Carbohydrates Flashcards
1
Q
simple carbohydrates
A
monosaccharides
- most common is glucose
- naturally occurring
- cannot be hydrolyzed into a smaller unit
- “reducing sugar” when the anomeric carbon is free
disaccharides
- most common is sucrose
- 2 monosaccharides joined by an acetyl bond (glycosidic)
2
Q
complex carbohydrates
A
oligosaccharideds
- short polymers
polysaccharides
- homo and hetero exist
- glycogen (animal), starch and cellulose (plant)
3
Q
ratio of organic molecules in carbohydrates
A
CHO = 1:2:1
4
Q
most common monosaccharides
A
Triose (3C)
- metabolites of glucose
- broken down into trioses in glycolysis
Pentose (5C)
- components of DNA and RNA
- sugars in nucleic acids
Hexose (6C)
- nutritionally the most important
- give our body energy
5
Q
what is an anomeric carbon
A
the carbon that is apart of the carbonyl (C=O) group
6
Q
what is an aldose
A
a sugar that contains an aldehyde (e.g. D-glucose)
7
Q
what is a ketose
A
a sugar that contains a ketone (e.g D-fructose)
8
Q
what is sterioisomerism?
A
CHO with the same molecular formula and sequence but differ in 3D space due to chiral carbonds
- come in D and L forms
- enzymes used for digestion can recognize stereoisomers (steriospecific)
9
Q
what is a chiral carbon?
A
a carbon attached to 4 DIFFERENT atoms or groups
10
Q
different types of isomers
A
enantiomers: mirror image
diasteromers: not mirror images
11
Q
Fischer projection
A
- linear form of a sugar
- counting of carbons begins at the anomeric carbon for an aldose
12
Q
Stereoisomers and Fischer projections
A
- D or L for is determined by the -OH group on the highest chiral carbon (furthest from C=O)
- OH on the right = D
- OH on the left = L
- the number of stereoisomers is 2^n (n = # of chiral carbons)
13
Q
why are D monosaccharides nutritionally important?
A
digestive enzymes are stereospecific for D sugars
14
Q
converting a linear sugar to a cyclical sugar
A
- the anomeric carbon allows the linear structure to turn cyclical
- carbon 1 becomes a new chiral center
- hemiacetal: made from an aldose
- hemiketal: made from a ketose
- reaction occurs spontaneously (no enzymes involved and reversible)
15
Q
alpha and beta configurations of cyclical sugars
A
- alpha = OH on anomeric carbon is down
- beta = OH on anomeric carbon is up
- beta is more common
- alpha is more nutritionally important for us
16
Q
Haworth model of sugars
A
- the non-acetyl/ketal Ch2OH always points up (C#6)
- OH group right in fischer = down in haworth, left in fischer = up in haworth
- alpha = OH on C1 is down, beta = OH on C1 is up
17
Q
disaccharides
A
- the most common oligosaccharide
- 2 monosaccharides attached by a glycosidic bond
- glycosidic bond can be alpha or beta
- configuration of -OH group on the anomeric carbon determines whether the disaccharide is alpha or beta
18
Q
common disaccharides
A
sucrose (alpha(1-2))
- found in sugar cane and fruits
- glucose + fructose
- non-reducing (both anomeric carbons are used)
lactose (beta(1-4))
- found in milk
- galactose + glucose
- reducing - free anomeric carbon
maltose (alpha(1-4))
- found in beer and liquor
- glucose + glucose
- reducing - free anomeric carbon
19
Q
polysaccharides
A
long strings or branches of monosaccharides (min 6) attached by glycosidic bonds
homo: all the same monosaccharides
hetero: different monosacharides
- both found in nature but homo ar more common in food
20
Q
advantage of branching in polysaccharides
A
- provides a larger number of ends from which to cleave glucose when energy is needed
21
Q
what is dietary fibre
A
- non-digestible CHO
- structureal part of plants
2 types: soluble and insoluble
22
Q
insoluble fibre
A
- includes cellulose, lignin and hemicellulose
- remains intact through the intestinal tract
- doesn’t dissolve in water
- reduced transit time (moves quick through gut)
- increased fecal bulk
23
Q
soluble fibre
A
- includes pectins, gums and mucilages
- forms a gel in intestinal tract
- dissolves in water
- delays gastric emptying (increased transit time)
- slows the rate of nutrient absorption
24
Q
dietary fibres and solubility
A
characteristics of solubility are…
water holding ability (ability of a fibre to hold water, becoming a viscous solution)
absorptive ability (ability of a fibre to bind enzymes and nutrients)
25
why is dietary fibre beneficial for our gut health
dietary fibre feeds our gut microbiota, which reduced inflammation; less stress on host
26
cellulose
- both a dietary fibre (naturally occuring) and functional fibre (natural but added
- homopolysaccharide of B-1,4 glucose units in a linear chain
- poorly fermented by human gut bacteria ( because humans lack cellulose-fermenting microbes in their gut microbiome
- rich in bran, legumes, nuts, peas, etc.
