Metabolic Pathways, Enzyme Function, and Energy Production: GOB Chemistry Study Guide

Enzymes

Enzyme Models and Function

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. Understanding their mechanisms is essential for grasping metabolic processes.

• Lock and Key Model vs. Induced Fit Model: The lock and key model suggests that the enzyme's active site is a perfect fit for the substrate, while the induced fit model proposes that the enzyme changes shape to accommodate the substrate.

• Enzyme Substrate Binding: Enzymes bind substrates at their active sites, forming an enzyme-substrate complex.

• Enzyme Cofactors: Cofactors are non-protein molecules (such as metal ions or organic molecules) required for enzyme activity. Coenzymes are organic cofactors, often derived from vitamins. Holoenzyme refers to the complete enzyme with its cofactor, while apoenzyme is the protein part alone.

Enzyme Activity and Regulation

Enzyme activity is influenced by several factors, including temperature, pH, and substrate concentration.

• Temperature and pH: Each enzyme has an optimal temperature and pH for activity. Deviations can decrease activity or denature the enzyme.

• Substrate Concentration: Increasing substrate concentration increases reaction rate until the enzyme is saturated.

• Enzyme Inhibition: Competitive inhibitors bind to the active site, blocking substrate binding. Noncompetitive inhibitors bind elsewhere, altering enzyme shape and reducing activity.

• Regulation: Enzymes are regulated by positive and negative modulators, which can increase or decrease activity.

Enzyme Kinetics and Inhibition

Understanding enzyme kinetics helps explain how enzymes function and are regulated in metabolic pathways.

• Reversible vs. Irreversible Inhibition: Reversible inhibitors can dissociate from the enzyme, while irreversible inhibitors permanently inactivate the enzyme.

• Example: Aspirin acts as an irreversible inhibitor of cyclooxygenase enzymes.

Carbohydrate Metabolism: Glycolysis, Gluconeogenesis, Glycogen Metabolism

Metabolic Pathways Overview

Metabolism consists of catabolic (breakdown) and anabolic (synthesis) pathways. Carbohydrate metabolism is central to energy production.

• ATP Production: ATP (adenosine triphosphate) is the main energy currency of the cell, produced during catabolic reactions.

• Glycolysis: The breakdown of glucose to pyruvate, generating ATP and NADH.

• Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors.

• Glycogen Metabolism: Glycogen is the storage form of glucose in animals. Glycogenolysis breaks down glycogen; glycogenesis synthesizes glycogen.

Key Steps and Regulation

• Feeder Pathways: Galactose and fructose are converted into glycolytic intermediates in muscle and liver.

• Pyruvate Fate: Pyruvate can be converted to lactate (anaerobic), acetyl-CoA (aerobic), or used in gluconeogenesis.

• Regulation: Hormones such as insulin and glucagon regulate glycolysis and gluconeogenesis.

• Key Enzymes: Hexokinase, phosphofructokinase, and pyruvate kinase are major regulatory enzymes in glycolysis.

Glycolysis Summary Table

Step	Enzyme	Product
1	Hexokinase	Glucose-6-phosphate
3	Phosphofructokinase	Fructose-1,6-bisphosphate
10	Pyruvate kinase	Pyruvate

Energy Production: Citric Acid Cycle and Electron Transport Chain

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of reactions that oxidize acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Main Purpose: To harvest high-energy electrons for the electron transport chain.

• Key Products per Turn: 3 NADH, 1 FADH2, 1 GTP (converted to ATP), 2 CO2

• Location: Mitochondrial matrix

Electron Transport Chain (ETC)

The ETC uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis.

• Complexes: Four main protein complexes (I-IV) transfer electrons to oxygen, the final electron acceptor.

• ATP Yield: Approximately 30-32 ATP per glucose molecule.

• Oxidative Phosphorylation: ATP is produced as protons flow back into the mitochondrial matrix through ATP synthase.

Summary Table: Citric Acid Cycle Products

Product	Per Acetyl-CoA
NADH	3
FADH2	1
GTP (ATP)	1
CO2	2

Lipid and Amino Acid Metabolism

Lipid Catabolism

Lipids are broken down to fatty acids and glycerol, which are further metabolized for energy.

• Beta-Oxidation: Fatty acids are oxidized in the mitochondria, producing acetyl-CoA, NADH, and FADH2.

• Energy Yield: Fatty acids are more reduced than carbohydrates, yielding more ATP per molecule.

• Transport: Fatty acids are activated and transported into the mitochondria via the carnitine shuttle.

Amino Acid Metabolism

Amino acids can be used for energy, especially during fasting or starvation.

• Deamination: Removal of the amino group, producing ammonia and a carbon skeleton.

• Glucogenic vs. Ketogenic Amino Acids: Glucogenic amino acids can be converted to glucose; ketogenic amino acids are converted to ketone bodies.

• ATP Calculation: The number of cycles in beta-oxidation and the resulting ATP can be calculated based on the fatty acid length.

Key Equations

• ATP Yield from Glucose: $\text{Glucose} + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + 30-32\,\text{ATP}$

• Beta-Oxidation: $\text{Fatty Acid} + \text{CoA} + \text{FAD} + \text{NAD}^+ \rightarrow \text{Acetyl-CoA} + \text{FADH}_2 + \text{NADH}$

Additional info: The notes cover topics from Chapters 20-24 of a typical GOB Chemistry course, focusing on enzymes, metabolic pathways, and energy production. These are essential for understanding biochemistry in health sciences.