Metabolism Overview

Chapter 5 discusses metabolism, focusing on the production of ATP as the primary energy currency for cellular processes.
Metabolism is a complex network of enzymatic reactions that convert food into energy, involving both catabolic and anabolic pathways.
Includes detailed discussions on enzyme functionality, types of biochemical reactions, and the intricate mechanisms of enzyme regulation, essential for maintaining homeostasis.

Definition: Biological catalysts crucial for increasing the rate of biochemical reactions by lowering the activation energy required.
Naming: Enzymes typically end in "-ase" (e.g., lactase, amylase) and are classified based on the reactions they catalyze.
Composition:
- 100% Protein: Some enzymes, known as simple enzymes, are fully made up of amino acids.
- Apoenzymes: The inactive form of an enzyme, primarily composed of proteins; many require cofactors to become active.
- Cofactors:
  - Inorganic: Essential metal ions such as calcium and iron, which aid the formation of holoenzymes, the active form of the enzyme.
  - Organic: Often termed coenzymes (e.g., NAD, FAD), these are crucial for various enzymatic reactions and energy transfer.
Active Site: The specific three-dimensional region of the enzyme where substrate binding occurs and the catalysis process is initiated.
Specificity: Enzymes exhibit high specificity for their substrates, often only catalyzing a single type of reaction, which is known as the lock-and-key model.

Endoenzymes: Synthesized within the cell and function there (e.g., metabolic enzymes involved in cellular respiration).
Exoenzymes: Formed inside the cell but secreted outside to perform functions such as digestion or defense against pathogens (e.g., proteases).

Constitutive Enzymes: Always present and produced at a constant rate without regard to the environmental conditions (e.g., enzymes for glucose metabolism).
Induced Enzymes: Synthesized in response to specific substrates; production is contingent on cellular needs (e.g., lactose-digesting enzymes induced when glucose availability is low).

Catabolic Reactions: Involve breaking down larger, complex molecules into smaller, simpler ones, releasing energy in the process, commonly referred to as decomposition.
Anabolic Reactions: Construct larger macromolecules from smaller subunits, requiring energy input, highlighted by biosynthetic pathways.
Redox Reactions:
- Oxidation: Defined as the loss of electrons from a substance.
- Reduction: The gain of electrons, which often occurs simultaneously in metabolic pathways, emphasizing the importance of electron carriers such as NAD.

Turnover Number: Refers to the number of substrate molecules converted to product by an enzyme in a specified time, which can vary significantly from one enzyme to another, ranging typically from 1 to 10,000+ conversions per second.

Temperature: Elevated temperatures can lead to enzyme denaturation, disrupting hydrogen bonds and rendering the enzyme inactive, while low temperatures may slow down enzymatic activity.
pH: The optimal pH varies for different enzymes; deviations can lead to denaturation or reduced activity.
Substrate Concentration: Enzymatic activity increases with substrate concentration until a saturation point is reached where further increases do not affect the rate of reaction, echoed in the Michaelis-Menten kinetics.

Enzyme Inhibition: Critical for the control of metabolic pathways; when products accumulate, inhibition can help regulate enzyme activity to restore balance.
- Competitive Inhibition: Occurs when an inhibitor competes with the substrate for the active site; if the inhibitor is reversible, its effects can be overcome by increasing substrate concentration.
- Noncompetitive Inhibition: An inhibitor binds to an allosteric site, altering the enzyme's shape and functionality; this type of inhibition can often be irreversible and reduces the enzyme’s overall activity even when substrate levels are high.

Definition: Encompasses all biochemical reactions within a living organism, often categorized into anabolism and catabolism, and referred to collectively as respiration regardless of oxygen presence.
- Aerobic Metabolism: Involves the use of oxygen as the terminal electron acceptor in the electron transport chain, generating large ATP yields.
- Anaerobic Metabolism: Utilizes alternative electron acceptors, allowing organisms to metabolize substrates in the absence of oxygen, producing various metabolites (e.g., acids, gases).
ATP (Adenosine Triphosphate): Considered the primary energy carrier within cells; its hydrolysis releases energy for cellular work and metabolic processes.
Phosphorylation Types:
1. Oxidative Phosphorylation: Occurs during cellular respiration in both aerobic and anaerobic conditions, involving the electron transport chain.
2. Photophosphorylation: A light-dependent process used by plants to convert solar energy into chemical energy.
3. Substrate-level Phosphorylation: Occurs during the conversion of glucose to pyruvate in glycolysis; less efficient than oxidative phosphorylation.

Location: Occurs in the cytoplasm of both eukaryotic and prokaryotic organisms.
Process Overview: Initiates the breakdown of a single 6-carbon glucose molecule into two 3-carbon pyruvate molecules, involving an initial investment of ATP, which is crucial for the preparatory phase of glycolysis (investment phase).

Takes place in the cytoplasm of prokaryotes and within the mitochondria of eukaryotes.
Cardinal Steps:
- Prepares the substrate by converting pyruvate into acetyl-CoA through decarboxylation (loss of CO2).
- Acetyl-CoA initiates the cyclical processes that produce high-energy carriers, NADH and FADH2, as well as ATP/GTP.
Each glucose molecule metabolized through glycolysis results in two cycles of the Krebs cycle.

Location: Embedded in the cytoplasmic membrane for prokaryotes and the inner mitochondrial membrane for eukaryotes.
Function: A series of redox reactions releasing electrons from reduced coenzymes to ultimately synthesize ATP via chemiosmosis.
- In aerobic respiration, oxygen serves as the terminal electron acceptor, resulting in water production; however, anaerobic conditions can yield various end products, highlighted in fermentation pathways.

The final metabolic process leading to the majority of ATP production facilitated by ATP synthase.
Mechanism:
- Protons are actively transported out of the inner mitochondrial membrane or cell membrane, resulting in a proton motive force.
- Protons flow back into the cell through ATP synthase, which drives the phosphorylation of ADP to ATP.
ATP Yield per glucose:
- Eukaryotes: Typically yield 36 ATP molecules per glucose molecule oxidized.
- Prokaryotes: Can produce up to 38 ATP per glucose due to differing membrane structures.

Functions as an alternative metabolic pathway when oxygen is limited; less efficient compared to aerobic processes.
It utilizes organic molecules as electron acceptors, resulting in the production of various metabolites (e.g., alcohol in yeast, lactic acid in muscle cells).