27
hemicellulose
- heteropolysaccharide that varies between plants
- a mixture of alpha and beta glycosidic linkages
- can contain both pentoses and hexoses (xylose is most common)
- exists as both branched and linear structures
- the solubility and fermentability of hemicellulose depends on the sugar composition
- found in bran, whole grains, nuts and some vegetables and fruits
28
pectin
- both a dietary and functional fibre
- part of the primary cell wall of plants
- backbone of unbranched alpha-1,4-linked D galacturonic acid
- stable at low pH
- highly fermented by gut bacteria (good bulking agent in animal feed)
- rich in fruits
29
resistant starch
- four main types: RS1-4 (found in different foods)
- typically found in plant cell walls
- resistant to amylase activity
- conveys some advantages of both soluble and insoluble fibres
30
what are the health benefits of fibre
- maintains function and health of the gut
- insoluble fibre decreases constipation: stimulates muscle contraction to break down waste, decreases the risk of bacterial infection
- soluble fibre increases satiety: delays gastric emptying, slows down nutrient uptake
31
soluble fibre and disease risk
- decreases cardiovascular disease risk by lowering blood cholesterol (gel binds cholestrol in the small intestine and caries it out of the body)
- lowers the risk of type II diabetes by binding some glucose in the digestive tract
32
carbohydrate digestion: mouth
- a-amylase gets the ball rolling by breaking a-1,4-glycosidic bonds
- produces only a few monosaccharides
- cellulose and lactose are resistant since they have a-1,6-bonds (branch in structure)
33
carbohydrate digestion: stomach
- a-amylase digestion continues until pH drops, and then is inactivated by stomach acid
- at this point the pool of dietary CHO consists of small polysaccharides and maltose
34
carbohydrate digestion: small intestine
- a-amylase digestion continues in the pancreas
- active at neutral pH
- a-1,6-bonds are resistant and eventually produce isomaltose
- bulk of digestion happens here
35
brush border enzyme activity
- by the time sugars reach the BBM there are disaccharides
specific enzymes break sugars down further...
isomaltase (alpha-dextrinase): isomaltose - 2 glucose
maltase: maltose - 2 glucose
invertase (sucrase): sucrose - glucose + fructose
lactase: lactose - glucose + galactose
36
lactose intolerance
- no lactase present in the intestine
- bacteria ferments the lactose and makes byproducts that cause discomfort for the individual
- age, ethnicity and genetics influence lactase enzyme activity
37
what are entrocytes?
- polarized cells that take up monosaccharides into the intestinal lumen for absorption
-
38
what happens to glucose after it is taken up by entrocytes?
