Structure and Properties of Amino Acids

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Questions and Answers

What is the charge of aspartic and glutamic acids at physiologic pH?

Positively charged

Weakly negatively charged

Negatively charged (correct)

Neutral

Which amino acid is an exception to being optically active due to its structure?

Lysine

Glycine (correct)

Alanine

Histidine

How are the D and L forms of amino acids classified?

They are different states of ionization.

They are mirror images of each other. (correct)

They represent different solubilities.

They differ by molecular weight.

What is a key characteristic of basic amino acids at physiologic pH?

They accept protons and are fully ionized positively. Signup and view all the answers

Which amino acids are referred to as aspartate and glutamate when proton donors?

Amino acids with acidic side chains Signup and view all the answers

What is true about the α-carbon of an amino acid?

It has four different chemical groups attached to it. Signup and view all the answers

Where are D-amino acids typically found?

In bacterial cell walls and some antibiotics Signup and view all the answers

How are the pK values for nonpolar amino acids at the α-carboxyl and α-amino groups described?

They are similar to those shown for glycine. Signup and view all the answers

What is a distinguishing feature of proline compared to other amino acids?

It has a secondary amino group. Signup and view all the answers

What occurs to the carboxyl group of amino acids at physiological pH?

It dissociates into a carboxylate ion. Signup and view all the answers

How are amino acids classified based on their side chains?

Based on their charge and hydrophilicity. Signup and view all the answers

In aqueous solutions, what best describes the behavior of nonpolar amino acids?

They tend to cluster together. Signup and view all the answers

Which group of amino acids contains side chains that can form hydrogen bonds due to their polar hydroxyl groups?

Amino acids with uncharged polar side chains Signup and view all the answers

What is the charge of uncharged polar amino acids at neutral pH?

Zero net charge Signup and view all the answers

What effect does the clustering of nonpolar side chains have on protein structure?

It stabilizes the folded structure of the protein. Signup and view all the answers

Which amino acids can lose a proton at an alkaline pH?

Cysteine and tyrosine Signup and view all the answers

Study Notes

Structure of Amino Acids

Proteins are the most abundant and functionally diverse molecules in living systems.
All proteins are linear polymers of amino acids.
All amino acids (except proline) have a carboxyl group, a primary amino group, and a distinctive side chain (R-group).
Proline has a secondary amino group.
The α-carbon atom is bonded to the carboxyl group, the amino group, the hydrogen atom, and the R-group.

Properties of Amino Acids

At physiological pH (7.4), the carboxyl group is dissociated, forming the negatively charged carboxylate ion (COO-), and the amino group is protonated (NH3+).
Almost all carboxyl and amino groups in proteins are combined through peptide linkage and are not available for chemical reactions, except for hydrogen bond formation.
Amino acids are classified according to the properties of their side chains into polar (uneven distribution of electrons) and nonpolar (even distribution of electrons).

Nonpolar Side Chains

Nonpolar amino acids do not gain or lose protons or participate in hydrogen or ionic bonds.
Their side chains promote hydrophobic interactions.
In aqueous solutions, the side chains of nonpolar amino acids tend to cluster together in the interior of the protein, known as the hydrophobic effect.

Uncharged Polar Side Chains

These amino acids have zero net charge at neutral pH.
Serine, threonine, and tyrosine contain a polar hydroxyl group (OH-) that can participate in hydrogen bond formation.
Asparagine and glutamine contain a carbonyl group and an amide group, both of which can participate in hydrogen bonds.
Cysteine and tyrosine can lose a proton at an alkaline pH.

Acidic Side Chains

Aspartic and glutamic acids are proton donors.
At physiological pH, their side chains are fully ionized and contain a negatively charged carboxylate group.
They are called aspartate or glutamate.

Basic Side Chains

The side chains of basic amino acids accept protons.
At physiological pH, the side chains of lysine and arginine are fully ionized and positively charged.
Histidine is weakly basic and largely uncharged at physiological pH.
When incorporated into a protein, histidine's side chain can be positively charged or neutral depending on the ionic environment.

Abbreviation and Symbols

Each amino acid name has a three-letter abbreviation and a one-letter symbol.

Optical Properties

The α-carbon of an amino acid is attached to four different chemical groups, making it a chiral or optically active carbon atom.
Glycine is the exception because it has two hydrogen substituents and is optically inactive.
Amino acids with an asymmetric center at the α-carbon can exist in two forms, designated D and L, which are mirror images of each other.
The two forms in each pair 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

This quiz covers the essential structure and properties of amino acids, which are the building blocks of proteins. Learn about the functional groups, classification of amino acids, and the significance of the α-carbon in their structure. Test your understanding of polar and nonpolar side chains as well.