Carbohydrates and metabolism Flashcards
generic form of common CHO
Cn(H20)n - they are carbon hydrates
- some may contains N, P or S
biochemical description of CHO
polyhydroxyl compounds that carry an aldehyde or ketone group, or substances that yield such compounds upon hydrolysis
three classes of CHO
- monosaccharides
- oligosaccharides
- polysaccharides
what are monosaccharides
simple sugars
e.g D-glucose, D-fructose
what are oligosaccharides
short-chain sugar units of 2-10 or 20
e.g fructans
what are polysaccharides
> 10 or 20 sugar units, usually a bioassembly and so not found on their own
e.g starch and cellulose
what characterises oligosaccharides and polysaccharides
glycosidic linkages
significance of CHO
- must abundant biomolecule in the world (structural & storage CHO)
- Supplies energy for animal & human nutrition (dietary staple)
- play an important role in human metabolism and health (fibre)
- wide industrial uses (oils, textures, paper & pharmaceuticals)
- green polymers (bioplastic, thermoplastc starch)
- films for wound dressing (chitosan films)
- biofules (cellulose and starch)
name the common monosaccharides
- Ara
- Fru
- Fuc
arabinose
fructose
fucose
name the common monosaccharides
- Gal
- Glc
- Man
Galactose
glucose
mannose
name the common monosaccharides
- Rib
- Rha
- Xyl
ribose
rhamnose
xylose
name the common monosaccharides
- GlcUA
- GalN
- GalNac
- GlcNac
- glucuronic acid
- galactosamine
- N-acteylgalactosamine
- N-actetylglucosamine
what does glucose need bc it is a reducing sugar
active carbonyl group
what happens to D-glucose in water
a mixture of alpha, beta and open chains form
four most common hexoses
glucose, mannose, galactose and fructose
what shape are pyrnaose rings
they are NOT flat as in Haworth projections
- occur in a variety of shapes
- chair is most common conformation,
- boat conformation
what are the two possible chair forms of glucose pyranose
4C1 and 1C4
which is the most stable chair form in sugars
4C1 because the bulk C-6 group is in equatorial locations
what are the differnet location descrptors
equatorial and axial
what are sugars linked by
O-glycosidic bonds
where is sucrose hydrolysed and what to
in the gut, to fructose and glucose
is frcutose a reducing sugar
no, becuase the C group is in the glycosidic linkage. But once hydrolysed it produced fructose and glucose which are reducing
solution used for testing if sugar is reducing
Fehling’s
what is sucrose used to make
sucralose - artificial sweetner AKA splenda
what is Splenda
sucralose, artifical sweetner
how is sucralose made
chlorine is added to the C4 of Glc and to C1 and C6 of the Fru of a sucrose molecule
properties of Splenda
- very stable
- resistant to heat treatment
- not hydrolysed to the gut
- thousands times sweetner than sucrose
- zero calories bc not hydrolysed
is maltose a reducing sugar
yes, aldehyde group is free to react with oxidants
where is maltose found
- rarely found in nature
- forms from hydrolysis of starch in human gut
- found in malted grains e.g barley
is lactose a reducing sugar
yes
where is lactose found
milk
what causes lactose intolerance
deficiency in lactase.
