Genes, Proteins, Enzymes, and Human Disease: Molecular Genetics Study Guide

Genes and Their Functions

Genetic and Biochemical Definitions of Genes

Genes are fundamental units of heredity that control the form, function, and behavior of organisms. They reside on chromosomes and segregate in defined ways between parents and offspring. Biochemically, a gene is a segment of DNA that contains the information to express a protein, which performs a specific function in the cell or body.

• Genetic definition: Controls phenotype, inherited in Mendelian fashion.

• Biochemical definition: Encodes a protein or functional RNA.

• Example: The gene responsible for alkaptonuria encodes an enzyme required for the breakdown of homogentisic acid.

Proteins: Structure and Function

Amino Acids and Protein Structure

Proteins are polymers made of amino acids, each with a unique sequence. The structure and chemical properties of amino acids determine how proteins fold and function.

• General structure: Each amino acid has an amino group, a carboxyl group, and a variable R group.

• Peptide bond: Covalent bond between the carboxyl group of one amino acid and the amino group of another.

• Polypeptide chain: Has an amino terminus (start) and a carboxyl terminus (end).

Levels of Protein Structure

Proteins fold into precise three-dimensional structures, which are essential for their function. There are four levels of protein structure:

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local structures formed by hydrogen bonding (e.g., alpha helix, beta sheet).

• Tertiary structure: Overall 3D shape formed by long-range interactions.

• Quaternary structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Enzymes: Catalysts of Biochemical Reactions

Enzyme Function and Active Site

Enzymes are proteins that catalyze specific chemical reactions. The active site is the region where substrate binding and catalysis occur.

• Enzyme-substrate complex: Substrate binds to the active site, reaction occurs, and products are released.

• Specificity: Each enzyme catalyzes a specific reaction.

Mutations in Genes and Human Disease

Alkaptonuria: An Inborn Error of Metabolism

Alkaptonuria is a genetic disease described by Archibald Garrod in 1902. It is caused by a mutation in a gene encoding an enzyme required for the breakdown of homogentisic acid, leading to its accumulation and black urine upon oxidation.

• Inheritance: Recessive Mendelian trait.

• Biochemical basis: Defective enzyme in the phenylalanine breakdown pathway.

Phenylketonuria (PKU)

PKU is caused by mutations in the gene encoding phenylalanine hydroxylase (PAH). Without PAH, phenylalanine accumulates and is converted into harmful products, leading to severe mental retardation if untreated.

• Inheritance: Recessive.

• Diagnosis: Newborn screening and dietary management.

Tay-Sachs Disease

Tay-Sachs disease is caused by a defect in the lysosomal enzyme HEXA, leading to the accumulation of its substrate and degeneration of neurons. It is common among Jewish people of European ancestry and has no cure.

• Symptoms: Paralysis, blindness, hearing loss, early death.

• Inheritance: Recessive.

Sickle Cell Anemia: A Molecular Disease

Sickle cell anemia is caused by a mutation in the gene for beta hemoglobin, resulting in a single amino acid change (E6V: glutamic acid to valine). This alters the protein's structure, causing red blood cells to adopt a sickle shape and leading to various health complications.

• Inheritance: Recessive.

• Diagnosis: Protein gel electrophoresis can distinguish genotypes.

• Population genetics: The sickle cell allele persists in populations exposed to malaria due to heterozygote advantage.

Biochemical Pathways and Genetic Screens

Beadle and Tatum: One Gene-One Enzyme Hypothesis

Beadle and Tatum used Neurospora crassa to show that mutations in single genes affect specific steps in biochemical pathways. Their work led to the 'one gene-one enzyme' hypothesis, later refined to 'one gene-one polypeptide.'

• Genetic screens: Mutagenesis and selection for auxotrophic mutants.

• Auxotrophs: Mutants unable to synthesize specific compounds.

The Central Dogma of Molecular Biology

Genotype to Phenotype

The central dogma describes the flow of genetic information: DNA is transcribed to RNA, which is translated into protein. Proteins determine cellular form and function.

• DNA → RNA → Protein

• Mutations: Changes in DNA can alter protein structure and function, leading to disease.

Population Genetics and Disease Alleles

Sickle Cell Anemia and Malaria

The sickle cell allele persists in populations exposed to malaria due to the heterozygote advantage. Heterozygotes (carriers) have increased resistance to malaria, explaining the high frequency of the allele in certain regions.

• Heterozygote advantage: Increased fitness in malarial environments.

• Historical context: Spread of the allele linked to agricultural practices and migration.

Summary Table: Amino Acid Categories

The chemical nature of the R group is used to group amino acids into categories:

Category	Amino Acids
Nonpolar	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Tryptophan, Phenylalanine, Proline
Polar	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Acidic	Aspartic acid, Glutamic acid
Basic	Lysine, Arginine, Histidine

Key Equations and Concepts

• Peptide bond formation:

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

• Central Dogma:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Conclusion

Mutations in genes can cause human diseases by altering the structure and function of proteins, especially enzymes. Understanding the molecular basis of these diseases, the structure and function of proteins, and the genetic principles underlying inheritance is fundamental to genetics and molecular biology.