Metabolic Reactions and Enzymes: Foundations of Biochemistry in Anatomy & Physiology

Metabolic Reactions and Enzymes

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within the body, essential for maintaining life. These reactions are broadly classified into two categories: anabolism (synthesis of complex molecules) and catabolism (breakdown of complex molecules). Metabolic reactions involve the making and breaking of chemical bonds, as well as the transfer of electrons between molecules.

• Anabolism: The synthesis of larger, complex molecules from simpler ones. Requires energy input.

• Catabolism: The breakdown of larger molecules into smaller ones. Releases energy.

• General Reaction: A + B ⇌ C + D (Reactants/Substrates ⇌ Products)

Types of Chemical Reactions in Metabolism

Chemical reactions in biological systems can be classified based on their energy requirements and the direction of energy flow.

• Exergonic Reactions: Energy-releasing reactions (catabolic/decomposition). Proceed spontaneously. Energy in reactants > energy in products.

• Endergonic Reactions: Energy-requiring reactions (anabolic/synthesis). Do not proceed spontaneously. Energy in reactants < energy in products.

• Exchange Reactions: Atoms or electrons are exchanged between molecules.

Comparison of Reaction Types

Type	Energy Flow	Example	Spontaneity
Exergonic (Catabolic)	Releases energy	ATP → ADP + Pi	Spontaneous
Endergonic (Anabolic)	Requires energy	ADP + Pi → ATP	Non-spontaneous

Activation Energy

Every chemical reaction requires an initial input of energy to proceed, known as activation energy. This energy is the 'push' that initiates the reaction, allowing reactants to reach a transition state before forming products.

• Activation Energy ($E_a$): The minimum energy required to start a chemical reaction.

• Enzymes lower the activation energy, increasing the rate of reaction.

Energy in Biological Reactions

Biological reactions involve the transfer and transformation of energy. The two main types are:

• Hydrolysis: Breakdown of molecules by adding water (catabolic).

• Dehydration Synthesis (Condensation): Formation of molecules by removing water (anabolic).

• Phosphorylation/Dephosphorylation: Addition/removal of phosphate groups, often regulating protein activity.

• Oxidation-Reduction (Redox): Transfer of electrons between molecules. LEO says GER: Loss of Electrons is Oxidation, Gain of Electrons is Reduction.

Chemical Equilibrium

Chemical equilibrium is reached when the rates of the forward and reverse reactions are equal, resulting in no net change in the concentration of reactants and products.

• Reversible Reactions: Can proceed in both directions (A + B ⇌ C + D).

• Irreversible Reactions: Proceed in one direction only (A + B → C + D).

Enzymes: Biological Catalysts

Structure and Function of Enzymes

Enzymes are proteins (and some RNA molecules) that catalyze biochemical reactions by lowering the activation energy required. They are highly specific for their substrates and are not consumed in the reaction.

• Active Site: The region on the enzyme where the substrate binds.

• Specificity: Each enzyme catalyzes a specific reaction or set of reactions.

• Lock-and-Key Model: Substrate fits exactly into the enzyme's active site.

• Induced-Fit Model: Binding of substrate induces a conformational change in the enzyme, enhancing catalysis.

Mechanism of Enzyme Action

• Enzyme binds substrate to form an enzyme-substrate complex.

• Transition state is stabilized, lowering activation energy.

• Products are released, and the enzyme is free to catalyze another reaction.

Examples of Enzyme-Catalyzed Reactions

Enzyme	Function
RNA polymerase	DNA unwinding and synthesis of RNA
DNA polymerase	DNA unwinding and synthesis of DNA
Protein kinase	Phosphorylation of proteins
Phosphoprotein phosphatase	Dephosphorylation of proteins
Catalase	Breakdown of hydrogen peroxide
Hexokinase	Phosphorylation of glucose
ATP synthase	Synthesis of ATP
Lactate dehydrogenase	Conversion of lactic acid to pyruvic acid
Glucose-6-phosphatase	Removal of phosphate from glucose-6-phosphate
Carbonic anhydrase	Conversion of carbonic acid to water and carbon dioxide
Amylase	Breakdown of complex carbohydrates
Lipase	Breakdown of triglycerides
Sucrase	Breakdown of sucrose

Cofactors and Coenzymes

Many enzymes require non-protein helpers for activity:

• Cofactors: Inorganic ions (e.g., Mg2+, Cu2+, Zn2+, Fe2+) that help maintain enzyme conformation.

• Coenzymes: Organic molecules (often derived from vitamins) that transfer chemical groups between molecules.

Coenzyme	Vitamin Source	Group Transferred
NAD+	Niacin (B3)	H+ (hydrogen)
FAD	Riboflavin (B2)	H+ (hydrogen)
Coenzyme A	Pantothenate (B5)	Acetyl groups

Regulation of Enzyme Activity

Enzyme activity is tightly regulated to meet the needs of the cell and organism.

• Allosteric Regulation: Binding of a molecule at a site other than the active site alters enzyme activity (can be inhibitory or activating).

• Covalent Regulation: Chemical groups (e.g., phosphate) are covalently attached or removed, changing enzyme activity.

• Feedback Inhibition: End product of a metabolic pathway inhibits an earlier enzyme, preventing overproduction.

• Feedforward Activation: Early substrate or intermediate activates a downstream enzyme, enhancing pathway efficiency.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; too high or too low reduces activity.

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme.

• Enzyme Concentration: Increasing enzyme concentration increases reaction rate (if substrate is not limiting).

• Substrate Concentration: Increasing substrate concentration increases reaction rate up to a maximum (Vmax).

• Affinity: The strength of the enzyme-substrate interaction affects reaction rate.

• Cofactor/Coenzyme Availability: Required for full enzyme activity.

Metabolic Pathways and Their Regulation

Metabolic pathways are sequences of enzyme-catalyzed reactions leading to a specific product. The slowest step is called the rate-limiting step, often regulated by feedback inhibition or allosteric mechanisms.

• Inborn Errors of Metabolism: Genetic mutations affecting enzymes can lead to accumulation of intermediates or deficiency of end-products, causing metabolic diseases.

Clinical Applications: Diagnostic Enzymes

Measurement of specific enzymes in plasma can aid in the diagnosis of various diseases.

Enzyme	Diagnostic Value
Alkaline phosphatase	Obstructive jaundice, Paget's disease, bone carcinoma
Acid phosphatase	Benign prostatic hypertrophy, prostate cancer
Amylase	Pancreatitis, perforated peptic ulcer
Aldolase	Muscular dystrophy
Creatine kinase	Muscular dystrophy, myocardial infarction
Lactate dehydrogenase (LDH)	Myocardial infarction, liver disease, renal disease, pernicious anemia
Transaminases (AST, ALT)	Myocardial infarction, hepatitis, muscular dystrophy

Additional info: Enzyme deficiencies can be inherited (inborn errors of metabolism) and may require dietary or medical management. Enzyme assays are important tools in clinical diagnostics.