Cellular Metabolism

Types of Chemical Reactions

· Decomposition (Catabolism) – breaks down molecules into smaller fragments;

o provides energy

· Synthesis (Anabolism) – assembles larger molecules;

o uses energy

Reversible Reactions – decomposition and synthesis occur simultaneously
Exchange Reaction – molecules are shuffled around to make new molecules

Hydrolysis and Condensation

· Hydrolysis – splitting by adding water

AB + H₂O ————→ A-OH + B-H

· Condensation – joining by losing water

A-OH + B-H ————→ AB + H₂O

Phosphorylation and Dephosphorylation

· – adding phosphate (HPO₄^-3, H₂PO₄^-2)

A + P_i ————→ A-P_i

· – removing phosphate

A-P_i ————→ A + P_i

Oxidation and Reduction

· Reduction – adding electrons or hydrogen atoms

A + H ————→ A-H

· Oxidation – removing electrons, hydrogen toms, or reacting with O₂

A-H————→ A + H

Energy of Reactions

When the potential energy of the products is greater than the potential energy of the reactants the reaction requires energy. (if ΔE is positive, the reaction requires energy)

ΔE = E _products – E _reactants

The rate of a reaction is determined by

· Concentration of the reactants and the products

o Reactions follow the Law of Mass Action – the reaction will proceed from highest concentration to lowest concentration

· Temperature

o Reactions will proceed more rapidly with higher temperature

· Activation energy – the transition state in a reaction possesses a higher potential energy than either the reactants or the products

o By lowering the activation energy the reaction will proceed more rapidly

o Enzymes lower activation energy

Cellular Metabolism

· Enzymes - specific proteins used as catalysts

· Reactants (Substrates) - reacting compounds

· Products - compounds that result from the reaction

enzyme

Reactants ————→ Product

· Enzymes usually act by attaching to a reactant and lowering the activation energy of the reaction

enzyme + Reactant—→ enzyme-Reactant—→ Product + enzyme

· Metabolic Pathway – A chemical reaction that often contains many steps

enzyme enzyme enzyme

A + X ————→ B ————→ C ————→ D + Y

intermediates

Regulation of Enzymatic activity

Enzymatic activity is affected by

· The catalytic rate of the enzyme

· Changes in enzyme concentration

· Affinity of the enzyme for the reactant (substrate)

· Allosteric regulation – a modulator binds to a regulatory site of an enzyme and changes its activity

o Enzyme Activation

o Enzyme Inhibition

o Permits feedback regulation

· Covalent regulation – covalent bonding of a chemical group to an enzyme, where the bonding is controlled by another enzyme

o Permits feedback regulation

Cofactors(Metal ions and vitamins) help an enzyme hold its normal conformation

Coenzymes

Coenzymes participate in reactions catalyzed by enzymes

· Usually carry particular chemical groups from one reaction to another

· NAD⁺(nicotinamide adenine dinucleotide) – carry hydrogen (electron)

· FAD (flavin adenine dinucleotide) – carry hydrogen (electron)

· CoA (coenzyme A) – carry acetyl groups (-CH₂COOH)

Control of Cell function

· Enzyme activity – regulatory enzymes provide a major mechanism for short term control of cell function

· Enzyme synthesis – control of gene expression provides a major mechanism for long term control of cell function

Carbohydrate metabolism

Glycolysis

breaks down glucose (a six carbon molecule) to two pyruvate molecules (each a three carbon molecule) and yields ATP and reduced coenzymes
requires ATP (adenosine triphosphate), ADP (adenosine diphosphate), NAD (nicotinamide adenine dinucleotide)

Glucose + 2 NAD⁺ + 2 ADP + 2 P_i →
2 pyruvate+ 2 NADH + 2 H⁺ + 2 ATP (net 2 ATP)

Decarboxylation

formation of acetyl-coenzyme A from pyruvate and yields carbon dioxide and reduced coenzymes
requires coenzyme A and NAD

2 Pyruvate+ 2 CoA + 2 NAD⁺ → 2 Acetyl-CoA + 2 CO₂ + 2 NADH + 2H⁺

Tricarboxylic acid (TCA) cycle (Krebs cycle)

uses acetyl-CoA to yield carbon dioxide, reduced coenzymes and ATP
requires a four carbon molecule, NAD and FAD (flavin adenine dinucleotide)

2 Acetyl-CoA + 6 NAD⁺ + 2 FAD + 2 ADP + 2 P_i →
4 CO₂ + 6 NADH + 6 H⁺ + 2 FADH₂ + 2 ATP

Electron transport system

uses oxygen to yield ATP
requires NADH, FADH₂, Oxygen and ADP

10 NADH₂ + 10 H⁺ + 2 FADH₂ + 6 O₂ + 34 ADP + 34 P_i →
10 NAD⁺ + 2 FAD + 12 H₂O + 34 ATP

Glycogenolysis

· glycogen → glucose

Glycogenesis

glucose → glycogen

Gluconeogenesis

glucose synthesis from pyruvate and other three carbon molecules

pyruvate → glucose
glycerol → glycerol phosphate
lactate →pyruvate
some amino acids → pyruvate
some amino acids → phosphenopyruvate

Lipid metabolism

Lipolysis

breakdown of lipids (triglycerides) to glycerol and fatty acids through hydrolysis

· Triglyceride → Glycerol + 3 Fatty acids

· Glycerol → pyruvate (for TCA cycle)

· Fatty acids → acetyl CoA (beta oxidation)

· Ketones are an alternate energy source for the nervous system

Protein metabolism

Amino acid catabolism

breakdown of amino acids by removal of amino groups to produce ketoacids

some amino acids → pyruvate
some amino acids → phosphenopyruvate

some amino acids → acetyl CoA