Carbohydrates Flashcards
simple carbohydrates
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)
complex carbohydrates
oligosaccharideds
- short polymers
polysaccharides
- homo and hetero exist
- glycogen (animal), starch and cellulose (plant)
ratio of organic molecules in carbohydrates
CHO = 1:2:1
most common monosaccharides
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
what is an anomeric carbon
the carbon that is apart of the carbonyl (C=O) group
what is an aldose
a sugar that contains an aldehyde (e.g. D-glucose)
what is a ketose
a sugar that contains a ketone (e.g D-fructose)
what is sterioisomerism?
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)
what is a chiral carbon?
a carbon attached to 4 DIFFERENT atoms or groups
different types of isomers
enantiomers: mirror image
diasteromers: not mirror images
Fischer projection
- linear form of a sugar
- counting of carbons begins at the anomeric carbon for an aldose
Stereoisomers and Fischer projections
- 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)
why are D monosaccharides nutritionally important?
digestive enzymes are stereospecific for D sugars
converting a linear sugar to a cyclical sugar
- 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)
alpha and beta configurations of cyclical sugars
- 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
Haworth model of sugars
- 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
disaccharides
- 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
common disaccharides
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
polysaccharides
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
advantage of branching in polysaccharides
- provides a larger number of ends from which to cleave glucose when energy is needed
what is dietary fibre
- non-digestible CHO
- structureal part of plants
2 types: soluble and insoluble
insoluble fibre
- 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
soluble fibre
- 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
dietary fibres and solubility
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)
why is dietary fibre beneficial for our gut health
dietary fibre feeds our gut microbiota, which reduced inflammation; less stress on host
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.
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
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
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
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
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
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)
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
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
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
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
what are entrocytes?
- ## polarized cells that take up monosaccharides into the intestinal lumen for absorption
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
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
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
6 processes in carbohydrate metabolism
- glycogenesis
- glycolysis
- hexose monophosphate shunt
- gluconeogenesis
- glycolysis
- Krebs cycle
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
what is glycogenesis?
- the formation of glycogen from glucose
- G6P becomes G1P and then glycogen synthase turns it to glycogen by removing a phosphate
what does the liver do to glycogen
breaks down glycogen to release glucose into the blood when needed
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
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
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
what is the role of glucagon?
- present when blood sugar is low
- activates glycogen phosphorylase to make glucose from glycogen
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
what are the main ways to produce energy in the cell?
- 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 - oxidative phosphorylation
- happens in the ETC of the mitochondria
- O2 is required and increased the proton motive force
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
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
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
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
fate of pyruvate after glycolysis
anaerobic = kreb’s cycle
aerobic = lactate
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
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
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
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
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
hexose monophosphate shunt: nonoxidative phase
- ribulose-5-phosphate is converted to ribose-5-phosphate which can do 1 of 2 things
- go onto nucleotide synthesis
- create C3-C7 intermediates which become F6P
- all cells use this phase for nucleotide synthesis to continue
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
pyruvate dehydrogenase complex
- several enzymes and cofactors are required to convert pyruvate to acetyl CoA
- 4 vitamins (micronutrients): thiamine, Niacin, Riboflavin, pantothenic acid
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
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
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
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
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
