Biochemistry Cheat Sheet

The core ideas of Biochemistry distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Enzyme Catalysis

Enzymes are biological catalysts, mostly proteins, that dramatically accelerate chemical reactions by lowering the activation energy required. They achieve this through an active site whose shape and chemical environment are complementary to the substrate, forming a transient enzyme-substrate complex.

ATP and Bioenergetics

Adenosine triphosphate (ATP) is the primary energy currency of cells. Energy released from the hydrolysis of its terminal phosphate bond drives endergonic reactions, muscle contraction, active transport, and biosynthetic processes throughout the cell.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein. DNA is transcribed into messenger RNA by RNA polymerase, and the mRNA is then translated into a polypeptide chain by ribosomes using the genetic code.

Protein Folding and Structure

Proteins fold into specific three-dimensional conformations determined by their amino acid sequence. The hierarchy of structure ranges from primary (sequence) to secondary (alpha-helices, beta-sheets), tertiary (overall 3D shape), and quaternary (multi-subunit assembly). Misfolding can lead to disease.

Glycolysis

Glycolysis is a ten-step metabolic pathway occurring in the cytoplasm that converts one molecule of glucose into two molecules of pyruvate, yielding a net gain of two ATP and two NADH. It is nearly universal across life and does not require oxygen.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of eight enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA to $\text{CO}_2$, generating NADH, $\text{FADH}_2$, and GTP. These reduced coenzymes then feed electrons into the electron transport chain for further ATP production.

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which electrons from NADH and $\text{FADH}_2$ pass through a series of protein complexes in the inner mitochondrial membrane, creating a proton gradient that drives ATP synthase to produce ATP. It is the most efficient energy-generating pathway in aerobic organisms.

DNA Replication

DNA replication is the semiconservative process by which a cell duplicates its entire genome before division. Helicase unwinds the double helix, primase synthesizes RNA primers, and DNA polymerase extends new strands, with additional enzymes handling proofreading and ligation.

Signal Transduction

Signal transduction refers to the cascade of molecular events by which a cell converts an extracellular chemical signal into an intracellular response. Ligands bind to cell-surface receptors, triggering second messengers and kinase cascades that ultimately alter gene expression or cellular behavior.

Lipid Bilayer and Membrane Transport

Biological membranes are composed of a phospholipid bilayer that forms a selectively permeable barrier around cells and organelles. Integral and peripheral membrane proteins facilitate transport of ions and polar molecules across this hydrophobic barrier through channels, carriers, and pumps.

Key Terms at a Glance

Active Site:The specific region of an enzyme where substrate molecules bind and undergo a chemical reaction. The shape and chemical properties of the active site are complementary to the substrate, following the induced-fit model.

Amino Acid:An organic molecule containing an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain) attached to a central alpha-carbon. Twenty standard amino acids serve as the building blocks of proteins.

Anabolism:The set of metabolic pathways that construct complex molecules from simpler ones, requiring an input of energy typically in the form of ATP. Examples include protein synthesis, gluconeogenesis, and fatty acid synthesis.

Catabolism:The set of metabolic pathways that break down complex molecules into simpler ones, releasing energy that is captured as ATP or reduced coenzymes (NADH, $\text{FADH}_2$). Examples include glycolysis, beta-oxidation, and the citric acid cycle.

Coenzyme:A small organic molecule, often derived from a vitamin, that transiently associates with an enzyme and participates in the reaction by carrying chemical groups between enzymes. Examples include $\text{NAD}^+$, FAD, and coenzyme A.

Denaturation:The loss of a protein's native three-dimensional structure due to disruption of non-covalent interactions by heat, extreme pH, detergents, or other agents. Denatured proteins typically lose their biological function.

Electron Transport Chain:A series of membrane-bound protein complexes (Complexes I-IV) and mobile electron carriers in the inner mitochondrial membrane that transfer electrons from NADH and $\text{FADH}_2$ to molecular oxygen, pumping protons to create the gradient used for ATP synthesis.

Enzyme:A biological catalyst, usually a protein, that accelerates a specific chemical reaction by lowering the activation energy. Enzymes are highly specific, regulated, and not consumed in the reactions they catalyze.

Fatty Acid:A long hydrocarbon chain with a terminal carboxyl group that serves as a major component of lipids. Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds) and are a rich source of metabolic energy via beta-oxidation.

Glycogen:A highly branched polysaccharide of glucose that serves as the principal storage form of carbohydrate energy in animals, found mainly in the liver and skeletal muscle. It is rapidly mobilized by glycogen phosphorylase when blood glucose levels fall.

Glycolysis:A ten-step cytoplasmic metabolic pathway that converts glucose into two molecules of pyruvate, producing a net gain of 2 ATP and 2 NADH. It is the most ancient and universal energy-yielding pathway and operates under both aerobic and anaerobic conditions.

Hydrophobic Interaction:The tendency of nonpolar molecules or nonpolar portions of molecules to aggregate in aqueous solution, minimizing their contact with water. Hydrophobic interactions are a major driving force in protein folding and membrane assembly.

Isomerase:An enzyme that catalyzes the structural rearrangement of a molecule, converting it from one isomer to another without adding or removing atoms. Examples include phosphoglucose isomerase in glycolysis and triose phosphate isomerase.

Km (Michaelis Constant):The substrate concentration at which an enzyme-catalyzed reaction proceeds at half its maximum velocity ($V_{max}$). It is an inverse measure of enzyme-substrate affinity; a lower $K_m$ indicates tighter binding between the enzyme and substrate.

Lipid:A diverse group of hydrophobic or amphipathic biological molecules including fats, phospholipids, steroids, and waxes. Lipids serve as energy storage, structural components of membranes, and signaling molecules.

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