Pentose Phosphate Pathway

Oxidative and Non-Oxidative Phases

The pentose phosphate pathway (PPP) is a metabolic pathway parallel to glycolysis, primarily responsible for generating NADPH and ribose-5-phosphate. It consists of two phases: oxidative and non-oxidative.

• Oxidative Phase: Produces NADPH and ribulose-5-phosphate via oxidation of glucose-6-phosphate.

• Non-Oxidative Phase: Converts ribulose-5-phosphate into ribose-5-phosphate and other sugars for nucleotide synthesis.

• Main Products: NADPH (oxidative), ribose-5-phosphate (non-oxidative).

• Regulation: If NADPH is needed, the oxidative phase is active; if nucleic acids are needed, the non-oxidative phase is used to generate ribose-5-phosphate; if both are sufficient, the pathway is downregulated.

Example: In rapidly dividing cells, ribose-5-phosphate is used for nucleotide synthesis.

Glycolysis and Gluconeogenesis

Irreversible Reactions and Their Modification

Three glycolysis reactions are irreversible and must be bypassed in gluconeogenesis:

• Hexokinase/Glucokinase:

• Phosphofructokinase-1:

• Pyruvate kinase:

In gluconeogenesis, these steps are bypassed by:

• Glucose-6-phosphatase: Converts glucose-6-phosphate to glucose.

• Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.

• Pyruvate carboxylase and PEP carboxykinase: Convert pyruvate to phosphoenolpyruvate via oxaloacetate.

Example: During fasting, gluconeogenesis maintains blood glucose levels.

Lipid Categories and Subcategories

Classification and Recognition

Lipids are classified into several categories and subcategories based on structure and function.

Example: Sphingomyelin is found in nerve cell membranes.

Lipid Structures and Saponification

Drawing and Predicting Products

Understanding the structure of lipids is essential for predicting their chemical behavior.

• Triglycerides: Composed of glycerol and three fatty acids.

• Glycerophospholipids: Glycerol backbone, two fatty acids, and a phosphate group with an alcohol.

• Sphingolipids: Sphingosine backbone, fatty acid, and a polar head group.

• Saponification: Hydrolysis of triglycerides with base produces glycerol and soap (fatty acid salts).

Example: Saponification of tristearin yields glycerol and sodium stearate.

Cell Membrane and Transport

Structure and Transport Mechanisms

The cell membrane is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Passive Transport: Diffusion and facilitated diffusion (no energy required).

• Active Transport: Requires energy (ATP) to move substances against concentration gradients.

• Endocytosis/Exocytosis: Bulk transport mechanisms.

Example: Glucose enters cells via facilitated diffusion.

Lipoproteins

Types and Roles

Lipoproteins transport lipids in the blood. There are four main types:

Example: High LDL is associated with increased risk of cardiovascular disease.

Beta Oxidation of Fatty Acids

Steps, Products, and Energy Yield

Beta oxidation breaks down fatty acids into acetyl-CoA, NADH, and FADH2.

• Each cycle: Produces 1 acetyl-CoA, 1 NADH, 1 FADH2.

• Type of Reaction: Includes oxidation, hydration, oxidation, and thiolysis.

• Energy Yield: ATP is generated from NADH and FADH2 via oxidative phosphorylation.

Formula: For a saturated fatty acid with n carbons:

Example: Palmitic acid (16C) yields 8 acetyl-CoA, 7 NADH, 7 FADH2.

Fatty Acid Synthesis

Steps, Products, and Comparison to Beta Oxidation

Fatty acid synthesis builds fatty acids from acetyl-CoA in the cytosol.

• Each cycle: Adds 2 carbons to the growing fatty acid chain.

• Type of Reaction: Includes condensation, reduction, dehydration, reduction.

• Differences: Synthesis occurs in cytosol, uses NADPH; beta oxidation occurs in mitochondria, produces NADH and FADH2.

Example: Synthesis of palmitic acid requires 7 cycles.

Nucleic Acids: Structure and Synthesis

Nucleosides, Nucleotides, and DNA/RNA Structure

Nucleosides consist of a nitrogenous base and a sugar; nucleotides add phosphate groups.

• Nucleoside: Base + sugar (ribose or deoxyribose).

• Nucleotide: Nucleoside + phosphate(s).

• DNA/RNA: Polymers of nucleotides; DNA uses deoxyribose, RNA uses ribose.

• Base Pairing: Hydrogen bonds between complementary bases (A-T/U, G-C).

Example: ATP is a nucleotide with three phosphates.

Genetic Information: Replication, Transcription, Translation

Processes and Steps

• DNA Replication: Semi-conservative process; each strand serves as a template.

• Transcription: DNA is used to synthesize mRNA.

• Translation: mRNA is decoded to synthesize proteins.

Example: The sequence of mRNA determines the amino acid sequence of a protein.

Mutations

Types and Effects

Mutations are changes in DNA sequence that can affect protein function.

• Point Mutation: Single nucleotide change.

• Insertion/Deletion: Addition or removal of nucleotides.

• Silent, Missense, Nonsense: Effects range from no change to premature stop codon.

Example: Sickle cell anemia is caused by a missense mutation.

Amino Acid Metabolism

Transamination, Deamination, and Urea Cycle

Amino acid metabolism involves synthesis and breakdown, including transamination and deamination reactions.

• Transamination: Transfer of amino group from amino acid to alpha-keto acid.

• Oxidative Deamination: Removal of amino group as ammonia (e.g., glutamate).

• Reductive Amination: Synthesis of amino acids from alpha-keto acids.

• Urea Cycle: Detoxifies ammonia; occurs in mitochondria and cytosol.

• Fates of Carbon Skeletons: Can be used for energy, glucose, or fatty acid synthesis.

Example: Glutamate undergoes oxidative deamination to produce ammonia and alpha-ketoglutarate.

Essential and Nonessential Amino Acids

Recognition and Synthesis

Essential amino acids cannot be synthesized by humans and must be obtained from diet; nonessential amino acids are synthesized in the body.

• Essential: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine.

• Nonessential: Synthesized via transamination and other pathways.

Example: Alanine is synthesized from pyruvate via transamination.

Additional info: This guide expands on brief exam guidelines to provide academic context and explanations for each topic, including definitions, examples, and formulas.