Not common in areas where dairy is requent consumed
methods of overcoming lactose intolerance
use of reduced lactose milk
addition of lactase to products - expensive
Trehalose
- oligosaccharide
- found in shrimp, fungi, yeast and blood of insects
- high water retenton useful for anhydrobiosis (tide)
- means that plants and invertebrate animals can withstand dessication (Removal of water)
Raffinose
- oligosaccharide
- found inn foods like cabbage, beans and brussel sprouts
- made up of galactose, glucose and fructose
- not digested in upper gut
- fermented in large intestine
- fermentation produces SCFA and gas (flatulence)
Satchyose
- oligosacharide
- naturally found in vegetables e.g beans and legumes
- used as a bulk sweetner
- not digested by upper gut in humans = no calories
- similar to raffinose but different strucure bc of one extra galactose
fructo-oligosaccharide
- oligosaccharide
- storage CHO in chicory root and jerusalem artichoke
- main source is cereals due to large consumption
- could be argued as polysaccharide bc 2-60 sugar units
- studied for pre-biotic effects, promotes growth of bifidobacteria
DP
degree of polymerisation
e.g DP >40 = polysaccharide
what controls chain conformaton of polysaccharide
chain geometry
- from flexible disordered coil through to packed linear arrays and helices
- 1,4 and/or 1,6 glycosidic bonds
different types of glycosidic bonds and conformation
- 1,4 diequatorial
- 1,4 diaxial
- 1,4 Ax-Eq
1,4 diequatorial
- linkage patterns are packed giving ribbon like strcuture packed in to tough fibriliar assemblies
- equatorial link is the 1,4 coming in and out of the ribbon structure
- ribbon strcutures themselves are held together with hydrogen bonds
1,4 diaxial
- ribbon like but hghly buckled structures that produce cavaties
- counter ions (calcium) sit in the cavaties and interact to hold structure together
1,4 Ax-Eq
- bonds of sugar units are not parallel
- twist in a chain
- mixture of axial and equatorial bonds
- helical structure
what is the main supplier of exogenous glucose in humans
starch
what us starch in plants
CHO reserve; tubers and seed endorsperm
size of starch granules
variable
1-100 micrometers
what is starch made from
2 glucose polymers
- amylopectin
-amylose
and some lipid, and protein for biosynthesis
dominate glucose polymer in starch
Amylopectin (70-90%)
Amylose is 10-30%
strcuture of amylose
- linear chains of alpha-D-glucopyranosyl units
- 1,4 glycosidic bonds
- low levels of branching
- likes to form single helix, stabilised by H bonds
- hydrophobic inner surface
- starch-lipid complexes arent easily digested in gut
structure of amylopectin
- very large molecle
- chains of alpha-D-glucopyranosyl units
- molecular weight of 10^6 -10^8
- 4-5% of Glc units are involved in 1,6 glycosidic bonds
- branched polymer
what are the essential features of the cluster model of starch
- A chains on the outside
- B chains on the inside
- reducing end inside
organisation of amorphous and crystalline regions of starch
amorphous regions are more readily attacked by amylase.
alternating strips of amophous and crystaline regions
difference between A types and B types of tarch
A are more tighly packed and bind less waer than B
what forms packed double helices
short chains of amylopectin (6 units)
- some helices will pack into crystalline lamella or crystalites
glycogen structure
- highly branched with alpha-D-glucopyranosyl units
- glycosidic linksages at both 1,4 and 1,4
- similar to amylopectin but more highly branched
- built on a protein backbone
- glycogen granules are 0.1 microm in hepatocytes
size of glycogen granules in hepatocytes
0.1microm
celluose structure
- glucan polymer, contains only Glc
- consists of D-glucose units linked in Beta-1,4 configuration
- forms a packed ribbon-like structure
- individual parallel chains held together by H bonds
- msot animals cannot digest Beta-1,4 linkages
- water insoluble
what does mixed 1,3 1,4 beta-D-glucan linkages show
fluorescence
where are mixed linkage beta glucans found
endosperm cell walls of cerelas e.g barley and oats
why is there interest in mixed linage beta glucans
- 1,3 links make it water soluble
- water solubilty and high molecular weight = viscocsity
- reduces blood glucose
- reduces cholesterol by lowering LDL
- beta glucan increase viscosity in the gut
what are pectins
- linear polymers of alpha-D-galacturonic acid linked by 1,4 bonds,
- a proprtion of COOH esterified with methanol
- molecular weight >100K
sugars involved in the main or branched chain of pecti