- small amounts leak back out into the lumen from the entrocyte
- small amounts diffuse into the blood through the basolateral membrane
- majority gets transported into the blood through glut2
39
transport of monosaccharides from the lumen into the blood
- glucose and galactose depent on basolateral Na-K ATPase activity
- ATP targets the ion pumps, not the glucose transporters
- ion pumps prevent Na+ from building up in the cell which maintains gradient
- if you don't have sodium you cant take up glucose
- fructose is taken up by facilitated transport (GLUT5 on apical surface)
- all enter the blood through GLUT2
40
functions of carbohydrates in the body
- glucose is the primary source of energy for cells (proper function of CNS and red blood cells)
- carbohydrates "spare" proteins: prevents breakdown of protein for energy (allows it to concentrate on building, repairing and maintaining body tissue)
- carbohydrates prevent ketosis: when limited, fats are broken down for energy which leads to production of ketone bones causing the bodies pH to become slightly acidic
41
6 processes in carbohydrate metabolism
- glycogenesis
- glycolysis
- hexose monophosphate shunt
- gluconeogenesis
- glycolysis
- Krebs cycle
42
what happens when glucose enters a cell
- glucose becomes glucose-6-phosphate (G6P)
- G6P undergoes either glucogenesis, hexose monophosphate shunt, or glycolysis based on needs
- enters glycogenesis for energy storage
- enters glycolysis for energy production
- enters hexose monophosphate shunt to generate precursors for biogenesis
43
what is glycogenesis?
- the formation of glycogen from glucose
- G6P becomes G1P and then glycogen synthase turns it to glycogen by removing a phosphate
44
what does the liver do to glycogen
breaks down glycogen to release glucose into the blood when needed
45
what role does glycogen play with insulin
- insulin activates glucokinase, hexokinase and glycogen synthase
- insulin favours the uptake and clearance of blood sugar to form glycogen
46
what is glycogenin
- an enzyme that serves as a scaffold on which to attach glucose molecules to build glycogen
- 30 000+ glucose molecules can be contained in a single glycogen structure
- process requires energy
47
what is glycogenolysis?
- the breakdown of glycogen to make glucose
- glycogen becomes glucose-1-phosphate by glycogen phosphorylase (breaks a-1,4-bond to release monomers)
- G1P becomes G6P
- G6P is converted to glucose by glucose-6-phosphotase and has 2 fates...
1. in the liver (ONLY) G6P is dephosphorylated to glucose and is released into the blood
2. G6P can go onto glycolysis
48
what is the role of glucagon?
- present when blood sugar is low
- activates glycogen phosphorylase to make glucose from glycogen
49
state of flux between glycogenesis and glycogenolysis
low blood sugar
- promotes glucagon release by pancreas
- glucagon release stimulates the breakdown of glycogen in the liver
- glycogen is broken down to glucose and released from the liver to increase blood sugar
high blood sugar
- promotes insulin release from the pancreas
- insulin either travels to tissue cells OR stimulates glycogen formation in the liver
- glucose is taken up from the blood and converted to glycogen to decrease blood sugar
50
what are the main ways to produce energy in the cell?
1. substrate level phosphorylation
- transfer of a high-energy phosphate group from one molecule to another
- usually attached a phosphate to ADP to make ATP
- the only way red blood cells can get energy
2. oxidative phosphorylation
- happens in the ETC of the mitochondria
- O2 is required and increased the proton motive force
51
what is glycolysis?
- breaking glucose to generate energy (ATP)
- happens in the cytoplasm by enzymes
- endpoint depends on anaerobic vs aerobic cell
- one glucose generates 2 pyruvates
52
important molecules and enzymes in glycolysis
glucokinase/hexokinase: converts glucose to G6P
phosphofructokinase: produces fructose-1,6-bisphosphate
- phosphofructokinase is split into 2 molecules of glyceraldehyde-3-phosphate
- end product is 2 pyruvate molecules
53
what is the importance of phosphofructokinase in glycolysis?
- the first committed (irreversible step) in glycolysis
- ATP and glucagon in the liver inhibit this enzyme (no need to breakdown glucose) and therefore the committed step
54
net energy yield from 1 glucose in glycolysis
2NADH and 2ATP ~ 8 ATP
- NADH will go on to the ETC
- 2 ATP are put in and 4 are generated, therefore net gain of 2
55
fate of pyruvate after glycolysis
anaerobic = kreb's cycle
aerobic = lactate
56
Anaerobic metabolism of glucose: lactate production
- occurs in muscles during prolonged exercise and in red blood cells
- pyruvate is converted into lactate in the cell's cytosol
- regenerates NAD+ to continue glycolysis
- net of 2 ATP is produced when glucose is converted to lactate
57
Anaerobic metabolism of glucose: ethanol production
- doesn't happen in the body
- basis of fermentation when you make wine and beer
- yeast breaks down pyruvate into CO2 and ethanol
- regenerates NAD+ which allows glycolysis to continue
58
The cori cycle
- occurs when oxygen isn't available in the muscle leading to the production of lactate
- how lactate is transported back to the liver where gluconeogenesis allows for the conversion of pyruvate back to glucose
- for 2 molecules of lactate to form 1 glucose the cell consumes 6 ATP
- not sustainable because more energy is being consumed than produced
59
what is the hexose monophosphate shunt?
