Proteins and Nucleic Acids: Structure, Function, and Biological Importance

Proteins

Introduction to Proteins

Proteins are essential macromolecules composed of amino acids, performing a wide variety of functions in living organisms. Their structure and function are determined by the sequence and arrangement of amino acids.

• Monomer: Amino Acid

• Polymer: Polypeptide

• Bond: Peptide Bond

Used for:

• Enzymes (catalysis)

• Structural support

• Transport

• Defense

• Signaling

Amino Acids

Amino acids are the building blocks of proteins. Each amino acid contains a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

• Basic structure: Central (alpha) carbon, amino group (–NH2), carboxyl group (–COOH), hydrogen atom, and R group.

• There are 20 different amino acids, each with a unique R group.

General formula:

$\text{Amino Acid:} \quad \mathrm{NH_2{-}CHR{-}COOH}$

Example: Glycine has a hydrogen as its R group; alanine has a methyl group.

Polypeptides

Polypeptides are chains of amino acids linked by peptide bonds. A protein may consist of one or more polypeptide chains.

• Peptide bond formation involves a dehydration reaction between the amino group of one amino acid and the carboxyl group of another.

• 20 amino acids can be arranged in countless sequences to form thousands of different proteins.

$\text{Peptide Bond:} \quad \mathrm{COOH} + \mathrm{NH_2} \rightarrow \mathrm{CONH} + \mathrm{H_2O}$

Protein Folding

Protein folding is the process by which a polypeptide chain acquires its functional three-dimensional structure.

• Structure determines function; misfolded proteins may not function correctly.

• Denaturation: The process in which a protein loses its structure due to changes in temperature, pH, or other factors.

Levels of Protein Structure

Level	Description	Stabilizing Bonds	Example/Diagram
Primary	Linear sequence of amino acids	Peptide bonds	Polypeptide chain
Secondary	Formation of α-helices and β-sheets	Hydrogen bonds	α-helix, β-sheet
Tertiary	3D folding due to interactions between R groups	Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions	Globular structure
Quaternary	Association of multiple polypeptide chains	Same as tertiary	Hemoglobin (tetramer)

Sickle Cell Anemia

Sickle cell anemia is a genetic disorder caused by a single amino acid substitution in the hemoglobin protein, leading to abnormal protein structure and function.

• Results in sickle-shaped red blood cells that can block blood flow.

• Demonstrates the importance of primary structure in protein function.

Nucleic Acids

Introduction to Nucleic Acids

Nucleic acids are macromolecules that store and transmit genetic information. The two main types are DNA and RNA.

• Monomer: Nucleotide

• Polymer: Nucleic Acid (DNA or RNA)

• Bond: Phosphodiester bond

Nucleotide Structure

Each nucleotide consists of three components:

• Phosphate group

• Five-carbon sugar (ribose in RNA, deoxyribose in DNA)

• Nitrogenous base (Adenine, Thymine, Cytosine, Guanine, Uracil)

Nitrogenous Bases

Type	Bases
Purines	Adenine (A), Guanine (G)
Pyrimidines	Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)

Five-Carbon Sugars

• Ribose: Found in RNA

• Deoxyribose: Found in DNA (lacks one oxygen atom compared to ribose)

Structure of DNA and RNA

• DNA: Double-stranded helix, sugar is deoxyribose, bases are A, T, C, G

• RNA: Usually single-stranded, sugar is ribose, bases are A, U, C, G

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Nucleotides	A, T, C, G	A, U, C, G
Strands	Double	Single

Base Pairing Rules

• In DNA: Adenine pairs with Thymine (A-T), Guanine pairs with Cytosine (G-C)

• In RNA: Adenine pairs with Uracil (A-U), Guanine pairs with Cytosine (G-C)

Chargaff's Rule: In DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine.

Example: If a DNA molecule is 19% adenine, it is also 19% thymine; the remaining 62% is split equally between guanine and cytosine (31% each).

DNA to Protein

Genes in DNA contain instructions for building proteins. The process involves transcription (DNA to RNA) and translation (RNA to protein).

• Central Dogma: DNA → RNA → Protein

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Practice Questions

• If a DNA molecule consists of 19% adenine, what percentage is thymine? 19%

• What percentage of that molecule would be guanine? 31%

• If a DNA strand contains the sequence ATCGGAA, what is the complementary strand? TAGCCTT

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double	Single

Additional info: Protein and nucleic acid structure and function are foundational topics in General Biology, relevant to chapters on cell molecules, gene expression, and inheritance.