L-Rha, D-Glc, L-Ara
what is apple pectin
rhamnogalacturonan with xylose and arabinogalactan side chains
where are pectins found
primary cell wall and middle lamella of plants
role of pectins
- ripening of fruit
- form of dietary fibre
- forms viscous solutions e.g jam
biological properties of pectins that are currently being studies
- known to reduce blood glucose (postprandial glycaemia) and cholesterol
- recetn studies show anti-cancer effects because pctis inhibit Galectin 3 and produces butyrate in the large intestine
what is chitin
linear polymer of N-acetylgucosamine units
- basically cellulose, but OH replaced with acetylated aminoo group at C2
uses of chitin
- principle component of hard exoskeleton of arthropods (insects, crabs)
- deacylated chitin forms chitosan used for pharmoceuticals (plant growth enhancer, anti-fungal)
what does deacylated chitin form
chitosan
what does available CHO mean
CHO is availble for digestion and absorption
-tubers, cereals, milk, fruit etc
what does unavailble CHO mean
CHO is unavailable for digestion and absoprtion = dietary fibre
Passes upper GI and will be fermented in lower GI
examples of available polysaccharides
starch and glycogeon
example of available disaccharides
sucrose and lactose
example of available monosaccharides
glucose and fructose
How are substrates converted to absorable forms
matrixes are disbaled and substrates are converted to lower molecular weight compounds such as glucose and fructose
what is the predominant CHO digested in huamn GI tract
starch = main source of exogenous glucose
effects of temperature on starch granules
- room temperature, native starch granules show maltese cross pattern (birefringence onvserved under polarised light)
-as temperature increases, starch absorbs water: - loss of birefringence
- granule swells
- amylose leaches and increases viscosity
- crystallinity
= gelatnisation process
occurs in the stomach and SI
what is the importance of gelatinisation
- increases amylolysis in the gut
- glycaemia
what happens when starch granules cool after heating
gel forms = retrogradation
resistant to amylosis and no effect on postgrandial glycaemia = resistant starch
where does digestion of CHO take place
- mouth and stomach
- small intestine
- intestinall mucosa
process of CHO digestion in mouth and stomach
- alpha amylase in saliva begins hydrolysis of starch
- Acidic pH of stomach inhibits action of amylase so hydrolysis is possible limited
- after food enters the stomach, pH rises from 2 to 4, and the pH inside food particles may be even higher allowing amyase action to continue (especially in soft foods like bread)
- stomach regulates emptying into the duodenum; recreates HCl, proteases and lipase under the neurol-humeral control of gastrin, CCk etc
process of CHO digestion in small intestine
- pancreatic alpha-amylase hydrolysis the alpha 1,4 bonds inthe gut lumen
- pancreatic alpha-amylase do not hydrolyse the alpha 1,6 branching points, terminal alpha 1,4 or the alpha 1,4 linkages that are next to the 1,6 branching points
- endoenzyme amylase carries ot multiple attacks on linear parts of free, mobile alpha glucan chains
- end product of starch hydrolysis are oligosaccharides - very little glucose is produced
- straight: maltose, maltotriose and higher M.Wt material glucose units with alpha 1,4 linkages
- branched: alpha-limit dextrins, contain 1,6 branching points as well as 1,4 linkages
what does pancreatic alpha amylase hydrolyse
alpha - 1,4 bonds
what does pancreatic alpha amylase not hydrolyse
alpha 1,6 branching points
terminal alpha 1,4 linkages
alpha 1,4 linkages that are next to the alpha 1,6 branching point
what carries out multiple attacks on linear parts of free, mobile alpha glucan chains
endoenzyme amylase
what is the end product of starch hydrolysis
oligosaccharides - very little glucose produced
- straight: maltose, maltotriose and higher M.Wt material glucose units with alpha 1,4 linkages
- branched: alpha-limit dextrins, contain 1,6 branching points as well as 1,4 linkages
types of oligosaccharides produced from starch hydrolysis
- straight: maltose, maltotriose and higher M.Wt material glucose units with alpha 1,4 linkages
- branched: alpha-limit dextrins, contain 1,6 branching points as well as 1,4 linkages
process of CHO digestion in the intestinal mucosa
- oligosaccharidases in the microvilli membrane continue starch digestion
- maltase-glucoamylase hydrolyses maltose to glucose
- sucrase-isomaltase hydrolyses alpha-limit dextrins 1,6 linkages to glucose
