Proteins: Structure, Function, and Amino Acids – Study Guide

Proteins and Amino Acids

Monomers of Proteins

Proteins are large biomolecules composed of smaller units called amino acids. These monomers link together to form polypeptide chains, which fold into functional proteins.

• Amino acids are the building blocks of proteins.

• Other biological macromolecules have different monomers: nucleotides for nucleic acids, monosaccharides for carbohydrates.

• Peptides are short chains of amino acids, not monomers.

Structure of Amino Acids

Each amino acid has a central carbon atom (the α-carbon), an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group).

• The α-carbon is the central carbon to which all other groups are attached.

• The α-amino group is the -NH2 group attached to the α-carbon.

• The α-carboxyl group is the -COOH group attached to the α-carbon.

• The R group (side chain) determines the identity and properties of each amino acid.

Functional Groups and Ionization States

The functional groups in amino acids can exist in different ionization states depending on the pH of the environment.

• Amino group: Can accept a proton and become positively charged at low pH.

• Carboxyl group: Can donate a proton and become negatively charged at high pH.

• R group: May be ionizable, affecting the overall charge of the amino acid.

• The ionization state of functional groups is crucial for protein structure and function.

Categorization of Amino Acids

Amino acids are classified based on the properties of their side chains (R groups).

• Non-polar: Hydrophobic side chains, often containing hydrocarbons (e.g., methyl group in alanine).

• Polar: Side chains that can form hydrogen bonds (e.g., serine, threonine).

• Charged: Side chains that are ionized at physiological pH (e.g., lysine, aspartic acid).

When categorizing amino acids, only the R group is considered, not the α-amino or α-carboxyl groups.

Effect of pH on Amino Acids

The charge state of amino acids can change with pH, affecting their interactions and solubility.

• At low pH, amino groups are protonated (positively charged), and carboxyl groups are uncharged.

• At high pH, carboxyl groups are deprotonated (negatively charged), and amino groups are uncharged.

• Changing the pH can alter the ionization state of the R group, shifting between charged and uncharged forms.

Peptide Bonds and Polypeptides

Formation of Peptide Bonds

Peptide bonds link amino acids together to form polypeptides, the primary structure of proteins.

• A peptide bond forms between the carboxyl group of one amino acid and the amino group of another.

• This reaction releases a molecule of water (condensation reaction).

Equation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

Polypeptide Structure

Polypeptides are chains of amino acids linked by peptide bonds. The sequence and number of amino acids determine the protein's properties.

• The N-terminus is the end with a free amino group.

• The C-terminus is the end with a free carboxyl group.

• The identity of the amino acid at the C-terminus can be determined by examining the side chain.

Levels of Protein Structure

Primary Structure

The primary structure is the linear sequence of amino acids in a polypeptide chain.

• Contains all the information necessary for the protein to fold into its proper structure.

• Remains unchanged when a protein is denatured.

Secondary Structure

Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms.

• Includes α-helices and β-sheets.

• Dependent on interactions between partially positively charged hydrogen and partially negatively charged oxygen of the backbone.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain.

• Stabilized by interactions between R groups of amino acids, including hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.

• Excludes interactions at the terminal ends.

Quaternary Structure

Quaternary structure arises when multiple polypeptide chains (subunits) assemble into a functional protein complex.

• Example: Hemoglobin consists of four polypeptide chains.

• Not all proteins have quaternary structure.

Protein Denaturation

Denaturation disrupts secondary, tertiary, and quaternary structures, but the primary structure remains intact.

• Denaturation can be caused by changes in temperature, pH, or chemical agents.

Protein Function and Classification

Types of Proteins

Proteins serve diverse functions in cells, including catalysis, transport, signaling, and structural support.

• Enzymes: Catalyze biochemical reactions (e.g., tyrosinase).

• G-protein coupled receptors (GPCRs): Involved in cell signaling (e.g., OPN1LW, TAS2R38).

• Ion channels: Transport ions across membranes (e.g., CFTR).

• Transport proteins: Carry molecules (e.g., hemoglobin binds oxygen).

Protein Examples Table

Protein	Type	Function
TYR	Enzyme	Catalyzes melanin synthesis
ABO	Enzyme	Glycosyltransferase for blood group antigens
FUT1	Enzyme	Fucosyltransferase in glycoprotein synthesis
OPN1LW	GPCR	Photoreceptor in vision
TAS2R38	GPCR	Taste receptor
CFTR	Ion channel	Chloride transport in epithelial cells
HBB	Transport protein	Oxygen transport in blood

Summary Table: Levels of Protein Structure

Level	Description	Stabilizing Interactions
Primary	Sequence of amino acids	Peptide bonds
Secondary	Local folding (α-helix, β-sheet)	Hydrogen bonds (backbone)
Tertiary	3D structure of polypeptide	R group interactions (hydrophobic, ionic, hydrogen, disulfide)
Quaternary	Assembly of multiple polypeptides	Same as tertiary, between subunits

Key Equations and Concepts

• Peptide bond formation: $\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Ionization of amino acids: $\text{R-NH}_2 + H^+ \rightarrow \text{R-NH}_3^+$ (low pH), $\text{R-COOH} \rightarrow \text{R-COO}^- + H^+$ (high pH)

Additional info:

• Protein structure and function are highly dependent on the chemical properties of amino acids and their environment.

• Understanding the levels of protein structure is essential for predicting protein folding and function.