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Metabolism > Carbohydrates and metabolism > Flashcards

Carbohydrates and metabolism Flashcards

1
Q

generic form of common CHO

A

Cn(H20)n - they are carbon hydrates

- some may contains N, P or S

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2
Q

biochemical description of CHO

A

polyhydroxyl compounds that carry an aldehyde or ketone group, or substances that yield such compounds upon hydrolysis

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3
Q

three classes of CHO

A
  • monosaccharides
  • oligosaccharides
  • polysaccharides
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4
Q

what are monosaccharides

A

simple sugars

e.g D-glucose, D-fructose

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5
Q

what are oligosaccharides

A

short-chain sugar units of 2-10 or 20

e.g fructans

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6
Q

what are polysaccharides

A

> 10 or 20 sugar units, usually a bioassembly and so not found on their own
e.g starch and cellulose

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7
Q

what characterises oligosaccharides and polysaccharides

A

glycosidic linkages

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8
Q

significance of CHO

A
  • 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)
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9
Q

name the common monosaccharides

  • Ara
  • Fru
  • Fuc
A

arabinose
fructose
fucose

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10
Q

name the common monosaccharides

  • Gal
  • Glc
  • Man
A

Galactose
glucose
mannose

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11
Q

name the common monosaccharides

  • Rib
  • Rha
  • Xyl
A

ribose
rhamnose
xylose

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12
Q

name the common monosaccharides

  • GlcUA
  • GalN
  • GalNac
  • GlcNac
A
  • glucuronic acid
  • galactosamine
  • N-acteylgalactosamine
  • N-actetylglucosamine
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13
Q

what does glucose need bc it is a reducing sugar

A

active carbonyl group

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14
Q

what happens to D-glucose in water

A

a mixture of alpha, beta and open chains form

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15
Q

four most common hexoses

A

glucose, mannose, galactose and fructose

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16
Q

what shape are pyrnaose rings

A

they are NOT flat as in Haworth projections

  • occur in a variety of shapes
  • chair is most common conformation,
  • boat conformation
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17
Q

what are the two possible chair forms of glucose pyranose

A

4C1 and 1C4

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18
Q

which is the most stable chair form in sugars

A

4C1 because the bulk C-6 group is in equatorial locations

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19
Q

what are the differnet location descrptors

A

equatorial and axial

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20
Q

what are sugars linked by

A

O-glycosidic bonds

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21
Q

where is sucrose hydrolysed and what to

A

in the gut, to fructose and glucose

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22
Q

is frcutose a reducing sugar

A

no, becuase the C group is in the glycosidic linkage. But once hydrolysed it produced fructose and glucose which are reducing

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23
Q

solution used for testing if sugar is reducing

A

Fehling’s

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24
Q

what is sucrose used to make

A

sucralose - artificial sweetner AKA splenda

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25
Q

what is Splenda

A

sucralose, artifical sweetner

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26
Q

how is sucralose made

A

chlorine is added to the C4 of Glc and to C1 and C6 of the Fru of a sucrose molecule

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27
Q

properties of Splenda

A
  • very stable
  • resistant to heat treatment
  • not hydrolysed to the gut
  • thousands times sweetner than sucrose
  • zero calories bc not hydrolysed
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28
Q

is maltose a reducing sugar

A

yes, aldehyde group is free to react with oxidants

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29
Q

where is maltose found

A
  • rarely found in nature
  • forms from hydrolysis of starch in human gut
  • found in malted grains e.g barley
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30
Q

is lactose a reducing sugar

A

yes

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31
Q

where is lactose found

A

milk

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32
Q

what causes lactose intolerance

A

deficiency in lactase.

Not common in areas where dairy is requent consumed

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33
Q

methods of overcoming lactose intolerance

A

use of reduced lactose milk

addition of lactase to products - expensive

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34
Q

Trehalose

A
  • 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)
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35
Q

Raffinose

A
  • 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)
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36
Q

Satchyose

A
  • 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
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37
Q

fructo-oligosaccharide

A
  • 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
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38
Q

DP

A

degree of polymerisation

e.g DP >40 = polysaccharide

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39
Q

what controls chain conformaton of polysaccharide

A

chain geometry

  • from flexible disordered coil through to packed linear arrays and helices
  • 1,4 and/or 1,6 glycosidic bonds
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40
Q

different types of glycosidic bonds and conformation

A
  • 1,4 diequatorial
  • 1,4 diaxial
  • 1,4 Ax-Eq
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41
Q

