Carbohydrates

Carbohydrates (Sugars)

• Can be simple sugars (monosaccharides), disaccharides, or polysaccharides.
• A disaccharide is two monosaccharides joined together;
• A polysaccharide is many monosaccharides joined together.

Structure of Monosaccharides

1. Carbon chain or ring
2. Functional groups off the side:
3. Hydroxyl (OH)
4. Carbonyl (C = O)
5. At end of chain: aldehyde
6. In the middle of chain: ketone Example: glucose (C6H12O6)

• Sometimes monosaccharides have the same chemical formula, but their functional groups are located in different places. We call all of these monosaccharides with the same formula isomers.
• Structural isomers have functional groups located on different carbons. Example: glucose vs fructose.
• Stereoisomers have functional groups are on the same carbon but different arrangements. Example: glucose vs galactose.

• Monosaccharides can be found as straight lines (linear) or rings.

Disaccharides

• Monosaccharides are joined together by a dehydration or condensation reaction to form a glycosidic linkage.
• Common disaccharides include lactose (milk), and sucrose (table sugar).

Carbohydrates

• Involved in post-translational modifications and cell recognition
• Referred to as "sugars"
• Monosaccharides are the simplest carbohydrate and have the formula: (CH2O)n
• Named based on the number of carbons they have
• eg. triose, tetrose, pentose, hexose, heptose ....
• Aldoses have a aldehyde at C1 and Ketoses have a ketone at C2
• eg. ketohexose, aldotriose

Cyclized Conformation:

• L and D are in reference to the different enantiomers of the sugar structure (asymmetry)
• Enantiomer dependent on the chiral carbon furthest from the carbonyl group
• These are optically active, meaning they rotate polarized light either to the right (+, D) or to the left (-, L)
• The number of chiral carbons (x) determine the possible number of stereoisomers: 2x

Carbohydrate Cyclization

• The hydroxyl group nucleophilically attacks the aldehyde or the ketone (very reactive) forming the ring
• Forms Furan (5 membered ring) or a pryan (6 membered ring)
• Bonds formed:
• Hemiacetal (from aldehyde)

• Hemiketal (from ketone)

• Cyclyzation results in two possible anomers ($\alpha$ and $\beta$)
• $\beta$ is more stable

Reducing Sugars

• Sugars able to act as a reducing agent due to a free aldehyde or free ketone group (all monosaccharides)
• Aldehydes are converted to carboxylic acids and ketones are tautomerized into aldehydes and then reduced to carboxylic acids
• Disaccharides are reducing sugars as long as one of the units have an open chain with an aldehyde

Complex Carbohydrates

• Modification:
• Phosphorylation
• Methylation
• Addition of N-containing groups
• Hydroxyl groups removed
• Naming sugar complexes:
• Monosaccharide = 1
• Disaccharide = 2
• Oligosaccharide = 3 - 20
• Polysaccharides = 1000s

• Condensation reaction forming a glycosidic bond (an O-glycosidic bond)
• There can be N-glycosidic bonds too
• O-linkages to proteins (Ser/Thr) or N-linkaged (Asn)

Glycosyltransferase

• The enzyme responsible for linking sugars via O-glycosidic bonds
• Uses UPD-glucose, a high energy sugar nucleotide to attache sugars onto other sugars or proteins

• Sugars are cleaved into monosaccharides via enzymes.
• Monosaccharides are a source of fuel
• Lactase, maltase, sucrase

Naming Disaccharides

1. Identify N or O linkage
2. Configuration of the first sugar pulse the name ending in -yl
3. The number identifiers of the carbons linked together
4. Configuration of the second sugar plus the name ending in -ose

Polysaccharides

• Also referred to as glycans
• Consist of 1000s of monosaccharides (linked either linearly or branched)
• Play roles in energy storage, cell structure, and cell recognition
• Homopolymers: made up of the same repeating monosaccharide
• Heteropolymers: made up of different monosaccharide repeats

• A energy storage homopolymer (glucose) with a multibranched structure
• Glycogen granules consists to branched glucose units surrounding a glycogenin protien at its core
• Structure: chain lengths of 8-12 of linearly linked glucose molecules ($\alpha$(1-4) glycosidic bonds) with branches forming $\alpha$(1-6) glycosidic bonds

• Amylose: Linear linkages of glucose ($\alpha$(1-4) glycosidic bonds)
• Amylopectin: Linear linkages of glucose ($\alpha$(1-4) glycosidic bonds) with $\alpha$(1-6) glycosidic linkages occurring every 30 glucose units

• $\alpha$ amylase: cleaves glycogen and starch at random locations yielding maltose and maltotriose
• It is secreted by the pancreas
• $\beta$ amylase: hydrolyzes glucose units in glycogen and starch systematically at non-reducing ends, producing maltose
• Not produced by mammals

• Structural components in plants
• Formed by $\beta$ (1-4) glycosidic linkages
• Linear chains
• Formula: (C6H10O5)n
• More crystalline than starch
• Can not be digested by humans

Common monosaccharides to know: