Amino Acids Structure and Classification

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Questions and Answers

What structural feature is common to all amino acids except proline?

A hydroxyl group

A carboxyl group (correct)

An aldehyde group

A secondary amino group

At physiologic pH (7.4), what charge does the carboxyl group of an amino acid acquire?

Negative charge (correct)

Positive charge

Variable charge

Neutral charge

Which amino acids primarily exhibit hydrophobic interactions due to their side chains?

Amino acids with ionizable side chains

Amino acids with polar side chains

Amino acids with nonpolar side chains (correct)

Amino acids with charged side chains

Which of the following amino acids contains a hydroxyl group that can participate in hydrogen bonding?

Tyrosine Signup and view all the answers

Which amino acids can lose a proton at alkaline pH?

Cysteine and tyrosine Signup and view all the answers

What type of bonding is primarily involved in the linkage of amino acids within proteins?

Peptide bonding Signup and view all the answers

How are amino acids classified based on their side chain properties?

Polar and nonpolar Signup and view all the answers

What determines the clustering behavior of nonpolar amino acids in aqueous environments?

The hydrophobic effect Signup and view all the answers

Which amino acids are considered as proton donors due to their acidic side chains?

Aspartic and Glutamic acids Signup and view all the answers

At physiologic pH, which amino acids possess fully ionized and positively charged side chains?

Lysine and Arginine Signup and view all the answers

What characterizes the α-carbon of most amino acids?

It has an asymmetric center. Signup and view all the answers

Which amino acids are NOT typically found in the L-configuration in proteins?

D-amino acids Signup and view all the answers

What would you find at the side chain of histidine at physiologic pH?

It can be either positively charged or neutral. Signup and view all the answers

Which of the following characteristics is true about glycine?

It is considered a nonpolar amino acid. Signup and view all the answers

What are the charges on the side chains of aspartate and glutamate at physiologic pH?

Both are negatively charged. Signup and view all the answers

What is the significant structural attribute of the D and L forms of amino acids?

They are mirror images of each other. Signup and view all the answers

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!