral amino acids exist as zwitterions and have no net charge.
The titration curve of an amino acid demonstrates how its functional groups react with hydrogen ions, culminating in the determination of isoelectric point (pI).

Isoelectric Point (pI)

The pI is the pH at which a molecule has no net electrical charge.
The formula for calculating pI is:
- pI = (pKa1 + pKa2) / 2, applicable for simple amino acids.
For glycine, the pI is approximately 5.97, derived from its carboxyl (pKa ~2.34) and amino (pKa ~9.60) groups.

Uncommon Amino Acids

Some posttranslational modifications yield uncommon amino acids, such as hydroxyproline and hydroxylysine found in collagen.
Thyroxine is derived from tyrosine and is specific to the thyroid gland.

Examples of Titration & pI Calculations

Example calculation for aspartic acid shows a net charge of -2 at high pH and the use of pKa values to establish pI.
pI for basic amino acids like lysine is calculated by averaging the pKa of their basic side chains.

pKa Values

A variety of amino acids have distinct pKa values, guiding their behavior in biological systems.

Charged vs. Uncharged State Ratios

The ratio of charged to uncharged side chains, particularly for histidine with a pKa of 6.0, indicates that at pH 7.0, the ratio is approximately 10:1.

These notes encapsulate the essential characteristics and biochemical significance of amino acids and their roles in proteins.

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Description

Explore the key concepts of amino acids and peptides in this quiz based on Chapter 3. It covers the three-dimensional structures, properties of amino acids, peptide bonds, and the physiological activities of small peptides. Test your understanding of these fundamental biochemical elements.