Bio 98 - Lecture 11 Carbohydrates a.k.a. Sugars, Carbs…

I. Definition of a carbohydrate General formula: (CH 2 O) n, “hydrated carbon” Example: C 6 H 12 O 6 is glucose Many carbohydrates have more complex formulas & contain amino, phosphate, sulfate & other groups II. Functions 1. Fundamental source of metabolic energy for most life forms. 2. Component of many important biomolecules.

animals, plants respiration ATP ADP sunlight CO 2 + H 2 O plants photosynthesis carbohydrate + O 2 1. Carbohydrates as an energy source bacteria

2. Biological molecules Cellulose - wood, plant fiber, etc. Chitin - exoskeleton of arthropods Cell walls of bacteria & yeast Glycoproteins, glycolipids - cell membranes DNA, RNA - deoxyribose and ribose

Carbohydrates are often polymers Monosaccharides: glucose, ribose, fructose, etc. Oligosaccharides: di-, tri-, tetra-, etc. Sucrose is a disaccharide: glucose + fructose Polysaccharides: can be linear or branched (i.e. starch)starch

Monosaccharide nomenclature 1.Carbon number: triose,.., pentose, hexose,.., octose 2. Aldoses and ketoses HC=O HC-OH HO-CH H 2 C-OH HC-OH C# D-glucose an aldohexose H 2 C-OH C=O HO-CH H 2 C-OH HC-OH D-fructose a ketohexose

Asymmetric (chiral) carbons generate many distinct monosaccharides; compare mannose, glucose, and galactose, which are all aldohexoses. How many chiral carbons are in glucose?

O-chem terms relevant to monosaccharide structure 2.epimers - differ in chirality at only one carbon. Glucose and galactose are epimers at carbon 4.epimers 1.diastereomers - identical structures except for configuration (chirality) at one or more carbons; e.g., all aldohexoses are diastereomers of each other.diastereomers 3.enantiomers - mirror images; designated as D- & L-; no name change (like amino acids)!enantiomers How many aldohexose names are possible? 2 configuration choices at each of 4 asymmetric carbons; however, half of these represent enantiomers (3. above). Number of unique names = 2 4 / 2 = 8.

Ring structures Most pentoses and hexoses spontaneously form ring (cyclized) structures in solution. 5-member ring: furanosefuranose 6-member ring: pyranosepyranose When forming a ring, a new chiral center is created, giving rise to 2 possible anomers.anomers HC=O HC-OH HO-CH H 2 C-OH HC-OH.. ++ D- glucose(linear form)

 and  anomers hemiacetal Linear form Haworth projection  -D-glucopyranose

Linear D-glucose  -D-glucopyranose  -D-glucopyranose In solution rapid mutarotation occursmutarotation (1%) (~66%) (~33%)

Disaccharides 1.Mainly found in plants 2.Three common disaccharides sucrose - sugar cane, sugar beets lactose - milk sugar maltose - malted (germinating) barley, wheat 3.2 monosaccharides joined covalently by an O-glycosidic bond

Formation of hemiacetal and acetals Glycosidic bond

Common disaccharides are produced by enzyme- catalyzed condensation/dehydration reactions Can be either  or  due to mutarotation

Notice: there is no hemiacetal

Polysaccharides - aka GlycansGlycans 1.homopolysaccharides vs heteropolysaccharides 2.can be branched or unbranched 3.used by animals and plants as a compact storage form of CHOs 4.common examples starch - plants, roots and seeds glycogen - liver of mammals cellulose - plant fiber, wood

Starch = amylose + amylopectinamylose found in corn, rice, potato, wheat and barley a linear polysaccharide of glucose units

Amylopectin - a branched form of amylose

Structure of starch glycogenglycogen (animal starch) is like starch, but more highly branched

High-fructose corn syrup High-fructose corn syrup is produced by milling corn to produce corn starch, then enzymatically processing that starch to yield corn syrup, which is almost entirely glucose, followed by adding other enzymes that change a portion of the glucose into fructose. corn starch corn syrupglucoseenzymes 1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.Corn starchalpha-amylaseoligosaccharides 2. Glucoamylase - which is produced by Aspergillus, a fungus, in a fermentation vat — breaks the sugar chains down even further to yield the simple monosaccharaide glucose.GlucoamylaseAspergillusglucose 3. The enzyme xylose isomerase (aka glucose isomerase) then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.xylose isomerase

High-fructose corn syrup 1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.Corn starchalpha-amylaseoligosaccharides

High-fructose corn syrup 3. The enzyme glucose isomerase then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.

Sugars covalently attached to the polypeptide as oligosaccharide chains containing 4 to 15 sugars Sugars frequently comprise 50% or more of the total molecular weight of a glycoprotein Most glycosylated proteins are either secreted or remain membrane- bound Glycosylation is the most abundant form of post-translational modification Glycosylation confers resistance to protease digestion by steric protection Important in cell-cell recognition Protein glycosylation: A post-translational modification

Blood groupBlood group antigens on the erythrocyte surface The O substance is a tetrasaccharide which is missing the 5 th residue and does not elicit an antibody response (non-antigenic). The A antigen and B antigen are pentasaccharides which differ in composition of the 5 th sugar residue