Chapter 18 Flashcards
biochemisty
is the study of chemicals and their interactions within living organisms.
These chemicals are called biochemical substances and are divided into two groups:
1. bioinorganic
2. bioorganic
monosaccharides
Contain single polyhydroxy aldehyde or ketone unit
Cannot be broken down into simpler substances by hydrolysis reactions
Contain 3–7 C atoms
5 and 6 carbon species are more common
Pure monosaccharides - Water soluble white, crystalline solids
Monosaccharides - Glucose and fructose
disaccharides
Contain 2 monosaccharide units covalently bonded to each other
Crystalline and water soluble substances
Common disaccharides - Table sugar (sucrose) and milk sugar (lactose)
Upon hydrolysis, they produce 2 monosaccharide units
Oligosaccharides
Contain three to ten monosaccharide units covalently bonded to each other
Free oligosaccharides are seldom encountered in biochemical systems
Usually found associated with proteins and lipids in complex molecules
Serve structural and regulatory functions
mirror image
reflection of an object in a mirror
achiral molecule
Superimposable mirror images: Images that coincide at all points when the images are laid upon each other
-do not have handedness
chiral molecule
Nonsuperimposable mirror images: Images where not all points coincide when the images are laid upon each other.
-have handedness
enantiomers
Stereoisomers whose molecules are nonsuperimposable mirror images of each other
Molecules with opposite configurations at all chiral centers
diastereomers
Stereoisomers whose molecules are nonsuperimposable non-mirror images of each other
Molecules with opposite configurations at some chiral centers
Example: Cis-trans isomers
Molecules that contain more than one chiral center can also exist in diastereomeric as well as enantiomeric forms
epimers
Diastereomers whose molecules differ only in the configuration at one chiral center
dextrorotatory
compound: Chiral compound that rotates light towards right (clockwise; +)
levorotatory
(L-) compound: Chiral compound that rotates light towards left (counterclockwise; -)
d-glucose
Most abundant in nature Most important source of human nutrition Grape fruit and ripe fruits are good sources of glucose (20–30% by mass) Also named grape sugar Other names Dextrose Blood sugar (70–100 mg/dL) Six-membered cyclic form
d- galactose
Milk sugar Synthesized in human beings Also called brain sugar Part of brain and nerve tissue Used to differentiate between blood types Six-membered cyclic form
D-Fructose
Ketohexose
Sweetest tasting of all sugars
Found in many fruits and in honey
Good dietary sugar due to
higher sweetness
Five-membered cyclic form
D-Ribose
Part of a variety of complex molecules which include: RNA ATP DNA Five-membered cyclic form
a form
the –OH of C1 and CH2OH of C5 are on opposite sides
b form
where the –OH of C1 and CH2OH of C5 are on the same side
anomers
Cyclic monosaccharide formation always produces two stereoisomers, an a and b form.
Cyclic monosaccharides that differ only in the position of the substituents on the anomeric carbon atom
pyranose
Cyclic monosaccharide containing a six-atom ring
furanose
Cyclic monosaccharide containing a five-atom ring
haworth projection formula
Two-dimensional structural notation that specifies the three-dimensional structure of a cyclic form of a monosaccharide.
The hemiacetal ring is viewed “edge on” – the oxygen ring atom is always show as follows:
6-member ring upper right corner
5-member ring at the top
- D-CH2OH is above ring
- L- CH2OH down
Haworth - B configuration
both groups point same direction
Haworth- A configuration
2 groups point in opposite directions
Haworth; OH
to the right- points down
to the left- points up
5 reactions of monosaccharides
- Oxidation to acidic sugars
- Reduction to sugar alcohols
- Glycoside formation
- Phosphate ester formation
- Amino sugar formation
Oxidation to produce acidic sugars: weak agent
oxidize the aldehyde end to give an aldonic acid
Glucose is oxidized to gluconic acid
In Tollens test glucose reduces the Ag+ ion to Ag. The aldose (glucose) acts as a reducing agent so it is called a reducing sugar.
A reducing sugar is a carbohydrate that gives a positive test with the Tollens solution.
Ketoses are also reducing sugars: in this situation, the ketose undergoes a structural rearrangement that produces an aldose – and then the aldose reacts.
Therefore, (almost) all monosaccharides, both aldoses and ketoses, are reducing sugars.
Oxidation to produce acidic sugars: strong agent
can oxidize both ends of a monosaccharides at the same time. The carbonyl group and the terminal primary alcohol are both oxidized to a carboxylic acid group.
A polyhydroxy dicarboxylic acid is called an aldaric acid.
For glucose, this oxidation produces glucaric acid:
Oxidation to produce acidic sugars: enzymes
will oxidize only the primary alcohol end of an aldose, such as glucose, without oxidizing the aldehyde group.
This type of acid is called an alduronic acid.
For glucose, this oxidation produces glucuronic acid:
reduction to produce sugar alcohols
The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent
The product is the corresponding polyhydroxy alcohol called sugar alcohol or alditol
Glycloside formation
Glycoside: An acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon –OH group with an –OR group
Cyclic forms of monosaccharides are hemiacetals so they react with alcohols to form acetals.
Glycoside is an acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon –OH group with an –OR group
Exist in both α and β forms
Phosphate ester formation
- Hydroxyl groups of a monosaccharide can react with inorganic oxyacids to form inorganic esters
- Enzymes catalyze the esterification of the hemiacetal group (C1) and the primary alcohol group (C6) in glucose to produce the following compounds:
- Phosphate esters of various monosaccharides are stable in aqueous solution and play important roles in the metabolism of carbohydrates
amino suga formation
Amino sugar - Formed when one of the hydroxyl groups of a monosaccharide is replaced with an amino group
In naturally occurring amino sugars (the three below), the C2 hydroxyl group is replaced by an amino group
maltose
Structurally made of 2 D-glucose units, one of which must be α-D-glucose, linked via an α(14) glycosidic linkage
Digested easily by humans because of an enzyme that can break α(14) linkages
Baby foods are rich in maltose
cellobiose
Produced as an intermediate in the hydrolysis of the polysaccharide cellulose
Contains two D-glucose monosaccharide units, one of which must have a β configuration, linked through a β(14) glycosidic linkage
Cannot be digested by humans
lactose
Made up of β-D-galactose unit and a D-glucose unit (two different monosaccharides) joined by a β(14) glycosidic linkage
Milk is rich in the disaccharide lactose
Lactase hydrolyzes β(14) glycosidic linkages
Sucrose (table suger and a disaccharide)
The most abundant of all disaccharides and found in plants
Produced from the juice of sugar cane and sugar beets
Sugar cane contains up to 20% by mass sucrose
Sugar beets contain up to 17% by mass sucrose
Glycosidic linkage is not a (14) linkage. Instead it is an a,b(12) glycosidic linkage.
