CARBOHYDRATE METABOLISM
Carbohydrate metabolism
Classification of carbohydrates
o Carbohydrates for energy and structural components
o DNA carbohydrate ribose and plant cell wall cellulose
o Carbon, hydrogen and oxygen atoms (1:2:1)
Sugar (1-2):
Monosaccharides – glucose, galactose and fructose
Disaccharides – sucrose, lactose and trehalose
Polyols – sorbitol and mannitol
Oligosaccharides (3-9):
Malito-oligosaccharides – maltodextrins
Other oligosaccharides – raffinose, stachyose and fructo-oligosaccharides
Polysaccharides (>9):
Starch – amylose, amylopectin and modified starches
Non-starch polysaccharides – cellulose, hemicellulose, pectins and hydrocolloids
Monosaccharides = smallest unit building block
Glucose contains 6 carbon atoms in a ring-like structure
α-carbohydrate – hydroxyl OH group attached to C1 points in opposite direction to C6
β-carbohydrate – hydroxyl OH group attached to C1 points in same direction to C6
Disaccharides = two monosaccharides
1. α-glucose + α-glucose -> condensation (-H2O) -> maltose
α(1-4)glycosidic bond
Linkage of C1 and C4 of different glucose molecules
Hydrolysis (+H2O) where water breaks bonds
2. β-galactose + β-glucose -> condensation (-H2O) -> lactose
β(1-4)glycosidic bond
Linkage of C1 and C4 of a galactose molecule and glucose molecule
Hydrolysis (+H2O) where water breaks bonds
3. α-glucose + β-fructose -> condensation (-H2O) -> sucrose (only plants)
Glucose α C1 and fructose β C2 therefore fructose flips over
Hydrolysis (+H2O) where water breaks bonds
, CARBOHYDRATE METABOLISM
Oligosaccharides = <20 monosaccharides in a short chain of monosaccharides
Maltose + another glucose molecule -> condensation -> maltotriose (i.e. 3 glucose)
Polysaccharides = >20 monosaccharides within the chain
Polysaccharides from oligosaccharides and occurs as glycans
Homopolysaccharide – single type of monosaccharide i.e. only glucose molecules
Heteropolysaccharide – two or more types of monosaccharides i.e. glucose + fructose
Unbranched and branched
Homopolysaccharides = storage forms of monosaccharides
STARCH – main carbohydrate
o Homopolysaccharide with only glucose
Unbranched linear contains ONLY a α(1-4)glycosidic bond between two glucose molecules =
amylose
Branched contains BOTH a α(1-6)glycosidic bond & α(1-4)glycosidic bond = amylopectin
GLYCOGEN – storage form
o Homopolysaccharide with only glucose
Unbranched and branched and both made of glucose – starch and glycogen
Amylose and amylopectin – starch and glycogen
BUT branch points in glycogen every 8-12 glucose residues and branch points in starch every 24-
30 glucose residues
Branch points occur more frequently in glycogen than in starch influencing the structure
DEXTRANS – structural component in bacteria and yeast
α(1-6)glycosidic bond and α(1-3)glycosidic bond within glucose units
α(1-2)glycosidic bond and α(1-4)glycosidic bond
CELLULOSE – structural components in plants
Unbranched homopolysaccharides with thousands glucose molecules
β-glucose configuration with β(1-4)glycosidic bond
Humans not hydrolyse β(1-4)glycosidic bond – no enzymes to break down
Hydrogen bonds = strong structure
General mechanisms of digestion and absorption
None – glucose substrate (may not require digestion) is absorbed by intestinal cells
through the lumen, epithelium and into interstitial space as ingested
Luminal hydrolysis of polymer to monomers – protein polymer digested in lumen to
constituent monomers i.e. amino acids by pancreatic enzymes before absorption
Brush border hydrolysis of oligomer to monomers – sucrose oligomer digested into
constituent monomers i.e. monosaccharides of glucose and fructose by brush border
enzymes before absorption
, CARBOHYDRATE METABOLISM
Intracellular hydrolysis – oligopeptide oligomer (peptide) directly broken absorbed by
cell and then broken down into monomers (amino acids) inside the cell
Luminal hydrolysis followed by intracellular resynthesis – TAG substance broken
down into constituent components i.e. glycerol and fatty acids before absorption and
cell resynthesises the original molecule
Digestive process for dietary carbohydrates
1. Intraluminal hydrolysis: starch forming oligosaccharides via salivary amylase and
pancreatic amylase
o Action of salivary amylase and pancreatic amylase – secreted in active forms
o Salivary amylase in the mouth initiates starch digestion
▪ Inactivated by gastric acid
▪ Partially protected by complexing with oligosaccharides
