Carbohydrate Metabolism: A Deep Dive into Digestion, Glycolysis, and Energy Production
Introduction
Carbohydrates are aldehydes or ketones of higher polyhydric alcohol or compounds that yield these derivatives (aldehydes or ketones) on hydrolysis. The general molecular formula of carbohydrate is Cn(H2O)n. Examples: starch, sucrose, lactose, fructose, galactose.
Digestion
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma (bloodstream).
Primary Stage Introduction
It is the breakdown and absorption of food along the gastrointestinal tract (GIT).
Mouth:
- Substrate: Starch
- Enzyme: Salivary amylase
- Product: Maltose and dextrin
Stomach
- Substrate: –
- Enzyme: No carbohydrate-splitting enzyme
- Product: –
Duodenum
- Substrate: Amylose
- Enzyme: α-amylase (pancreatic)
- Product: Dextrin, maltose, and maltotriose
Small Intestine
- Substrate: Amylose
- Enzyme: Pancreatic amylase
- Product: Free glucose molecule, maltose, dextrin
- Substrate: Lactose
- Enzyme: Lactase
- Product: Glucose + galactose
- Substrate: Maltose
- Enzyme: Maltase
- Product: Glucose + glucose
- Substrate: Sucrose
- Enzyme: Sucrase
- Product: Glucose + fructose
Regulation
- Mouth: Activated by Cl-, Optimum pH of 6-7
- Stomach: NIL
- Duodenum: Activated by Cl-, Optimum pH of 7.1
- Small Intestine pH:
- Lactase: 5.4-6.0
- Maltase: 5.8-6.2
- Sucrase: 5.0-7.0
Secondary Stage (Glycolysis) Introduction
This is the degradation of glucose to form pyruvic acid in the presence of oxygen and lactic acid in the absence of oxygen with the release of ATP.
Pathway
| NADH | 3ATP | |
|---|---|---|
| • | 1,3 bisphosphoglycerate | 1 ATP |
| • | PEP | 1ATP |
| • | NADH (3ATP × 2) | 6ATP |
| • | Substrate level | |
| • | 1,3 bisphosphoglycerate (1ATP × 2) | 2ATP |
| • | PEP (1ATP × 2) | 2ATP |
| • | TOTAL | 10ATP |
| • | USED | 2ATP |
| • | NET | 8ATP |
Regulation
- Enzymes
- Hexokinase
- Pyruvate kinase
- Glucokinase
- PFK
- Hormones
- Insulin
- Glucagon
- Glucocorticoids
Tertiary Stage Introduction
This is the series of reactions in mitochondria which oxidizes acetyl CoA to CO₂, H₂O, and reduced H₂ carriers that are oxidized through respiratory chains for ATP synthesis.
Kinetics
GTP, FADH2.
NADH
- 3NADH (3ATP × 3)
- 1FADH₂ (2ATP × 1).
| 1ATP |
|---|
| . |
GTP (1ATP × 1)
TOTAL
| 2ATP |
|---|
| * 12ATP |
| 9ATP |
|---|
1ATP
| ЗАТР |
|---|
| 2ATP |
| ——– |
Regulation
- Citrate synthase
- Mitochondrial isocitrate dehydrogenase
- Succinate dehydrogenase
- α-ketoglutarate dehydrogenase
- Inhibition
- Flouroacetate
- Arsenate
- Malonate
