Carbohidratos: Estructura y función #BIOQUÍMICA

What Are Carbohydrates?

Introduction to Carbohydrates

• The discussion begins with an overview of carbohydrates as one of the main biomolecules studied in biochemistry, highlighting their composition of carbon, hydrogen, and oxygen.

Monosaccharides: The Building Blocks

• Monosaccharides are defined as the simplest form of carbohydrates, consisting of single sugar units that cannot be further divided.

• The structure of monosaccharides resembles a skeleton made up of 3 to 7 carbon atoms, which is essential for their classification.

Chemical Structure and Functional Groups

• Monosaccharides contain functional groups such as aldehydes and hydroxyl groups that characterize them chemically.

• Glucose is highlighted as a key example of a monosaccharide that serves as an immediate energy source through cellular respiration.

Functions of Carbohydrates

• Carbohydrates play crucial roles in cell recognition and communication by interacting with receptors on cell membranes.

• They also contribute structurally to cells; for instance, cellulose provides rigidity to plant cell walls.

Classification Based on Carbon Count

• Monosaccharides can be classified based on the number of carbon atoms they contain: from three (trioses) to seven (heptoses).

• Each category has specific names derived from Greek prefixes indicating the number of carbons present.

Identifying Carbohydrate Types

• It’s important to note that terms like "sugar" and "carbohydrate" can be used interchangeably throughout discussions.

• The distinction between different types is based on their functional groups—those with aldehyde groups are termed aldoses while those with ketone groups are called ketoses.

Summary of Key Concepts

• A carbohydrate's identity hinges on its hydrocarbon skeleton combined with specific functional groups (aldehyde or ketone).

Understanding the Structure of Carbohydrates

The Concept of Final Carbon in Glucose

• The discussion begins with identifying the "final carbon" in glucose, which is crucial for understanding its structure. This carbon must be bonded to four different atoms or functional groups.

• The first carbon is connected to a hydrogen atom and an oxygen atom, but it does not meet the criteria for chirality due to its bonding configuration.

• The second carbon meets the chirality requirement as it bonds with an aldehyde group, a hydrogen atom, a hydroxyl group, and another carbon.

• The fifth carbon is identified as the most distant from the functional group and satisfies all conditions for being chiral by connecting to four distinct groups: hydrogen, another carbon, a hydroxyl group, and H2O.

Importance of Chiral Carbons

• Chiral carbons lead to molecules that have identical molecular structures but differ in function; they can be distinguished by their interaction with polarized light.

• When polarized light hits these structures, some will deflect right while others deflect left based on the orientation of their hydroxyl groups attached to chiral carbons.

• For example, if the hydroxyl group on a chiral carbon points right, light bends right; if it points left, light bends left. This creates mirror-image molecules known as enantiomers.

Nomenclature of Sugars Based on Chirality

• The nomenclature differentiates between D-glucose (hydroxyl on the right at final carbon) and L-glucose (hydroxyl on the left).

• "D" indicates dextrorotatory (light rotates clockwise), while "L" indicates levorotatory (light rotates counterclockwise).

Classification of Monosaccharides

• Monosaccharides are classified based on their number of carbons and functional groups. For instance:

• A triose has three carbons,

• A tetrose has four,

• Pentoses have five.

Structural Variations Among Monosaccharides

• As more carbons are added to monosaccharides like tetroses or pentoses, structural variations increase significantly due to different configurations of hydroxyl groups.

• Examples include erythrose and threose for tetroses; increasing complexity leads to various forms such as ribose and glucose among pentoses.

Cyclic Structures in Biological Systems

• In biological systems, monosaccharides do not exist solely in linear form; they adopt cyclic structures when dissolved in water due to interactions with water molecules.

Understanding the Transition from Linear to Cyclic Structures in Monosaccharides

The Role of Glycosidic Linkage

• The transition from a linear to a cyclic structure in monosaccharides occurs through a glycosidic linkage, primarily between the aldehyde group and an alcohol group.

• In most cases, this interaction involves the hydroxyl group on carbon 5, although it can occasionally involve carbon 4.