1,4 diequatorial

A
  • 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
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42
Q

1,4 diaxial

A
  • ribbon like but hghly buckled structures that produce cavaties
  • counter ions (calcium) sit in the cavaties and interact to hold structure together
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43
Q

1,4 Ax-Eq

A
  • bonds of sugar units are not parallel
  • twist in a chain
  • mixture of axial and equatorial bonds
  • helical structure
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44
Q

what is the main supplier of exogenous glucose in humans

A

starch

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45
Q

what us starch in plants

A

CHO reserve; tubers and seed endorsperm

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46
Q

size of starch granules

A

variable

1-100 micrometers

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47
Q

what is starch made from

A

2 glucose polymers
- amylopectin
-amylose
and some lipid, and protein for biosynthesis

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48
Q

dominate glucose polymer in starch

A

Amylopectin (70-90%)

Amylose is 10-30%

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49
Q

strcuture of amylose

A
  • 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
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50
Q

structure of amylopectin

A
  • 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
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51
Q

what are the essential features of the cluster model of starch

A
  • A chains on the outside
  • B chains on the inside
  • reducing end inside
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52
Q

organisation of amorphous and crystalline regions of starch

A

amorphous regions are more readily attacked by amylase.

alternating strips of amophous and crystaline regions

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53
Q

difference between A types and B types of tarch

A

A are more tighly packed and bind less waer than B

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54
Q

what forms packed double helices

A

short chains of amylopectin (6 units)

- some helices will pack into crystalline lamella or crystalites

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55
Q

glycogen structure

A
  • 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
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56
Q

size of glycogen granules in hepatocytes

A

0.1microm

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57
Q

celluose structure

A
  • 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
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58
Q

what does mixed 1,3 1,4 beta-D-glucan linkages show

A

fluorescence

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59
Q

where are mixed linkage beta glucans found

A

endosperm cell walls of cerelas e.g barley and oats

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60
Q

why is there interest in mixed linage beta glucans

A
  • 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
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61
Q

what are pectins

A
  • linear polymers of alpha-D-galacturonic acid linked by 1,4 bonds,
  • a proprtion of COOH esterified with methanol
  • molecular weight >100K
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62
Q

sugars involved in the main or branched chain of pecti

A

L-Rha, D-Glc, L-Ara

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63
Q

what is apple pectin

A

rhamnogalacturonan with xylose and arabinogalactan side chains

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64
Q

where are pectins found

A

primary cell wall and middle lamella of plants

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65
Q

role of pectins

A
  • ripening of fruit
  • form of dietary fibre
  • forms viscous solutions e.g jam
66
Q

biological properties of pectins that are currently being studies

A
  • 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
67
Q

what is chitin

A

linear polymer of N-acetylgucosamine units

- basically cellulose, but OH replaced with acetylated aminoo group at C2

68
Q

uses of chitin

A
  • principle component of hard exoskeleton of arthropods (insects, crabs)
  • deacylated chitin forms chitosan used for pharmoceuticals (plant growth enhancer, anti-fungal)
69
Q

what does deacylated chitin form

A

chitosan

70
Q

what does available CHO mean

A

CHO is availble for digestion and absorption

-tubers, cereals, milk, fruit etc

71
Q

what does unavailble CHO mean

A

CHO is unavailable for digestion and absoprtion = dietary fibre
Passes upper GI and will be fermented in lower GI

72
Q

examples of available polysaccharides

A

starch and glycogeon

73
Q

example of available disaccharides

A

sucrose and lactose

74
Q

example of available monosaccharides

A

glucose and fructose

75
Q

How are substrates converted to absorable forms

A

matrixes are disbaled and substrates are converted to lower molecular weight compounds such as glucose and fructose

76
Q

what is the predominant CHO digested in huamn GI tract

A

starch = main source of exogenous glucose

77
Q

effects of temperature on starch granules

A
  • 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
78
Q

what is the importance of gelatinisation

A
  • increases amylolysis in the gut

- glycaemia

79
Q

what happens when starch granules cool after heating

A

gel forms = retrogradation

resistant to amylosis and no effect on postgrandial glycaemia = resistant starch

80
Q

where does digestion of CHO take place

A
  1. mouth and stomach
  2. small intestine
  3. intestinall mucosa
81
Q

process of CHO digestion in mouth and stomach

A
  • 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
82
Q

process of CHO digestion in small intestine

A
  • 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
83
Q

what does pancreatic alpha amylase hydrolyse

A

alpha - 1,4 bonds

84
Q

what does pancreatic alpha amylase not hydrolyse

A

alpha 1,6 branching points
terminal alpha 1,4 linkages
alpha 1,4 linkages that are next to the alpha 1,6 branching point

