Amino Acids and Proteins: Structure and Properties

Chapter 3: Amino Acids and Proteins

Overview

Proteins are the most abundant and versatile macromolecules in living organisms. They are composed of 20 different amino acids, each with unique side chains, and their structure determines their diverse functions in cells.

Amino Acids: Structure and Properties

Core Structure of Amino Acids

All amino acids share a common core structure, which consists of a c entral carbon atom (the alpha-carbon) bonded to four different groups:

• Hydrogen atom (H)

• Amino group (NH2) – a functional group that acts as a base

• Carboxyl group (COOH) – a functional group that acts as an acid

• R group (side chain) – a variable group that determines the identity and properties of the amino acid

Example: The general structure can be represented as:

H | NH2—C—COOH | R

Ionization of Amino Acids

In aqueous solutions, the amino and carboxyl groups of amino acids ionize:

• The amino group gains a proton to become NH3+

• The carboxyl group loses a proton to become COO-

This ionization helps amino acids stay in solution and affects their chemical reactivity.

The Nature of Side Chains (R Groups)

The R group or side chain is the part of the amino acid that varies among the 20 standard amino acids. The properties of amino acids are determined by their R groups:

• Some R groups contain functional groups that can participate in chemical reactions.

• Other R groups are composed only of carbon and hydrogen, making them nonreactive and hydrophobic.

Classification of Amino Acids by Side Chain Properties

Amino acids can be classified based on the properties of their side chains:

Type	Properties	Examples
Electrically charged (Acidic)	Side chains have a negative charge; can form ionic bonds; hydrophilic	Aspartate (Asp), Glutamate (Glu)
Electrically charged (Basic)	Side chains have a positive charge; can form ionic bonds; hydrophilic	Lysine (Lys), Arginine (Arg)
Polar uncharged	Side chains have partial charges; can form hydrogen bonds; hydrophilic	Serine (Ser), Threonine (Thr), Tyrosine (Tyr), Asparagine (Asn)
Nonpolar	Side chains lack charged or highly electronegative atoms; hydrophobic	Glycine (Gly), Alanine (Ala), Valine (Val), Leucine (Leu), Methionine (Met), Phenylalanine (Phe), Tryptophan (Trp)

Polarity and Charge of R Groups Affect Solubility

The solubility of an amino acid in water depends on the nature of its R group:

• Polar and charged R groups interact with water and are hydrophilic.

• Nonpolar R groups do not interact with water and are hydrophobic.

Example: In a watery environment, hydrophilic R groups will interact with water, while hydrophobic R groups will cluster away from water.

Determining Amino Acid Type from Structure

To classify an amino acid based on its side chain, ask:

1. Does the side chain have a negative charge? (Acidic)

2. Does the side chain have a positive charge? (Basic)

3. If uncharged, does it contain an oxygen atom? (Polar uncharged)

4. If none of the above, it is nonpolar.

Summary Table: The 20 Major Amino Acids

Category	Amino Acids	Properties
Acidic (Negatively charged)	Aspartate (Asp), Glutamate (Glu)	Hydrophilic, can form ionic bonds
Basic (Positively charged)	Lysine (Lys), Arginine (Arg), Histidine (His)	Hydrophilic, can form ionic bonds
Polar uncharged	Serine (Ser), Threonine (Thr), Tyrosine (Tyr), Asparagine (Asn), Glutamine (Gln)	Hydrophilic, can form hydrogen bonds
Nonpolar	Glycine (Gly), Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), Phenylalanine (Phe), Tryptophan (Trp), Proline (Pro), Cysteine (Cys)	Hydrophobic, do not form hydrogen bonds with water

Additional info: The above tables are reconstructed from standard biology knowledge and the provided images. Some amino acids and their properties are inferred for completeness.