Mechanism of Cyclization

• When the hydroxyl group on carbon 5 interacts with water, it breaks a double bond with oxygen at carbon 1, creating a free valence for bonding.

• This free valence allows the hydroxyl group from carbon 5 to bond with carbon 1, forming a cyclic structure.

Structural Changes and Outcomes

• Removing the hydroxyl group leads to an available valence on carbon 1 while oxygen retains its connection to carbon 5.

• The final result is that the hydroxyl from carbon 5 bonds with carbon 1, establishing a cyclic monosaccharide structure.

Characteristics of Different Sugars

• For hexoses (six-carbon sugars), cyclization results in six-membered rings; however, pentoses (five-carbon sugars) form five-membered rings due to their functional groups' positions.

• In ketoses like fructose (a type of sugar), cyclization typically occurs at different carbons compared to aldoses like glucose.

Formation of Furanose and Pyranose Structures

• Furanose structures have five sides while pyranose structures have six sides. The difference arises based on which functional groups are involved during cyclization.

• In ketoses such as fructose, cyclization often involves interactions starting from different carbons than those seen in aldoses.

Emergence of New Stereocenters

• Upon forming cyclic structures, new stereocenters emerge. The anomeric carbon becomes significant as it connects four distinct atoms/groups when in cyclic form.

• Carbon one serves as the anomeric center because it binds to four different entities: another oxygen atom, another carbon atom, a hydroxyl group, and hydrogen.

Asymmetry in Monosaccharides

Understanding Monosaccharides and Their Importance

Structure and Classification of Monosaccharides

• The positioning of hydroxyl groups (-OH) on carbon atoms in monosaccharides determines their classification as alpha or beta; if the hydroxyl group is below the carbon, it is classified as alpha, while above indicates beta.

• The biological significance of this classification lies in the differing functions that alpha and beta forms can perform within biological systems.

• In a solution, approximately 38% of glucose molecules adopt an alpha form (hydroxyl group below), while 62% take on a beta form (hydroxyl group above).

Chemical Variability Among Monosaccharides

• Some monosaccharides can have different chemical natures beyond just hydrocarbon skeletons due to substitutions at hydroxyl groups with other atoms or functional groups.

• An example is glucosamine, where a hydroxyl group on the second carbon is replaced by an amino group (-NH2), leading to a new structure with distinct properties.

Formation of Oligosaccharides and Polysaccharides

• When multiple monosaccharides are linked together, they form larger structures: one monosaccharide is termed a "monosaccharide," two to ten linked units are called "oligosaccharides," and ten or more units create "polysaccharides."

• These connections occur through glycosidic bonds formed between the hydroxyl group of one monosaccharide and the carbon atom of another.

Mechanism of Glycosidic Bond Formation

• Glycosidic bonds are established via condensation reactions catalyzed by enzymes; these reactions require specific conditions for occurrence.

• During bond formation, water molecules are released as two monosaccharides join through an oxygen atom linking them together.

Characteristics of Disacáridos

• The resulting disacárido from two joined monosaccharides retains unique characteristics based on how their respective hydroxyl groups interact during bonding.

Understanding Disaccharides and Their Structures

Alpha and Beta Disaccharides

• The discussion begins with the classification of disaccharides, focusing on the alpha configuration where the hydroxyl group is positioned below the anomeric carbon.

• If the hydroxyl group were above, it would indicate a beta disaccharide. This distinction is crucial for understanding carbohydrate structures.

• The specific bonding between carbon atoms in disaccharides is highlighted, particularly noting that carbon 1 (anomeric carbon) bonds with other carbons such as carbon 4 to form an alpha 1,4 linkage.

Importance of Maltose

• Maltose is identified as a significant product of starch digestion, which is a major polysaccharide found in plants.

• The process of carbohydrate digestion leads to maltose formation before further breakdown into glucose molecules for absorption in the digestive system.

Lactose Structure and Function

• Lactose consists of two different monosaccharides: galactose and beta-glucose. Galactose has its hydroxyl group above the anomeric carbon.

• The bond formed between lactose's anomeric carbon and glucose's fourth carbon results in a beta 1,4 linkage, essential for its classification as a disaccharide.