- lactose is hydrlysed by lactase to galactose and glucose
- sucrose is hydrolysed by sucrase to fructose and glucose
what hydrolyses maltose in the intestinal mucosa
maltase-glucoamylase
maltose to glucose
what hydrolyses alpha limit dextrin 1,6 linkages in the intestinal mucosa
sucrase-isomaltase
- to glucose
what hydrolyses lactose in the intestinal mucosa
lactase
lactose to glucose and galactose
what hydrolyses sucrose in the intestinal mucosa
sucrase
sucrose to fructose and glucose
Where are sugars absorbed following starch digestion
- SI wall to enter the enterocytes
sugars produced by digestion of starch and oligosaccharides are rapidly absorbed across the SI
process of sugar absorption
sugar molecules are transported across the mucusal barrier into the capillary blood supplying the villi and then draining into the hepatic portal vein
what does the hepatic portal vein do
it is the main blood vessel carrying nutrients away from the gut
what is the traditional view of speed of CHO absorption and does this still stand
glucose and other hexoses are removed before the remains of the meal (chyme) reach the terminal ileum
BUT more recent data since 1970s suggests that not all availbles CHO is digested to the same extent or at the same rate e.g starch
what happens to resitant starch in digestion
it cannot be digested and so is pushed down to the large bowl for fermentation by the microbiota
mechanisms of sugar absoprtion
- during active phase of digestion glucose concentration on the luminal surface of brush border membrane is high (~200mmol/L)
- facilitated diffusion and active transport occur
when is the active phase of digestion
fed state
what is the concentration of glucose during active phase
~200mmol/L on luminal surface of BBM
what happens to some glucose in enterocytes
metabolised to lactate, because they use glucose as a fuel source
facilitated diffusion
- involves GLUT2 and only occurs when [glucose] is hight
- fructose transport independent of GLUT2 and uses GLUT5, and some fructose will be converted to glucose
active transport
- glucose and galactose cross membrane coupled to Na+ using SGLT, dependent on energy from Na gradient
where are GI hormones secreted and travel
mucosa and transported in circulation, infuecning the functions of the stomach, intestines, pancreas and gall bladder
what do GI hormones effect
- water, electrolyte and enzyme secretion
- motility (peristaltic movement)
- growth
- release of other hormones
- intestinal absorption
main action of gastrin
gastric acid secretion
primary effects of secretin
stimulation of pancreatic fluid and bicarbonate secretion
main action of CCK
potentiates the primary effect of secretin
stimulates gall bladder contraction
inhibits gastric emptying
what is CCK
cholecystokinin
main action of GIP and GLP-1
stimulate insulin secretion at meal times
what happens to CHO that cannot be digest
fermentation
where does fermentation occur
lower gut
what is the fermentation of CHO
anaerobic degradation in the colon, by enxymes of the microbiota of olig/polysaccharides and mucins
products of CHO fermentation
- SCFA mainly acetate, butyrate, proponate
- gas CO2, H2 and CH4
microbiota population of human colon
very large
10^11 organisms per g of contents (poop)
predominatent organisms of the microbiota
genera Bacteroides, Fusobacterium, Eubacterium and Bifidobacterium
how much CHO is needed to sustain gut microbiota
10-70g (depending on proprtion of fibre)
average CHO entering colon / day
10-70g CHO made up of:
- 8-40g resistant starch
- 8-20g NSP
- 2-8g oligosaccharides
- 2-10g sugars
what is endogenous CHO
mucins - highly bransed glycoprotein from the mucus
how much endogenous CHO enters colon
2-3g/day of mucin
what happens to SCFA produced by fermentation
- absorbed by the epithelial cells
what happens to acetate from fermentation
- enters blood stream
- converted to acetyl CoA in lieer and other tissues
precursor for lipogenesis
substrate for oxidation
what happens to propionate from fermentation
- extracted by the liver for oxidation
- effects gluconeogenesis
- effects cholesterol synthesis
what happens to butyrate from fermentation
- one of main fuels for colonocytes
- therefore little enters the bloodstream
- increased butryate is suggested to reduce risk of colon cancer
important part of regualtion of glucose homeostasis
insulin and incretins
how does glucose enter the blood stream
- intestinal absorption
- glycogen breakdown in the liver
- gluconeogenesis in the liver
is blood glucose response the same for all starch containing foods?
