Chapter 5: Metabolism—Enzymes, Reactions, Regulation, and Core Pathways

A set of vocabulary flashcards covering enzymes, regulation, and the main metabolic pathways (glycolysis, Krebs cycle, electron transport chain, and chemiosmosis) discussed in Chapter 5.

Enzyme

Biological catalyst, often ending in -ase; can be a 100% protein or part protein (apoenzyme) with a nonprotein cofactor, forming a holoenzyme when combined.

Apoenzyme

The protein portion of an enzyme without its nonprotein cofactor.

Cofactor

Nonprotein component required for enzyme activity; can be inorganic (e.g., Ca2+, Fe2+) or organic (coenzyme).

Coenzyme

Organic cofactor; example: NAD; essential for certain enzyme activities.

Holoenzyme

Active enzyme formed when an apoenzyme binds its cofactor.

Active site

Region of the enzyme where the substrate binds and catalysis occurs.

Substrate

Molecule that binds to the enzyme at the active site to undergo a chemical reaction.

Activation energy

Energy required to start a reaction; enzymes lower this energy barrier.

Endoenzyme

Enzyme produced within the cell and functions inside the cell.

Exoenzyme

Enzyme produced inside the cell but secreted outside the cell.

Constitutive enzyme

Enzymes that are continuously produced regardless of substrate presence.

Induced enzyme

Enzymes produced only when a specific substrate is present.

Catabolic reaction

Decomposition—breaking down a larger substrate into smaller products.

Anabolic reaction

Synthesis—joining smaller subunits to form a macromolecule.

Oxidation

Loss of electrons in a chemical reaction.

Reduction

Gain of electrons in a chemical reaction.

Redox reaction

Coupled oxidation-reduction reactions; electrons and hydrogen ions are transferred.

NAD+

Oxidized form of nicotinamide adenine dinucleotide; electron carrier.

NADH

Reduced form of NAD+; carries electrons earlier in metabolism.

FAD

Flavin adenine dinucleotide; electron carrier involved in redox reactions.

Turnover number

Maximum number of substrate molecules converted to product per enzyme per second.

Temperature effect

High temperature can denature enzymes by disrupting hydrogen bonds; reduces activity.

pH effect

pH changes can denature enzymes by altering structure and charge distribution.

Saturation

Point at which all enzyme active sites are bound with substrate; adding more substrate no longer increases rate.

Competitive inhibition

Inhibitor binds to the enzyme’s active site, blocking substrate binding; often reversible.

Noncompetitive inhibition

Inhibitor binds to the allosteric site, changing enzyme shape and reducing activity; often irreversible.

Allosteric site

Site on an enzyme, separate from the active site, used for noncompetitive regulation.

Feedback inhibition

End product inhibits an earlier enzyme to regulate the pathway and prevent overaccumulation.

Ribozymes

RNA molecules with catalytic activity, not proteins.

Metabolism

All chemical reactions in a cell, including anabolic and catabolic pathways.

Aerobic metabolism

Metabolic processes that use oxygen as the terminal electron acceptor.

Anaerobic metabolism

Metabolic processes that use a terminal electron acceptor other than oxygen (e.g., sulfur, iron).

Terminal electron acceptor

Final electron recipient in the electron transport chain; oxygen in aerobic metabolism.

ATP

Adenosine triphosphate; the energy currency of the cell.

Phosphorylation

Addition of a phosphate group to ADP to form ATP.

Oxidative phosphorylation

ATP production via the electron transport chain and proton motive force across a membrane.

Photophosphorylation

ATP production using light energy; occurs in photosynthetic organisms.

Substrate-level phosphorylation

Direct synthesis of ATP by transferring a phosphate from a substrate to ADP; occurs in glycolysis and Krebs; also linked to fermentation.

Glycolysis

Initial cytoplasmic breakdown of glucose to pyruvate; produces little/no net ATP in the step shown and prepares substrates for the Krebs cycle.

Pyruvate

End product of glycolysis; converted to acetyl-CoA to enter the Krebs cycle.

Decarboxylation

Removal of CO2 from a substrate, producing a two-carbon acetyl group in metabolism.

Acetyl-CoA

Acetyl group bound to coenzyme A; substrate that enters the Krebs cycle.

Coenzyme A

Carrier that binds acetyl groups and transports them into the Krebs cycle.

Oxaloacetate

Four-carbon molecule that combines with acetyl-CoA to form citrate in the Krebs cycle.

Citrate

Six-carbon molecule formed from oxaloacetate and acetyl-CoA; first step in the Krebs cycle.

Krebs cycle

Cyclic set of reactions in which acetyl-CoA is oxidized, producing CO2, NADH, FADH2, and ATP/GTP; occurs in mitochondria (eukaryotes) or cytoplasm (prokaryotes).

Electron transport chain

Series of carrier molecules (flavoproteins, cytochromes, coenzyme Q) that transfer electrons from NADH/FADH2 to the terminal acceptor, creating a proton gradient.

Proton motive force

Electrochemical gradient of protons across a membrane that drives ATP synthesis.

ATP synthase

Enzyme that uses the proton motive force to phosphorylate ADP to ATP.

Cristae

Folds of the inner mitochondrial membrane where proton accumulation creates the gradient in eukaryotes.

Periplasmic space

Space between membranes in Gram-negative bacteria where proton gradient can be established.

Fermentation

Anaerobic process that generates ATP by substrate-level phosphorylation without Krebs or ETC; final electron acceptor is an organic molecule.

Lactic acid fermentation

Fermentation producing lactate as the end product.

Alcohol fermentation

Fermentation producing ethanol and CO2 as end products.

Water formation

In aerobic respiration, water is produced when oxygen accepts electrons at the end of the electron transport chain.