- important for NADPH production and ribose synthesis from glucose (G6P)
- occurs in the cytoplasm of a cell
- molecules produced will go on to perform other functions in the cell such as biosynthesis
60
Hexose monophosphate shunt: oxidative phase
- G6P releases NADPH and becomes 6-PG
- 6-PG releases NADPH and CO2 to give ribulose-5-phosphate which will go onto the nonoxidative phase
- NADPH produced can be used for fatty acid biosynthesis in the liver, oxidative phosphorylation and oxidative defence system in red blood cells
- G6P dehydrogenase is the rate limiting step which converts the first intermediate - can be inhibited by NADPH
61
hexose monophosphate shunt: nonoxidative phase
- ribulose-5-phosphate is converted to ribose-5-phosphate which can do 1 of 2 things
- 1. go onto nucleotide synthesis
- 2. create C3-C7 intermediates which become F6P
- all cells use this phase for nucleotide synthesis to continue
62
Pyruvate dehydrogenase
- the "gatekeeper" to Kreb's which converts pyruvate to acetyl CoA
- happens in the mitochondria (pyruvate is committed to staying there)
- CoA-SH is added to pyruvate (3C) and NAD+ comes in and NADH + CO2 comes out to give acetyl CoA (2C)
- happens twice per 1 glucose
- net energy yield: 2NADH ~ 6ATP
63
pyruvate dehydrogenase complex
- several enzymes and cofactors are required to convert pyruvate to acetyl CoA
- 4 vitamins (micronutrients): thiamine, Niacin, Riboflavin, pantothenic acid
64
the Kreb's cycle
- over 90% of the energy in food is released in this biochemical process
- final catabolic pathway for products of protein, lipid and carbs
- takes place in the mitochondrial matrix
- some AAs can be converted to acetyl CoA or other compounds in the kreb's cycle in a fasted state if your body needs to break down protein for energy
- energy yield of 1 cycle: 3NADH, 1FADH2, 1GTP ~ 12ATP
65
how can pyruvate get converted to Oxaloacetate (instead of acetyl CoA)?
- pyruvate carboxylase converts pyruvate (3C) to oxaloacetate (4C)
- required the input of 1 ATP
- this step is inhibited if enough Acetyl CoA (2C) is present
66
How much energy can you get from one glucose molecule?
1 NADH ~ 3ATP, 1 FADH2 ~ 2ATP, 1 GTP ~ 1ATP
- glycolysis: 8 ATP
- pyruvate dehydrogenase: 6 ATP (since x2 pyruvate)
- Kreb's cycle 24 ATP (since x2 acetyl CoA)
TOTAL ENERGY: 38 ATP
- theoretical maximum possible since some NADH and FADH2 go onto contribute to other processes
67
What is gluconeogenesis?
- making glucose from non-CHO sources
- active when glucose is needed - such as a fasting state
- very active in the liver, can also happen in the kidney during starvation
- doesn't happen in muscle or adipose tissue since they lack the enzymes
- when lactate travels from muscle back to the liver (cori cycle) gluconeogenesis can take place
68
gluconeogenesis: mitochondria
- lactate gets converted back to pyruvate which enters the mitochondrial via pyruvate translocase
- Pyruvate carboxylase converts pyruvate to oxaloacetate
- oxaloacetate cannot leave the mitochondria so it is converted to malate via malate dehydrogenase, the crosses the membrane and is converted back to oxaloacetate in the cytosol
