The Molecular Formula: C₆H₁₂O₆
The fundamental formula for galactose is C₆H₁₂O₆. This molecular formula indicates it is a hexose with six carbon atoms. Galactose belongs to the monosaccharide class of carbohydrates, which have a general formula of $(CH_2O)_n$. For galactose, $n=6$, fitting this general formula. However, this formula is shared with isomers like glucose and fructose, which have different structural arrangements.
Open-Chain and Cyclic Structures
Beyond the molecular formula, the structural formula shows how atoms are arranged. Galactose exists in an open-chain form (Fischer projection) and a more stable cyclic ring form (Haworth projection).
- Open-Chain Form: Galactose is an aldohexose with an aldehyde group (CHO) at one end, making it a reducing sugar.
- Cyclic Form: In water, galactose is mainly cyclic. The ring forms when the aldehyde group on carbon-1 reacts with the hydroxyl on carbon-5, creating a six-membered pyranose ring.
Alpha (α) and Beta (β) Anomers
Cyclization creates anomers at carbon-1: alpha ($α$) and beta ($β$). These differ in the hydroxyl group's position on carbon-1 and affect how enzymes process them.
- Alpha ($α$)-Galactose: Hydroxyl at carbon-1 points down (axial).
- Beta ($β$)-Galactose: Hydroxyl at carbon-1 points up (equatorial).
The C-4 Epimerism: A Key Structural Distinction
A crucial difference between galactose and glucose is their epimeric relationship. Galactose is a C-4 epimer of glucose, meaning they differ only at the fourth carbon's hydroxyl group orientation. This seemingly small difference allows the body's enzymes to distinguish and process them via different metabolic pathways. In D-glucose, the C-4 hydroxyl is down in the Haworth projection, while in D-galactose, it's up.
Comparing Galactose and Glucose
Despite the same formula, the structural difference gives galactose and glucose distinct properties.
| Feature | Galactose | Glucose |
|---|---|---|
| Molecular Formula | C₆H₁₂O₆ | C₆H₁₂O₆ |
| Structural Relationship | C-4 epimer of glucose | C-4 epimer of galactose |
| Hydroxyl at C-4 (Haworth) | Positioned upwards (axial) | Positioned downwards (equatorial) |
| Stability | Less stable than glucose, converted to it by the liver. | More stable than galactose. |
| Sweetness | Approximately 65% as sweet as sucrose, and roughly the same as glucose. | Approximately 70-80% as sweet as sucrose. |
| Metabolism | Metabolized via the Leloir pathway. | A primary energy source for cellular respiration. |
| Biological Role | Part of lactose; component of glycolipids and glycoproteins. | Primary source of cellular energy; forms starch and glycogen. |
Biological Significance of Galactose
Galactose has several vital functions in the body.
- Lactose Formation: It combines with glucose to form lactose, the disaccharide in milk.
- Brain Function: It's a key part of glycolipids and glycoproteins essential for the central nervous system, sometimes called "brain sugar".
- Metabolism: The liver converts galactose to glucose for energy via the Leloir pathway. Galactosemia is a disorder where the body cannot metabolize galactose.
Natural Sources of Galactose
Galactose comes from diet and is produced internally. Sources include:
- Dairy Products: A major source as part of lactose.
- Avocados: Contain smaller amounts.
- Sugar Beets: Another natural source.
- Gums and Mucilages: Contain galactans, polymeric forms of galactose.
The Empirical Formula
The empirical formula for many carbohydrates, including galactose, is $(CH_2O)$. This shows the simplest atom ratio (1:2:1 for C₆H₁₂O₆). However, this formula doesn't distinguish isomers like glucose and galactose; the full molecular and structural formulas are needed for that.
Conclusion
The formula for galactose is C₆H₁₂O₆. Although identical to glucose's formula, a key structural difference at the C-4 carbon makes them distinct molecules with varied properties and biological roles, from milk digestion to brain function. For more on its properties, consult authoritative chemical databases like the one from NIH.
