Carbohydrates Biochemistry Carbohydrate metabolism is a fundamental biochemical process that provides energy for the body and supports various physiological functions. Glucose, the primary carbohydrate, undergoes multiple metabolic pathways to meet the body’s energy requirements, store energy, and synthesize other vital molecules. This article explores the fate of glucose, the glycolytic pathway, its biomedical importance, and specific enzymes such as hexokinase and glucokinase.

Fate of Glucose

The metabolic fate of glucose includes several key pathways:

1. Oxidation:
   • Glycolysis: This is the primary pathway for the oxidation of glucose or glycogen to pyruvate and lactate.
   • HMP Shunt: An alternative oxidative pathway for glucose, providing NADPH for reductive synthesis and pentoses for nucleic acid synthesis.
   • Uronic Acid Pathway: Another alternative pathway that produces D-glucuronic acid, necessary for mucopolysaccharide synthesis and detoxification reactions.
2. Storage as Glycogen:
   • Glycogenesis: The process of converting excess glucose into glycogen, which is stored primarily in the liver and muscles for future energy needs.
3. Conversion to Fat:
   • Lipogenesis: When glycogen storage capacity is exceeded, glucose is converted into fatty acids and stored as triacylglycerol in adipose tissue.
4. Conversion to Amino Acids:
   • Glucose serves as a precursor for synthesizing certain non-essential amino acids required by the body.
5. Conversion to Other Sugars:
   • Ribose, Fructose, Mannose, Galactose: Glucose is converted into these sugars, which play specific roles in the body, such as forming nucleic acids, sperm metabolism, and mucopolysaccharides.

Glycolysis

Definition:
Glycolysis is the process by which glucose or glycogen is oxidized to pyruvate and lactate. Described by Embden, Meyerhof, and Parnas, this pathway is also known as the Embden-Meyerhof pathway. Glycolysis occurs in almost all tissues, particularly in erythrocytes and nervous tissues, where it is the primary source of energy. This pathway is unique because it can function both aerobically (in the presence of oxygen) and anaerobically (in the absence of oxygen).

Two Phases of Glycolysis

1. Aerobic Phase:
   In the aerobic phase, oxidation is carried out through dehydrogenation, where reducing equivalents are transferred to NAD+. Reduced NAD+ is then oxidized in the electron transport chain in the presence of oxygen, producing ATP.
2. Anaerobic Phase:
   In the anaerobic phase, NADH cannot be oxidized through the electron transport chain, resulting in no ATP production via this pathway. Instead, NADH is oxidized to NAD+ by converting pyruvate to lactate, without ATP production. This phase limits the energy yield per mole of glucose oxidized. To produce the same amount of energy, more glucose must undergo glycolysis anaerobically than aerobically.

Biomedical Importance of Glycolysis

• Provision of Energy:
  Glycolysis is crucial for providing energy, especially in tissues with limited oxygen supply.
• Skeletal Muscle:
  In skeletal muscle, glycolysis provides ATP even in the absence of oxygen, enabling muscles to survive brief periods of anoxia.
• Heart Muscle:
  Unlike skeletal muscle, the heart muscle is better suited for aerobic metabolism. It has lower glycolytic activity and is less capable of surviving ischemic conditions.
• Role in Cancer Therapy:
  In fast-growing cancer cells, the rate of glycolysis is significantly increased, producing more pyruvic acid than the tricarboxylic acid (TCA) cycle can process. The accumulation of pyruvic acid leads to excess lactic acid, causing local lactic acidosis, which may be beneficial for certain cancer therapies.
• Hemolytic Anemias:
  Inherited enzyme deficiencies, such as hexokinase or pyruvate kinase deficiency, can disrupt glycolysis, leading to hemolytic anemia.

Reactions of the Glycolytic Pathway

Glycolysis involves a series of reactions that convert glucose or glycogen to pyruvate or lactate. These reactions can be divided into four stages for better understanding:

Stage I: Preparatory Phase

Before glucose can be split, it is converted from an asymmetric molecule to a nearly symmetrical form, fructose 1,6-bisphosphate, through the addition of two phosphate groups from ATP.