o Pancreatic α-amylase completes starch digestion in lumen of small intestine
2. Membrane digestion: hydrolysis of oligosaccharides forming monosaccharides via
brush border disaccharidases
, CARBOHYDRATE METABOLISM
INTRALUMINAL HYDROLYSIS: lumen
Salivary α-amylase and pancreatic α-amylase are endoenzymes:
Amylase digests the linear internal α-1,4 linkages between glucose residues in
amylose and amylopectin
Not break terminal α-1,4 linkages (between the last two sugars in the chain) in amylose
and amylopectin
Not split α-1,6 linkages at branch points or the adjacent α-1,4 linkages in amylopectin
Products of α-amylase action are linear glucose oligomers, maltotriose (a linear glucose
trimer), maltose (a linear glucose dimer), and α-limit dextrin’s (α-1,6 branching linkage)
MEMBRANE DIGESTION: lumen-epithelium border
The brush border oligosaccharides are intrinsic membrane proteins with their catalytic
domains facing the lumen
The sucrase-isomaltase is 2 enzymes. 4 oligosaccharidases split oligosaccharides
produced by α-amylase into monosaccharides
o Small intestine contains 3 brush border oligosaccharidases: lactase,
glucoamylase (maltase) and sucrase-isomaltase
1. Lactase only 1 substrate i.e. breaks lactose into glucose and galactose
Lactase splits lactose into monomers i.e. glucose and galactose where it is
transported via SGLT1 [utilising 2 Na+]
2. Maltase degrades the α-1,4 linkages in straight chain oligosaccharides up to 9
monomers in length – cannot split either sucrose or lactose
Glucoamylase (maltase) removes glucose monomers i.e. maltotriose or maltose for
transportation via SGLT1 [utilising 2 Na+]
3. Sucrase moiety of sucrase-isomaltase splits sucrose into glucose and fructose
4. Isomaltase moiety of sucrase-isomaltase crucial – ONLY enzyme that splits the
branching α-1,6 linkages of α-limit dextrin’s
Sucrase-isomaltase contains two enzymes:
Sucrase splits sucrose (glucose and fructose), maltose and maltotriose via GLUT5
Isomaltase splits α-limit dextrin’s, maltose and maltotriose via SGLT1
Absorption in the digestive system
o Digestion converts ingested materials into molecules small for absorption
o Absorption occurs through: active transport, simple diffusion, facilitated
diffusion, secondary active transport and endocytosis
Carbohydrate metabolism
Classification of carbohydrates
o Carbohydrates for energy and structural components
o DNA carbohydrate ribose and plant cell wall cellulose
o Carbon, hydrogen and oxygen atoms (1:2:1)
Sugar (1-2):
Monosaccharides – glucose, galactose and fructose
Disaccharides – sucrose, lactose and trehalose
Polyols – sorbitol and mannitol
Oligosaccharides (3-9):
Malito-oligosaccharides – maltodextrins
Other oligosaccharides – raffinose, stachyose and fructo-oligosaccharides
Polysaccharides (>9):
Starch – amylose, amylopectin and modified starches
Non-starch polysaccharides – cellulose, hemicellulose, pectins and hydrocolloids
Monosaccharides = smallest unit building block
Glucose contains 6 carbon atoms in a ring-like structure
α-carbohydrate – hydroxyl OH group attached to C1 points in opposite direction to C6
β-carbohydrate – hydroxyl OH group attached to C1 points in same direction to C6
Disaccharides = two monosaccharides
1. α-glucose + α-glucose -> condensation (-H2O) -> maltose
α(1-4)glycosidic bond
Linkage of C1 and C4 of different glucose molecules
Hydrolysis (+H2O) where water breaks bonds
2. β-galactose + β-glucose -> condensation (-H2O) -> lactose
β(1-4)glycosidic bond
Linkage of C1 and C4 of a galactose molecule and glucose molecule
Hydrolysis (+H2O) where water breaks bonds
3. α-glucose + β-fructose -> condensation (-H2O) -> sucrose (only plants)
Glucose α C1 and fructose β C2 therefore fructose flips over
Hydrolysis (+H2O) where water breaks bonds
, CARBOHYDRATE METABOLISM
Oligosaccharides = <20 monosaccharides in a short chain of monosaccharides
Maltose + another glucose molecule -> condensation -> maltotriose (i.e. 3 glucose)
Polysaccharides = >20 monosaccharides within the chain
Polysaccharides from oligosaccharides and occurs as glycans
Homopolysaccharide – single type of monosaccharide i.e. only glucose molecules
Heteropolysaccharide – two or more types of monosaccharides i.e. glucose + fructose
Unbranched and branched
Homopolysaccharides = storage forms of monosaccharides
STARCH – main carbohydrate
o Homopolysaccharide with only glucose
Unbranched linear contains ONLY a α(1-4)glycosidic bond between two glucose molecules =
amylose
