Biochemistry I: Carbohydrates & Biomolecules

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically in a ratio where hydrogen and oxygen are in the same proportion as in water, hence the formula (CH₂O)n. This indicates that for every carbon atom, there are two hydrogen atoms and one oxygen atom. The 'n' represents the number of repeating units in the carbohydrate, which can vary, leading to different types of carbohydrates such as sugars, starches, and cellulose. This formula reflects the basic structure of monosaccharides, the simplest form of carbohydrates.

Carbohydrates are defined as organic compounds consisting of carbon, hydrogen, and oxygen, typically with a hydrogen-oxygen atom ratio of 2:1. Chemically, they can be classified as polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). Aldoses contain one aldehyde group (-CHO), while ketoses contain one ketone group (C=O), both of which are paired with multiple hydroxyl groups (-OH). This structural characteristic is essential for their classification and function in biological systems, distinguishing them from other macromolecules like proteins and lipids.

Oligosaccharides are carbohydrates composed of a small number of monosaccharide units, typically ranging from 2 to 10. This classification distinguishes them from monosaccharides (single units) and polysaccharides (more than 10 units). Oligosaccharides play important roles in biological processes, including cell recognition and signaling, making their specific range of units significant in biochemistry.

Fructose is classified as a ketohexose because it is a six-carbon sugar (hexose) that contains a ketone functional group. Unlike glucose and galactose, which are aldoses with an aldehyde group, fructose has its carbonyl group located at the second carbon atom, making it a ketose. This structural difference is key in distinguishing fructose from other monosaccharides, as it influences its reactivity and metabolic pathways in the body.

Sucrose is classified as a non-reducing sugar because it does not have a free aldehyde or ketone group that can participate in redox reactions. In its structure, the glucose and fructose units are linked by a glycosidic bond, which prevents the carbonyl group from being accessible for oxidation. Consequently, sucrose does not react with Benedict's or Fehling's solutions, which are used to test for reducing sugars. This characteristic distinguishes it from other sugars like glucose and fructose, which are reducing sugars due to their free carbonyl groups.

D-ribose is a five-carbon sugar that serves as a crucial component of ribonucleic acid (RNA), forming the backbone of its structure. Additionally, it is a vital part of various coenzymes, including adenosine triphosphate (ATP), which is essential for energy transfer in cells, as well as nicotinamide adenine dinucleotide (NAD) and its phosphate form (NADP), which play key roles in metabolic processes. Thus, D-ribose's involvement in both RNA and important coenzymes underscores its significance in cellular function and metabolism.

D-glucose and D-galactose are both six-carbon sugars (hexoses) that differ in the configuration around only one specific carbon atom, which is carbon 4. This difference in stereochemistry at carbon 4 classifies them as epimers, a subtype of diastereomers. While they share the same molecular formula and structure, the variation in the hydroxyl group orientation at this carbon leads to distinct biochemical properties and functions in biological systems.

In the Haworth configuration of cyclic sugars, the α-anomer is characterized by the orientation of the hydroxyl group at the anomeric carbon. For α-anomers, this hydroxyl group is positioned below the plane of the ring structure. In a typical representation, when the ring is drawn, the anomeric carbon is at the right side, and the hydroxyl group appears below the plane, indicating its specific stereochemistry. This orientation is crucial for distinguishing between the α and β forms of sugars in carbohydrate chemistry.

Invertase, also known as sucrase, specifically catalyzes the hydrolysis of sucrose into its constituent monosaccharides, D-glucose and D-fructose. This enzyme facilitates the breakdown of the glycosidic bond in sucrose, allowing the sugar to be utilized by organisms for energy. Unlike maltase and lactase, which act on other sugars, invertase is uniquely suited for sucrose, making it essential in various biological and industrial processes, such as in the production of sweeteners and in the digestion of sugars in the body.

Amylose is a component of starch and consists of long chains of α-D-glucose units. These glucose units are connected by α-1,4 glycosidic bonds, which link the first carbon of one glucose molecule to the fourth carbon of the next. This bonding arrangement results in a linear structure, enabling amylose to form helical shapes. In contrast, other types of glycosidic bonds, such as α-1,6 or β-1,4, lead to different structures and properties, which are not characteristic of amylose.

Glycogen is a polysaccharide that serves as a primary energy storage molecule in animals. Its structure features a highly branched arrangement, with branches occurring every 8 to 12 glucose units, allowing for rapid mobilization of glucose when energy is needed. In contrast, amylopectin, a component of starch found in plants, has fewer branches, occurring every 24 to 30 glucose units. This difference in branching contributes to glycogen's efficiency in energy release, making it more highly branched than amylopectin.

Normal plasma glucose levels in humans typically range from 70 to 100 mg/dL when fasting. This range is crucial for maintaining metabolic functions and overall health. Levels below 70 mg/dL may indicate hypoglycemia, while levels above 100 mg/dL can suggest hyperglycemia, potentially leading to diabetes. Monitoring glucose levels within this range is essential for diagnosing and managing various health conditions.

Carbohydrates serve several vital functions in the human body. They are primarily an essential source of energy, providing fuel for cellular processes. Additionally, certain carbohydrates contribute to the structural integrity of cell walls in bacteria, which is crucial for their survival. Moreover, carbohydrates play a role in cell recognition and signaling, as seen in cell surface antigens like blood group substances, which are important for immune response and compatibility in blood transfusions. However, they do not serve as a primary source of genetic information, which is the role of nucleic acids.

Lactase is an enzyme that facilitates the hydrolysis of lactose, a disaccharide sugar found in milk. During this process, lactase breaks down lactose into its constituent monosaccharides, glucose and galactose, making it easier for the body to absorb. Individuals who are lactose intolerant lack sufficient lactase, leading to difficulty in digesting lactose and resulting in gastrointestinal discomfort. Thus, lactase plays a crucial role in lactose digestion and metabolism.