Nutrition, Metabolism, and Body Temperature Regulation: Metabolic Pathways and Diabetes Mellitus

Nutrition, Metabolism, and Body Temperature Regulation

Overview

This section covers the biochemical pathways of nutrient metabolism, focusing on the roles of redox reactions, ATP synthesis, and the metabolic fates of carbohydrates, lipids, and proteins. It also discusses the interconversion of nutrients and the pathophysiology of diabetes mellitus.

Redox Reactions & Coenzymes

Oxidation-Reduction (Redox) Reactions

• Redox reactions are coupled reactions involving the simultaneous transfer of electrons (e-) or entire hydrogen atoms (H).

• Oxidation is the loss of electrons or hydrogen; reduction is the gain of electrons or hydrogen.

• Enzymes involved: dehydrogenases (oxidation), oxidases (reduction).

Process	Loss	Gain	Energy	Enzyme
Oxidation	Hydrogen	Oxygen	Lost	Dehydrogenase
Reduction	Oxygen	Hydrogen	Gained	Oxidase

• Coenzymes are organic molecules required by redox enzymes to function as hydrogen acceptors (e.g., NAD+, FAD).

• Coenzymes are reduced to NADH and FADH2 during key metabolic reactions.

• They are essential for Acetyl CoA formation and Krebs Cycle intermediary steps.

Additional info: Redox reactions are fundamental to cellular energy production, as they allow the transfer of energy via electrons in metabolic pathways.

Cellular Respiration: ATP Synthesis

Mechanisms of ATP Synthesis

1. Substrate-level phosphorylation: Direct transfer of a high-energy phosphate group to ADP from a substrate, producing ATP and a product. Occurs in both cytoplasm and mitochondria. This process is anaerobic and does not require oxygen.

2. Oxidative phosphorylation: Coupled reactions involving the movement of hydrogen ions (H+) across a membrane via the electron transport chain. Produces much more ATP than substrate-level phosphorylation and occurs only in mitochondria. This process is aerobic and requires oxygen.

Equation for ATP synthesis (oxidative phosphorylation):

$\text{ADP} + P_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

Additional info: Substrate-level phosphorylation is responsible for ATP production in glycolysis and the Krebs cycle, while oxidative phosphorylation is the main source of ATP in cells.

Glucose Utilization

Pathways of Glucose Metabolism

• All carbohydrates are ultimately converted to glucose or its metabolites.

• Glucose enters cells by facilitated diffusion, a process enhanced by insulin.

• Three main pathways generate ATP from glucose:

  1. Glycolysis: Net yield of +2 ATP per glucose molecule.

  2. Krebs cycle: Net yield of +2 ATP (1 ATP per turn, two turns per glucose).

  3. Electron transport chain/oxidative phosphorylation: Net yield of ~28 ATP.

• Total ATP produced from one glucose molecule: $32-34$

Glucose Catabolism

Glycolysis

• Oxygen-independent catabolism of glucose in the cytoplasm via substrate-level phosphorylation.

• Net yield: +2 ATP (2 consumed, 4 made).

• NAD+ is reduced to NADH.

• Produces 2 pyruvic acids.

• Fate of pyruvate depends on oxygen availability:

  • With O2: Pyruvic acid is oxidized to acetyl CoA.

  • Without O2: Pyruvic acid is reduced to lactic acid by NADH.

Glucose Oxidation

Krebs Cycle (Citric Acid Cycle)

• Occurs in the mitochondrial matrix via substrate-level phosphorylation.

• Pyruvate is transformed into acetyl-CoA, which enters the cycle.

• Citric acid is converted through several intermediates, producing CO2, reduced coenzymes (NADH, FADH2), and ATP.

• Each glucose yields two turns of the cycle, netting +2 ATP.

Electron Transport Chain & Oxidative Phosphorylation

• Occurs between mitochondrial membranes and is O2-dependent.

• Reduced NADH and FADH2 transfer electrons to enzyme complexes, creating a proton (H+) gradient.

• Energy from electron transfer pumps H+ into the intermembrane space, making it more positive.

• H+ flows back through ATP synthase, powering ATP production.

• Equation:

$\text{ADP} + P_i + 2H^+ + 1/2 O_2 \rightarrow \text{ATP} + H_2O$

Carbohydrate Metabolism (Other than Glycolysis)

Key Processes

• Glycogenesis: Formation of glycogen from glucose when ATP needs are met (storage in liver/muscle).

• Glycogenolysis: Breakdown of glycogen to glucose in response to low blood glucose (hypoglycemia).

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (pyruvate, glycerol, amino acids, fatty acids, proteins).

Additional info: These processes ensure a continuous supply of glucose for ATP synthesis, especially during fasting or intense exercise.

Lipid Metabolism

Key Processes

• Lipogenesis: Synthesis of triglycerides from glycerol and fatty acids when dietary intake exceeds energy needs (storage in adipose tissue).

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids during hypoglycemia.

• Glycerol is converted into a glucose intermediary and metabolized into pyruvic acid.

• Fatty acids are converted into acetyl CoA, which enters the Krebs cycle.

Protein Metabolism

Key Processes (Primarily in Liver)

• Transamination: Transfer of an amino group from one amino acid to another, generating a keto acid (e.g., α-ketoglutarate, a Krebs cycle component).

• Keto acids can be converted into glucose during hypoglycemia.

• Deamination: Removal of the amino group from an amino acid when excess amino acids are present; residual carbon/hydrogen is oxidized for energy.

• Produces keto acids and releases ammonia (NH3), which is converted to urea in the liver and excreted by the kidneys.

Nutrient Interconversion (Switching Gears)

Metabolic Flexibility

• Fats and proteins can be converted into glucose (or intermediates) and/or acetyl CoA when carbohydrates are insufficient.

• Excessive lipid catabolism (lipolysis) can lead to ketoacidosis due to accumulation of ketone bodies.

• Increased protein catabolism results in loss of body mass and increased keto acids.

Additional info: This metabolic flexibility is crucial during fasting, starvation, or uncontrolled diabetes.

Diabetes Mellitus Pathology

Metabolic Consequences of Insulin Deficiency

• Insulin deficiency causes hyperglycemia (elevated blood glucose).

• Results from increased glycogenolysis and uninhibited gluconeogenesis.

• Diabetic ketoacidosis: Metabolic state where ketone bodies are formed by excessive fatty acid/amino acid breakdown.

• Accumulation of keto acids lowers blood pH (metabolic acidosis).

• Without insulin control:

  • Type 1 DM adults: May progress to coma or death.

  • Type 1 DM children: Rapid weight loss and risk of death.

• Three cardinal signs of diabetes:

  • Polyuria (excessive urination)

  • Polydipsia (excessive thirst)

  • Polyphagia (excessive hunger)

Additional info: Diabetes mellitus is a major metabolic disorder with systemic effects due to impaired glucose utilization and increased fat/protein catabolism.