85
Q

what carries out multiple attacks on linear parts of free, mobile alpha glucan chains

A

endoenzyme amylase

86
Q

what is the end product of starch hydrolysis

A

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
87
Q

types of oligosaccharides produced from starch hydrolysis

A
  • 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
88
Q

process of CHO digestion in the intestinal mucosa

A
  • 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
89
Q

what hydrolyses maltose in the intestinal mucosa

A

maltase-glucoamylase

maltose to glucose

90
Q

what hydrolyses alpha limit dextrin 1,6 linkages in the intestinal mucosa

A

sucrase-isomaltase

- to glucose

91
Q

what hydrolyses lactose in the intestinal mucosa

A

lactase

lactose to glucose and galactose

92
Q

what hydrolyses sucrose in the intestinal mucosa

A

sucrase

sucrose to fructose and glucose

93
Q

Where are sugars absorbed following starch digestion

A
  • SI wall to enter the enterocytes

sugars produced by digestion of starch and oligosaccharides are rapidly absorbed across the SI

94
Q

process of sugar absorption

A

sugar molecules are transported across the mucusal barrier into the capillary blood supplying the villi and then draining into the hepatic portal vein

95
Q

what does the hepatic portal vein do

A

it is the main blood vessel carrying nutrients away from the gut

96
Q

what is the traditional view of speed of CHO absorption and does this still stand

A

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

97
Q

what happens to resitant starch in digestion

A

it cannot be digested and so is pushed down to the large bowl for fermentation by the microbiota

98
Q

mechanisms of sugar absoprtion

A
  • 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
99
Q

when is the active phase of digestion

A

fed state

100
Q

what is the concentration of glucose during active phase

A

~200mmol/L on luminal surface of BBM

101
Q

what happens to some glucose in enterocytes

A

metabolised to lactate, because they use glucose as a fuel source

102
Q

facilitated diffusion

A
  • 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

103
Q

active transport

A
  • glucose and galactose cross membrane coupled to Na+ using SGLT, dependent on energy from Na gradient
104
Q

where are GI hormones secreted and travel

A

mucosa and transported in circulation, infuecning the functions of the stomach, intestines, pancreas and gall bladder

105
Q

what do GI hormones effect

A
  • water, electrolyte and enzyme secretion
  • motility (peristaltic movement)
  • growth
  • release of other hormones
  • intestinal absorption
106
Q

main action of gastrin

A

gastric acid secretion

107
Q

primary effects of secretin

A

stimulation of pancreatic fluid and bicarbonate secretion

108
Q

main action of CCK

A

potentiates the primary effect of secretin
stimulates gall bladder contraction
inhibits gastric emptying

109
Q

what is CCK

A

cholecystokinin

110
Q

main action of GIP and GLP-1

A

stimulate insulin secretion at meal times

111
Q

what happens to CHO that cannot be digest

A

fermentation

112
Q

where does fermentation occur

A

lower gut

113
Q

what is the fermentation of CHO

A

anaerobic degradation in the colon, by enxymes of the microbiota of olig/polysaccharides and mucins

114
Q

products of CHO fermentation

A
  • SCFA mainly acetate, butyrate, proponate

- gas CO2, H2 and CH4

115
Q

microbiota population of human colon

A

very large

10^11 organisms per g of contents (poop)

116
Q

predominatent organisms of the microbiota

A

genera Bacteroides, Fusobacterium, Eubacterium and Bifidobacterium

117
Q

how much CHO is needed to sustain gut microbiota

A

10-70g (depending on proprtion of fibre)

118
Q

average CHO entering colon / day

A

10-70g CHO made up of:

