carbohydrates

Carbohydrates

Definition: Carbohydrates can be linked to proteins and lipids, serving various functions within biological systems.
Functions:
- Serve as fuels, metabolic intermediates, and energy stores.
- Form the basis of most organic matter on Earth.
- Act as structural frameworks for nucleic acids (DNA and RNA) and polysaccharides.
- Participate in signaling and structure by linking with proteins and lipids.

General Characteristics:
- Molecular formula: $(CH2O)n$, where n ranges from 3 to 6.
- Classes of monosaccharides include:
- Triose (n=3) e.g., glyceraldehyde
- Tetrose (n=4)
- Pentose (n=5) e.g., ribose, deoxyribose
- Hexose (n=6) e.g., glucose, galactose, mannose, fructose
Types:
- Aldoses: Have an aldehyde group at one end (e.g., glucose).
- Ketoses: Have a keto group, usually at C2 (e.g., fructose).

Asymmetry:
- The designation of D (right) or L (left) is based on the configuration around the single asymmetric carbon in glyceraldehyde, the reference compound.
- For sugars with more than one chiral center, D or L refers to the asymmetric carbon farthest from the carbonyl carbon.
- Most naturally occurring sugars are D isomers.
Enantiomer Definition:
- D and L isomers of a sugar are enantiomers, meaning they are mirror images of each other.

Standard Sugars:
- Ribose: Standard five-carbon sugar.
- Glucose: Standard six-carbon sugar.
Epimers:
- Mannose: Epimer of glucose.
- Galactose: Epimer of glucose.
Fructose:
- Ketose form of glucose.

Reaction Mechanism:
- Aldehyde reacts with alcohol to form a hemiacetal.
- Ketone reacts with alcohol to form a hemiketal.
Distal Hydroxyl:
- Pentoses and hexoses cyclize as the ketone or aldehyde reacts with a distal hydroxyl group (OH).
Formation of Pyranose Ring:
- In glucose, C1 aldehyde and C5 OH react to form a six-member ring known as a pyranose.

Formation of Different Rings:
- Forms either a 6-member pyranose ring (pyran) via reaction of C2 keto group with OH on C6, or a 5-member furanose ring (furan) via reaction of C2 keto group with OH on C5.
Anomeric Carbon:
- Cyclization produces a new asymmetric center at C1, called the anomeric carbon.
- If C1’s hydroxyl (OH) group is trans to the CH2OH moiety, it is designated as α; if cis, it is designated as β.
- Examples:
  - α-D-glucose
  - β-D-glucose

Fehling’s Solution:
- Used to differentiate between reducing and non-reducing sugars.
Reducing Mechanism:
- Cupric ion ($Cu^{2+}$) is reduced to cuprous ion ($Cu^{+}$) by glucose.
Reducing Sugar Classification:
- All monosaccharides are reducing sugars; only some disaccharides are (e.g., sucrose is not).

Formation:
- Two monosaccharides are joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon.
Examples:
- Maltose: Formed from the condensation of two glucose molecules via a 1-4 bond.
- Common disaccharides include sucrose, lactose, and maltose.
Non-reducing Characteristics:
- Sucrose is non-reducing since the glycosidic bond is between two anomeric carbons, lacking reducing ends.

Definition:
- Polymers formed by multiple linked monosaccharides to minimize osmotic effects.
Types:
- Homopolysaccharides: Composed of one type of monosaccharide.
- Heteropolysaccharides: Composed of multiple types.
- Linear or branched forms.
Molecular Weight:
- Polysaccharides lack defined molecular weight.
Endpoints:
- The end of the polysaccharide with an anomeric C1 not in a glycosidic bond is the reducing end.
Common Polysaccharides:
- Starch (amylose and amylopectin), glycogen, and cellulose.

Storage and Composition:
- Main storage polysaccharide in plants; a mixture of two homopolysaccharides of glucose:
- Amylose: Unbranched glucose polymer with $( ext{α}1
  ightarrow 4)$ linked residues.
- Amylopectin: Glucose polymer with mainly $( ext{α}1
  ightarrow 4)$ linkages; branched with $( ext{α}1
  ightarrow 6)$ linkers occurring every 24-30 residues.
Hydrolysis:
- Both amylose and amylopectin can be hydrolyzed by α-amylase.

Storage Form:
- Storage form of glucose in animals.
- Structure similar to amylopectin but with more $( ext{α}1
  ightarrow 6)$ branches; branches occur every 8-12 residues.
- Forms $( ext{α}1
  ightarrow 4)$ linked chains with branch-points.

Structure:
- Major constituent of plant cell walls; an unbranched homopolysaccharide of glucose with $( ext{β}1
  ightarrow 4)$ linkages.
- Every other glucose unit is flipped, promoting intra-chain and inter-chain H-bonding, leading to a straight and rigid structure, which is water-insoluble.
Biological Relevance:
- Most animals cannot utilize cellulose due to the lack of enzymes to hydrolyze $( ext{β}1
  ightarrow 4)$ linkages.
Microbial Interaction:
- Fungi, bacteria, and protozoa produce cellulase to break down cellulose, allowing them to utilize wood as a glucose source.

Definition:
- Glycoprotein: A protein with small oligosaccharides attached via asparagine (N-linked) or serine/threonine (O-linked).
Function:
- Glycoproteins are significant in protein-protein recognition and signaling.
Proteoglycans:
- Composed of glycosaminoglycans attached to a core protein; constitute ~95% by weight of the molecule, functioning as structural components and lubricants.
Glycolipids:
- Lipids with carbohydrates attached; provide energy and markers for cellular recognition (e.g., ABO blood types).
Lipopolysaccharides:
- Large molecules of lipids and polysaccharides joined by covalent bonds; found in Gram-negative bacteria as endotoxins, eliciting strong immune responses.

Oligosaccharides:
- Monosaccharides can form a vast variety of oligosaccharides differing in stereochemistry, glycosidic bonds, substituent groups, and branching patterns, containing biological information referred to as the "sugar code."
Lectins:
- Proteins that bind carbohydrates on glycoproteins, promoting cell-cell interactions via non-covalent interactions.
- Found in animals, plants, and microorganisms, lectins use specific oligosaccharide binding to exert biological effects.