no, different postprandial rises in blood glucose and insulin occur - formalised in GI concept
what does the study of effects of two different starches on postprandial plasma glucose show
- large difference in plasma glucose, related to rate of digestion in gut lumen
- difference is amplified in diabetics
- difference in glucose relates to difference in insulin
- diabetics have a different insulin respose profle - shifted to the right
- but diabetics still show a difference in insulin response between the two starches
why is there lots of variences in glycaemic index values
processing and cooking
values still helpful to guage general glucose response to different foods
what does a lower GI indicate
CHO more slowly digested
benefits of low GI
- epidemiological studies suggest low GI diets have protective effects in the development of T2D
- Low gi diets are known to have therapeutic advantages in treatment of diabetes comapred with high GI diets
absoprtion and digestion factors that affect GI values
available CHO
- structure and processing (SA)
- presence of lipid (lipid starch complexes)
- presence of natural inhibitors of digestive enzymes, glucose transporters and insulin secretion
- presence of non-digestible dietary fibre
- Sructure and properties of starch (raw v gelatinised)
examples of natural nhibitors of digestive enzymes, glucose transporters and insulin secretion
polyphenolics
saponins
protein inhibitors of amylase
example of non-digestible diary fibre and effect on GI value
cell wall encapsulation - increases viscoty of digesta
how do kinetics link to starch digestion
kinetic parameters of alpha amylase effect action on starches
what is Km
michaelis-menten constant
the [substrate] at which the velocity is half the Vmax
= index of substrate availabilty
what is catalytic efficiency
Kcat/Km
what does a lower Km mean in terms of starch digestion
greater hydrolysis of starch
what effects Km of starch
cooking - Km decreases from ~0.6 to ~0.03 in potatoes and rice
gelatinised starch is more accessible and so reduces to only 0.13 giving a higher Kcat/Km
what happens to hydrolysis as temperature increases
starch becomes more disordered and so less crystalised = more hydrolysis
how does fibre affect digestion and absorption of other macros
- multiple effect of fibre at different sites of the gut (gastric function, digestion kinetics and fermentation
- effects of fibre on metabolism (post prandial rise in blood glucose and insulin, gut hormones and lipaemia)
- bioligcal activity varies and mechanisms of action not well understood (heterogenity of fibre are key)
what is the problem with fibre understanding
- biological activities and responses vary
- mechanisms of the action are not well understood
what is dietary fibre
mainly non-digestible cell walls of plant foods.
complex network of mainly NSP
examples of dietary fibre
CHO: celluose, hemicellulose, pectic substances
NonCHO: lignin and phenolics
what part of dietary fibre has marked effects
non-CHO like lignin
why do some NSP form viscous solutions
becuase they are water soluble
e.g: legume galactomannans & xyloglucans, pectins, cereal β-glucans
benefit of water soluble NSP
lower the increase in blood glucose
possibly due to viscosity
why are values for dietary fibre intakes not very helpful
bc little is actually known about the interaction betweeen polymers of pant cell wall network
example of fibre rich african plant food
Detarium Senegalese Gemlin - legume
used in rural nigeria to thicken soups and stews
- seed cotyledon: ~60g/100g of water-soluble xyloglucan
- M.Wt ~2.7 mil
what did WHO investigate fibre rich african plant food for
treatment of T2D
- lowers fasting blood glucose in human subjects
- lowers fasting blood cholesterol in rats
mechanisms of digestion and absoprtion of dietary fibre
- water soluble NSP increase viscosity
- NSP interact with starch, alpha-amylase and mucus
- encapsulation by intact cell walls in legumes
effects of water souble NSP increasing viscosity
- viscosity is an important predictor of blood-glucose and cholesterol lowering effects of NSP
- Increased viscosity leads to slower gastric emptying, intestinal transit, chyme flow and mixing and digestion rate
- rheology of human chyme is not well understood
- access to gut is problems, problems of lumps
effects of NSP interacting with starch, alpha-amylase and mucus
- Galactomannan barrier on starch surface = hindered access of alpha amylase
- soluble NSP restrict swelling and amylose leaching during hydrothermal processing of starch = less susceptible to amylolysis
- GM binds to alpha amylase and inhibits action on starch through non-competitive inhibitor
- soluble fibre interacts with mucus to decrease mucus permeabilty
effects of encapsulation by intact cell walls
- cell walls hinder starch digestion and postprandial glycaemia (low GI)
- noah et al 1998 report that cell walls protect starch from amylolysis
- legume cotyledon cells seperate after cooking
- intact cells recovered from terminal ileum of humans - mechaisms of cell wall still uncertain
- cell wall barrier effect is important
- restriction to swelling?
proof that bioaccessibitly of starch has effects
study done with course and smooth porridge, where only difference was that smooth porridge had increased number of ruptured cells. there was 30-50% reducions in responses to glucose, insulin, c-peptide and GIP inthose with LESS ruptured cells (coarse porridge)
how would improved understanding of CHO help
lead to design of functional foods or ingredients with enhanced health benefits, primarly for CVD and diabetes
what do mechanistic studies help to understand
the behaviour of CHO in gut in terms of:
- starch digestion
- gut behaviour
- prebiotics
what do we lack understanding on for CHO
- structure and properties of CHO at different sits of the GIT
- subsequent effects on metabolism
what is a naturally- occurring disaccharide
maltose
Which glucose transporter is responsible for the uptake of glucose by pancreatic beta cells
GLUT2