1. Uptake of Glucose by Cells and Phosphorylation:
   Glucose is freely permeable to liver cells. Insulin facilitates glucose uptake in skeletal muscle, cardiac muscle, diaphragm, and adipose tissue. Once inside the cell, glucose is phosphorylated to form glucose-6-phosphate (G-6-P), catalyzed by glucokinase in liver cells and by non-specific hexokinase in other tissues.
2. Conversion of G-6-P to Fructose-6-P:
   G-6-P is isomerized to fructose-6-phosphate (F-6-P) by the enzyme phosphohexose isomerase, involving an aldose-ketose isomerization.
3. Conversion of Fructose-6-P to Fructose-1,6-Bisphosphate:
   F-6-P is phosphorylated with ATP at the 1-position by phosphofructokinase-1, forming the symmetrical molecule fructose-1,6-bisphosphate.

Energetics of Glycolysis in Stage I:

During this stage, no useful energy is generated; rather, two ATP molecules are consumed for phosphorylation, resulting in a net energy expenditure of -2 ATP.

Stage II: Splitting Phase

Fructose-1,6-bisphosphate is split by aldolase into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP). These molecules are interconvertible.

Stage III: Energy-Yielding Reactions

This stage includes reactions where an aldehyde group is oxidized to an acid, releasing significant energy.

1. Oxidation of Glyceraldehyde-3-Phosphate to 1,3-Bisphosphoglycerate:
   The enzyme glyceraldehyde-3-phosphate dehydrogenase, which is NAD+ dependent, catalyzes this reaction.
2. Conversion of 1,3-Bisphosphoglycerate to 3-Phosphoglycerate:
   The enzyme phosphoglycerate kinase catalyzes this reaction, where a high-energy phosphate bond at the 1-position donates its phosphate to ADP, forming ATP. This is a substrate-level phosphorylation, an example of ATP production without the electron transport chain.

Inhibitors of Glycolysis:

• Arsenite: Competes with inorganic phosphate in the reaction converting glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, forming 1-arseno-3-phosphoglycerate instead, which hydrolyzes spontaneously, preventing ATP production.
• Iodoacetate and Iodoacetamide: These bind irreversibly to the –SH groups of glyceraldehyde-3-phosphate dehydrogenase, inhibiting glycolysis and leading to the accumulation of glyceraldehyde-3-phosphate.

Energetics of Glycolysis in Stage III:

1. The first reaction produces 2 NADH molecules, which can generate 6 ATP molecules in the presence of oxygen via the electron transport chain.
2. The second reaction produces 2 ATP molecules directly. The net gain at this stage per molecule of glucose oxidized is +8 ATP.

Stage IV: Recovery Phase

This stage involves the recovery of phosphate groups from 3-phosphoglycerate.

1. Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate:
   Catalyzed by phosphoglycerate mutase, this reaction likely involves 2,3-bisphosphoglycerate as an intermediate.
2. Conversion of 2-Phosphoglycerate to Phosphoenolpyruvate (PEP):
   The enzyme enolase catalyzes this reaction, requiring Mg++ or Mn++ for activity.
3. Conversion of Phosphoenolpyruvate to Pyruvate:
   Catalyzed by pyruvate kinase, this reaction transfers the high-energy phosphate of PEP to ADP, forming ATP.

Clinical Importance and Functions of Fluoride:

• Fluoride: Inhibits the enzyme enolase, preventing glycolysis. Sodium fluoride is used in blood collection to prevent in vitro glycolysis and ensure accurate glucose measurements.

Functions of Fluoride:

• Inhibits in vitro glycolysis by inhibiting enolase.
• Acts as an anticoagulant.
• Serves as an antiseptic.

Conclusion:

Understanding carbohydrate metabolism, particularly glycolysis, is essential for healthcare professionals, as it underlies many physiological processes and clinical conditions. Glycolysis provides a critical source of energy for cells, especially those with limited oxygen supply, and is pivotal in various medical conditions, including cancer therapy and hemolytic anemias. The enzymes involved, their inhibitors, and the specific biochemical pathways provide insight into how glucose metabolism is regulated and manipulated in clinical practice.