  • 8-40g resistant starch
  • 8-20g NSP
  • 2-8g oligosaccharides
  • 2-10g sugars
119
Q

what is endogenous CHO

A

mucins - highly bransed glycoprotein from the mucus

120
Q

how much endogenous CHO enters colon

A

2-3g/day of mucin

121
Q

what happens to SCFA produced by fermentation

A
  • absorbed by the epithelial cells
122
Q

what happens to acetate from fermentation

A
  • enters blood stream
  • converted to acetyl CoA in lieer and other tissues
    precursor for lipogenesis
    substrate for oxidation
123
Q

what happens to propionate from fermentation

A
  • extracted by the liver for oxidation
  • effects gluconeogenesis
  • effects cholesterol synthesis
124
Q

what happens to butyrate from fermentation

A
  • one of main fuels for colonocytes
  • therefore little enters the bloodstream
  • increased butryate is suggested to reduce risk of colon cancer
125
Q

important part of regualtion of glucose homeostasis

A

insulin and incretins

126
Q

how does glucose enter the blood stream

A
  • intestinal absorption
  • glycogen breakdown in the liver
  • gluconeogenesis in the liver
127
Q

is blood glucose response the same for all starch containing foods?

A

no, different postprandial rises in blood glucose and insulin occur - formalised in GI concept

128
Q

what does the study of effects of two different starches on postprandial plasma glucose show

A
  • 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
129
Q

why is there lots of variences in glycaemic index values

A

processing and cooking

values still helpful to guage general glucose response to different foods

130
Q

what does a lower GI indicate

A

CHO more slowly digested

131
Q

benefits of low GI

A
  • 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
132
Q

absoprtion and digestion factors that affect GI values

available CHO

A
  • 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)
133
Q

examples of natural nhibitors of digestive enzymes, glucose transporters and insulin secretion

A

polyphenolics
saponins
protein inhibitors of amylase

134
Q

example of non-digestible diary fibre and effect on GI value

A

cell wall encapsulation - increases viscoty of digesta

135
Q

how do kinetics link to starch digestion

A

kinetic parameters of alpha amylase effect action on starches

136
Q

what is Km

A

michaelis-menten constant
the [substrate] at which the velocity is half the Vmax
= index of substrate availabilty

137
Q

what is catalytic efficiency

A

Kcat/Km

138
Q

what does a lower Km mean in terms of starch digestion

A

greater hydrolysis of starch

139
Q

what effects Km of starch

A

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

140
Q

what happens to hydrolysis as temperature increases

A

starch becomes more disordered and so less crystalised = more hydrolysis

141
Q

how does fibre affect digestion and absorption of other macros

A
  • 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)
142
Q

what is the problem with fibre understanding

A
  • biological activities and responses vary

- mechanisms of the action are not well understood

143
Q

what is dietary fibre

A

mainly non-digestible cell walls of plant foods.

complex network of mainly NSP

144
Q

examples of dietary fibre

A

CHO: celluose, hemicellulose, pectic substances
NonCHO: lignin and phenolics

145
Q

what part of dietary fibre has marked effects

A

non-CHO like lignin

146
Q

why do some NSP form viscous solutions

A

becuase they are water soluble

e.g: legume galactomannans & xyloglucans, pectins, cereal β-glucans

147
Q

benefit of water soluble NSP

A

lower the increase in blood glucose

possibly due to viscosity

148
Q

why are values for dietary fibre intakes not very helpful

A

bc little is actually known about the interaction betweeen polymers of pant cell wall network

149
Q

example of fibre rich african plant food

A

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

150
Q

what did WHO investigate fibre rich african plant food for

A

treatment of T2D

  • lowers fasting blood glucose in human subjects
  • lowers fasting blood cholesterol in rats
151
Q

mechanisms of digestion and absoprtion of dietary fibre

A
  • water soluble NSP increase viscosity
  • NSP interact with starch, alpha-amylase and mucus
  • encapsulation by intact cell walls in legumes
152
Q

effects of water souble NSP increasing viscosity

A
  • 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
153
Q

effects of NSP interacting with starch, alpha-amylase and mucus

A
  • 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
154
Q

effects of encapsulation by intact cell walls

A
  • 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?
155
Q

proof that bioaccessibitly of starch has effects

A

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)

156
Q

how would improved understanding of CHO help

A

lead to design of functional foods or ingredients with enhanced health benefits, primarly for CVD and diabetes

157
Q

what do mechanistic studies help to understand

A

the behaviour of CHO in gut in terms of:

  • starch digestion
  • gut behaviour
  • prebiotics
158
Q

what do we lack understanding on for CHO

A
  • structure and properties of CHO at different sits of the GIT
  • subsequent effects on metabolism
159
Q

what is a naturally- occurring disaccharide

A

maltose

160
Q

Which glucose transporter is responsible for the uptake of glucose by pancreatic beta cells

A

GLUT2

Metabolism (11 decks)

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