Biological Significance of Lactose

• Lactose plays a vital role as it is a primary component of milk, crucial for lactation and development in mammals.

Sucrose Complexity

• Sucrose comprises glucose and fructose; however, it presents complexities due to their structural differences—glucose being an aldohexose while fructose is a ketohexose.

• The unique connection between their anomeric carbons forms either an alpha or beta linkage depending on their orientation during bonding.

Reducing vs Non-reducing Sugars

• When both anomeric carbons are linked together in sucrose, it results in non-reducing sugar properties since they lose their ability to act as reducing agents.

• This characteristic distinguishes sucrose from other sugars that can reduce other molecules due to free aldehyde or ketone groups present when not bonded.

Overview of Polysaccharides

Polysaccharides: Structure and Function

Types of Polysaccharides

• Polysaccharides are large molecules formed by the union of multiple monosaccharides. They can be homopolysaccharides, consisting of identical monosaccharide units (e.g., 100 glucose units), or heteropolysaccharides, which contain different types of monosaccharides.

• Heteropolysaccharides may include various combinations of monosaccharides, such as glucose and galactose, or a wide variety of other sugars like trehalose and lujosa.

Biological Importance of Homopolysaccharides

• Starch is a significant homopolysaccharide composed mainly of alpha-glucose. It exists in two forms: linear (amylose) and branched (amylopectin). Approximately 25% is amylose while 75% is amylopectin.

• The branching in amylopectin occurs through specific linkages every 20 to 40 glucose monomers, allowing for efficient energy storage.

Dietary Carbohydrates

• About 60% of dietary carbohydrates should come from starches. Humans do not store glucose as starch; instead, they consume it and convert it into energy.

• Glycogen serves as the primary storage form of glucose in humans, contrasting with plants that store it as starch.

Glycogen Structure and Function

• Glycogen consists entirely of amylopectin without any linear structures like amylose. Its branches occur every 12 to 20 glucose monomers via alpha-1,6 linkages.

• The highly branched structure allows glycogen to occupy less space while storing more glucose compared to linear structures.

Interaction with Water

• Glycogen's structure enables it to interact effectively with water due to its polar hydroxyl groups on glucose molecules. This interaction causes glycogen to absorb water and increase in weight significantly.

• The effective storage capacity means that the perceived weight of glycogen can be three times its original weight when hydrated.

Cellulose: A Unique Polysaccharide

• Cellulose is another important homopolysaccharide made up entirely of beta-glucose units. Unlike starch and glycogen, cellulose cannot be digested by humans due to the presence of beta linkages.

• The linear structure facilitates hydrogen bonding between chains, contributing to cellulose's rigidity but making it indigestible for human enzymes.

Understanding the Structure and Function of Cellulose

The Formation of Hydrogen Bonds in Glucose

• The structure of glucose consists of hydroxyl groups and hydrogen atoms, which can form hydrogen bonds when a hydroxyl group interacts with a hydrogen atom. This results in weak interactions that are crucial for molecular stability.

Properties of Cellulose

• Due to extensive hydrogen bonding, cellulose adopts a rigid and durable structure. All functional groups are interconnected through these bonds, leading to its rigidity and water impermeability.

• When water is introduced to cellulose molecules, it cannot interact due to all functional groups being engaged in hydrogen bonding, making cellulose impermeable to water.

Human Digestion and Cellulose

• Humans do not store or consume glucose in the form of cellulose because we lack the necessary enzymes to break down beta-glycosidic bonds found in cellulose chains.

Diversity Among Polysaccharides

• Discussing polysaccharides like glycogen, amylopectin, and cellulose reveals that variations in monosaccharide composition lead to unique structural properties. A diverse composition enhances informational capacity within cells.

Informational Capacity of Carbohydrates

• High variability in carbohydrate structures contributes to their uniqueness; this diversity allows cells to convey specific information essential for cellular recognition processes.

Blood Group Classification Based on Carbohydrates

• Blood types are primarily defined by carbohydrates present on red blood cell membranes. For example, type O has only one type of polysaccharide while type AB contains all three major types (A, B).

Immune Response Related to Blood Types