Branched contains BOTH a α(1-6)glycosidic bond & α(1-4)glycosidic bond = amylopectin
GLYCOGEN – storage form
o Homopolysaccharide with only glucose
Unbranched and branched and both made of glucose – starch and glycogen
Amylose and amylopectin – starch and glycogen
BUT branch points in glycogen every 8-12 glucose residues and branch points in starch every 24-
30 glucose residues
Branch points occur more frequently in glycogen than in starch influencing the structure
DEXTRANS – structural component in bacteria and yeast
α(1-6)glycosidic bond and α(1-3)glycosidic bond within glucose units
α(1-2)glycosidic bond and α(1-4)glycosidic bond
CELLULOSE – structural components in plants
Unbranched homopolysaccharides with thousands glucose molecules
β-glucose configuration with β(1-4)glycosidic bond
Humans not hydrolyse β(1-4)glycosidic bond – no enzymes to break down
Hydrogen bonds = strong structure
General mechanisms of digestion and absorption
None – glucose substrate (may not require digestion) is absorbed by intestinal cells
through the lumen, epithelium and into interstitial space as ingested
Luminal hydrolysis of polymer to monomers – protein polymer digested in lumen to
constituent monomers i.e. amino acids by pancreatic enzymes before absorption
Brush border hydrolysis of oligomer to monomers – sucrose oligomer digested into
constituent monomers i.e. monosaccharides of glucose and fructose by brush border
enzymes before absorption
, CARBOHYDRATE METABOLISM
Intracellular hydrolysis – oligopeptide oligomer (peptide) directly broken absorbed by
cell and then broken down into monomers (amino acids) inside the cell
Luminal hydrolysis followed by intracellular resynthesis – TAG substance broken
down into constituent components i.e. glycerol and fatty acids before absorption and
cell resynthesises the original molecule
Digestive process for dietary carbohydrates
1. Intraluminal hydrolysis: starch forming oligosaccharides via salivary amylase and
pancreatic amylase
o Action of salivary amylase and pancreatic amylase – secreted in active forms
o Salivary amylase in the mouth initiates starch digestion
▪ Inactivated by gastric acid
▪ Partially protected by complexing with oligosaccharides
o Pancreatic α-amylase completes starch digestion in lumen of small intestine
2. Membrane digestion: hydrolysis of oligosaccharides forming monosaccharides via
brush border disaccharidases
, CARBOHYDRATE METABOLISM
INTRALUMINAL HYDROLYSIS: lumen
Salivary α-amylase and pancreatic α-amylase are endoenzymes:
Amylase digests the linear internal α-1,4 linkages between glucose residues in
amylose and amylopectin
Not break terminal α-1,4 linkages (between the last two sugars in the chain) in amylose
and amylopectin
Not split α-1,6 linkages at branch points or the adjacent α-1,4 linkages in amylopectin
Products of α-amylase action are linear glucose oligomers, maltotriose (a linear glucose
trimer), maltose (a linear glucose dimer), and α-limit dextrin’s (α-1,6 branching linkage)
MEMBRANE DIGESTION: lumen-epithelium border
The brush border oligosaccharides are intrinsic membrane proteins with their catalytic
domains facing the lumen
The sucrase-isomaltase is 2 enzymes. 4 oligosaccharidases split oligosaccharides
produced by α-amylase into monosaccharides
o Small intestine contains 3 brush border oligosaccharidases: lactase,
glucoamylase (maltase) and sucrase-isomaltase
1. Lactase only 1 substrate i.e. breaks lactose into glucose and galactose
Lactase splits lactose into monomers i.e. glucose and galactose where it is
transported via SGLT1 [utilising 2 Na+]
2. Maltase degrades the α-1,4 linkages in straight chain oligosaccharides up to 9
monomers in length – cannot split either sucrose or lactose
Glucoamylase (maltase) removes glucose monomers i.e. maltotriose or maltose for
transportation via SGLT1 [utilising 2 Na+]
3. Sucrase moiety of sucrase-isomaltase splits sucrose into glucose and fructose
4. Isomaltase moiety of sucrase-isomaltase crucial – ONLY enzyme that splits the
branching α-1,6 linkages of α-limit dextrin’s
Sucrase-isomaltase contains two enzymes:
Sucrase splits sucrose (glucose and fructose), maltose and maltotriose via GLUT5
Isomaltase splits α-limit dextrin’s, maltose and maltotriose via SGLT1
Absorption in the digestive system
o Digestion converts ingested materials into molecules small for absorption
o Absorption occurs through: active transport, simple diffusion, facilitated
diffusion, secondary active transport and endocytosis
