Introduction to Carbohydrates

Carbohydrates: An Introduction

• Carbohydrates (or saccharides) are the most abundant biological molecules
• Chemically simpler than nucleotides or amino acids, they contain just three elements: C, H, and O
• These elements combine following the formula (CH2O)n, where n is greater than or equal to 3
• Monosaccharides are the basic carbohydrate units
• Different types of monosaccharides are present, differing in C atoms, and H and O atom arrangement
• Monosaccharides join together in almost limitless ways, forming polysaccharides

Sources and Functions of Carbohydrates

• Carbohydrates serve as the main energy source for living organisms
• Brain cells and red blood cells heavily rely on carbohydrates for energy
• Carbohydrates act as starting materials in the synthesis of fatty acids and amino acids
• They exist in every living organism
• Sugar, starch in food, cellulose found in wood, paper, and cotton are nearly pure carbohydrates
• Modified carbohydrates form part of the coating around living cells
• Roles include energy storage and transport in the form of starch and glycogen
• Structural components exist as cellulose in plants and chitin/cartilage in animals
• Important functions occur in the immune system, fertilization, pathogenesis, blood clotting, and development
• They are used as flavors and sweeteners, such as fructose, which is twice as sweet as sucrose
• Many immunoglobulins (antibodies) and peptide hormones contain carbohydrates
• Ribose and deoxyribose carbohydrates are in RNA and DNA structures
• Fructose is found in fruits and lactose is found in milk

Classification of Carbohydrates

• Carbohydrates are called saccharides and divided into three groups
• Monosaccharides, which are simple sugars
• Oligosaccharides, which are short chain sugars
• Polysaccharides, which are complex sugars
• Both oligosaccharides and polysaccharides contain more than one sugar molecule

Monosaccharides

• Monosaccharides are aldehyde or ketone derivatives of straight-chain polyhydroxy alcohols
• These contain a minimum of three carbon atoms
• Monosaccharides get classified by the chemical nature of their carbonyl group, and the number of C atoms
• When the carbonyl group is an aldehyde, the sugar becomes an aldose
• If the carbonyl group is a ketone, the sugar becomes a ketose
• Trioses are the smallest monosaccharides with three carbon atoms
• Others are tetroses, pentoses, hexoses, and heptoses with four, five, six, and seven carbon atoms, respectively
• The aldohexose D-glucose has the formula (CH2O)6

Common Ketoses

• Common ketoses are those with their ketone function at C2
• The carbonyl group position gives ketoses one less asymmetric center than their isomeric aldoses
• Ketohexose has only 8 possible stereoisomers
• Dihydroxyacetone, ribulose, and fructose are the most common ketoses

Cyclization of Glucose and Fructose

• Glucose and fructose can undergo cyclization to form ring structures
• This process results in the formation of α and β anomers

Structure in Hexose Monosaccharides

• If the carbonyl group (C=O) is on carbon 1, forms an aldose, where a monosaccharide becomes an aldehyde
• if carbonyl group is on a different carbon, there is a ketone formed and the monosaccharide is called a ketose

Functions of Monosaccharides

• Hexoses are the most commonly found monosaccharides in the body
• The two pentoses ribose, and deoxyribose, are found in RNA and DNA
• Glyceraldehyde, a triose, serves as a major intermediate in glycolysis
• Glucose, fructose, and galactose contain C6H12O6 in chemical formula

Disaccharides

• Disaccharides, the simplest polysaccharides, are fairly common
• Many result as the hydrolysis products of larger molecules
• Lactose occurs naturally only in milk, with a concentration ranging from 0% to 7% based on species
• Sucrose, our typical table sugar, is the most abundant disaccharide and major form in which carbohydrates transport in plants
• The systematic name for sucrose, a--D-glucopyranosyl-D-fructofuranoside shows the anomeric carbon of each sugar involved (C₁ in glucose and C2 in fructose)
• Sucrose is not a reducing sugar

Oligosaccharides

• Oligosaccharides are created of 2 - 10 sugar molecules via a glycosidic bond and can be hydrolyzed into di- and trisaccharides
• Disaccharide forms when two sugar molecules link by a glycosidic bond
• When 3 sugars link together, forms a trisaccharide
• When 4 sugars link together, forms a tetrasaccharide
• Carbohydrates in membranes are oligosaccharides, covalently attaching to form glycoproteins and glycolipids
• Oligosaccharides are contained in antibodies and clotting factors

Functions of Oligosaccharides in Glycoproteins

• Oligosaccharides determine glycoprotein structure, function, and recognition and help define protein structure
• Oligosaccharides also mediate recognition events
• These serve as antigenic determinants

Polysaccharides: Composition and Classification

• Polysaccharides, (glycans) are monosaccharides linked via glycosidic bonds
• These are classified as homopolysaccharides or heteropolysaccharides
• Determined by if they consist of one or more than one type of monosaccharide
• Polysaccharides create branched, and linear polymers, differing from proteins and nucleic acids
• Most polysaccharides are linear, and those that branch do so in defined ways

Cellulose

• Plants have rigid cell walls that have a high load-bearing functions
• Cellulose is the primary structural component of plant cell walls accounting for > half the carbon in the biosphere
• Approximately 10^15 kg of cellulose get synthesized/degraded annually
• Cellulose is a linear polymer of up to 15,000 D-glucose residues linked by (1-4) glycosidic bonds

Starch and Glycogen

• Starch is a mixture of glycans made by plants as a primary energy reserve
• Starch is deposited in cell chloroplasts and in insoluble granules of a-amylose/amylopectin
• Amylose is a linear polymer of several thousand glucose residues linked by a(1-4) bonds
• Glycogen is the storage polysaccharide of animals, present in skeletal muscle and liver as cytoplasmic granules
• The primary structure of glycogen resembles amylopectin
• Glycogen is more highly branched, w/ branch points appearing every 8-14 glucose residues

Glycoproteins

• Many proteins are glycoproteins, with carbohydrate contents ranging from 1-90% by weight
• Glycoproteins, w/ varied functions, hold enzymes, receptors, transport proteins, and structural proteins
• Their carbohydrate chains, are covalently linked to polypeptide chains
• Variable carbohydrate composition occurs within glycoproteins, a phenomenon called microheterogeneity

Laboratory Tests for Carbohydrates

• Molisch tests are for all carbohydrates
• Benedict's tests recognize reducing sugars
• Barfoed's tests distinguish mono- and disaccharides
• Bial's tests recognize pentoses
• Seliwanoff's tests distinguish aldohexoses from ketohexoses