carbohydrates Flashcards
carbohydrate definition
Groups of compounds, molecules of which contain aldehydes or ketones linked to a chain of alcohol groupings.
Carbohydrate Classification
monosaccharides : glucose , fructose , galactose
disaccharides ; maltose , lactose , sucrose
polysaccharides : starch , glycogen , cellulose
The Monosaccharides
Extremely rich in hydroxyl (-OH groups)
Empirical formula: (CH2O)n – carbon hydrate
Essentially are aldehydes or ketones that contain one or more –OH groups
Monosaccharide with an aldehyde group is an aldose while a monosaccharide containing a ketone is a ketose
Monosaccharides are the simplest carbohydrates and they not only serve as fuel molecules but as fundamental components of living systems (e.g. DNA is built from a backbone of deoxyribose – a five carbon sugar)
The smallest known monosaccharides which are comprised of just 3 carbons are glyceraldehyde and dihydroxyacetone
Naming monosaccharides
In addition to referring to monosaccharides by aldehyde or ketone groups – named according to number of C atoms
Glyceraldehyde and dihydroxyacetone have carbon of n=3 – they are termed triose
Number of carbons
n=3 (triose)
n=4 (tetrose)
n=5 (pentose)
n=6 (hexose)
n=7 (heptose)
The most familiar monosaccharides are probably the hexoses glucose and fructose
Glucose is an essential source of energy for all forms of life
Fructose is commonly used as a sweetener and once inside the cell is converted to usable glucose derivatives
monosaccharides - D/L forms
Carbohydrates exist in a large number of isomeric forms
Monosaccharides with more than three carbon atoms have multiple asymmetric carbon atoms
They can exist as enantiomers (non-superimposable mirror images) and as diastereoisomers (not mirror images)
By convention, enantiomers / diastereoisomers are designated either D- or L-
Pairs of stereoisomers are quite often indistinguishable in chemical reactions – but can be distinguished by examining their optical properties
D- and L-forms are dictated by the chiral carbon atom furthest from C1 (at the top)
D stands for dextrorotatory so that when plane polarised light is shone on a carbohydrate in D-form it will rotate the light in a clockwise direction (designated as +)
L stands for levorotatory so that when plane polarised light is shone on a carbohydrate in L-form it will rotate the light in an anti-clockwise direction (designated as -)
Monosaccharides contain both D- and L-forms and the rotation values are dictated by the abundance of D- and L-
If the rotation value is zero (ie. optically inactive) this means that you have a racemic mixture and this is designated as +/-
epimers
sugars that are diastereoisomers and only differ in configuration at a single asymmetric centre
anomers
differ in the location of the -OH group attached to the hemiacetal - new carbon chiral centre
Glycosidic linkages
Monosaccharides can be modified by reaction with molecules that significantly increase their versatility and biochemical properties and allow them to serve in many more important biochemical reactions
3 common reactants are alcohols, amines and phosphates
Bond between anomeric carbon of a sugar and oxygen atom of an alcohol is called a glycosidic bond, specifically an O-glycosidic bond and the product is called a glycoside – see dissacharides later
Bond between anomeric carbon of a sugar and nitrogen atom of an amine is called an N-glycosidic bond
Sugars can also form ester linkages with phosphates – most prominent modifications in carbohydrate metabolism
Glucose – Biological Importance
Metabolised (used as a fuel) in the body to produce energy
Glucose levels regulated in the body by number of homeostatic mechanisms (insulin)
Hyper and hypo-glycaemia
Excess glucose can be stored in liver and muscle in the form of glycogen (precious and not wasted)
Used industrially to make Vitamin C, citric acid, bioethanol, gluconic acid and sorbitol
Sorbitol used as a glucose substitute in chewing gum (still sweet-tasting but prevents bacteria building up and dental plaque
Fructose – Biological Importance
Also known as levulose or fruit sugar – sweetest of all simple sugars found in honey and corn syrup
Biologically fructose is derived from digestion of table sugar (sucrose)
Can be produced anaerobically by fermentation of yeast/bacteria (also produces ethanol)
Can undergo Maillard reaction with amino acids which is important in food industry
Fructose malabsorption leading to increased fructose in intestine causes irritable bowel syndrome
Some plants store fructose as a polymeric form known as inulin (not insulin) – food reserve a bit like starch
Galactose – Biological Importance
Less sweet than glucose, found in hemicellulose as a polymer called galactan– can be hydrolysed to galactose
Together with a modified form (N-acetylgalactosamine) is an important component of blood group antigens
Galactose can be metabolised to glucose by the Leloir Pathway
Glucose can be converted to galactose in humans by hexogenesis
Galactosaemia – defects in galactose metabolism (very serious for newborns)
Complex Carbohydrates: disaccharides
Oligosaccharides are built by the linkage of two or more monosaccharides by O-glycosidic bonds
For oligosaccharides, synthesis occurs through the action of specific enzymes which are called glycosyltransferases
The sugar to be added comes in the form of an activated (energy-rich) sugar nucleotide, such as UDP-glucose (UDP is uridine diphosphate)
The attachment of a nucleotide to enhance the energy content of a molecule is a common strategy in biosynthesis and biochemistry
Maltose
Maltose is derived from the condensation of glucose and glucose which forms an (1-4) glycosidic linkage)
2 glucose units are joined by an alpha-1,4-glycosidic linkage
Is a reducing sugar that is about half as sweet as glucose and often used in confectionary (Malt sugar)
Maltose comes from the hydrolysis of larger polymeric oligosaccharides such as starch and glycogen – occurs by enzyme maltase
Maltose is found in germinating barley seeds – useful in the brewing process
Lactose
Lactose is derived from the condensation of galactose and glucose (forms a -1-4 glycosidic linkage)
Galactose joined to glucose by a beta-1,4-glycosidic linkage
Is a reducing sugar and principal sugar found in milk (sometimes referred to as milk sugar)
Lactose can be hydrolysed to galactose and glucose by the enzyme lactase in humans and by beta-galactosidase in bacteria
Can lead to lactose intolerance – cramps, nausea, diarrohea
Sucrose
Sucrose is derived from the condensation of glucose and fructose but the anomeric C of both sugars is used in the formation of the glycosidic bond
The –OH of C1 in D-glucose and –OH of C2 in D-fructose
There is no free –OH: sucrose is a non-reducing sugar
Glucose joined to fructose by an alpha-1,2-glycosidic linkage
Is a non-reducing sugar commonly found in plants and often known as table sugar, cane sugar and beet sugar)
Sucrose can be hydrolysed to glucose and fructose by the enzyme sucrase
Sucrose increases osmotic pressure which can inhibit the growth of microorganisms – adopted as a food preservative
