week 3 carbs revised Flashcards
what are the major carbs in our diet
- starch (rice)
- glycogen (meat)
- cellulose and hemicellulose (plant cell wall, can’t digest)
- oligosaccharides containing (a1-6) linked galactose (peas, beans)
- lactose, sucrose, maltose
- glucose, fructose
what are the monosaccharides which are major carbs in our diet
- glucose
- fructose
- galactose = (oligosaccharide containing (a1-6) linked galactose)
what kind of sugars are monosaccharides
- hexoses (6 carbons sugars)
what are the disaccharides which are major carbs in our diet
- maltose
- lactose
- sucrose
what are the bonds linking monosaccharides called
- glycosidic bonds
what are the characteristics of maltose
- major carb in diet
- disaccharide
- anomeric, C-1 is available for oxidation therefore it is a reducing sugar
what are the characteristics of lactose
- major carb in diet
- disaccharide
- formed from glycosidic bond between galactose and glucose
- it is a reducing sugar
what are the characteristics of sucrose
- major carb in diet
- disaccharide
- non-reducing sugar
what is the starch formed from
amylose and amylopectin linked by glycosidic bond (two types of glucose polymer), form alpha helices with few reducing ends and many non-reducing ends
- amylopectin contains (a1-6) branches
where is glycogen stored
90% in liver and skeletal muscle
what is the function of glycogen in the liver
- replenish blood glucose when fasting
- if blood glucose falls then glycogen > G-6-P > glucose into blood
- G6P > glucose via glucose 6 phosphatase
what is the function of glycogen in the skeletal muscle
- catabolism produces ATP for contraction
- no G-6-P
- glycogen > lactate via glycolysis
what is the structure of glycogen
- polymer of glucose (a1-4) and (a1-6) branch every 8-12 residues
- stores glucose
- major carb in diet
what is a proteoglycan
- carb > protein
- in connective tissues
what is a glycoprotein
- protein > carb
- in membrane, ECM, blood, Golgi complex
what is a glycosaminoglycan (GAG)
- un branched polymers
- repeating units of hexuronic acid and an amino sugar
- in mucous and synovial fluid
what is the digestion of carbs in the mouth
- salivary amylase hydrolyses (a1-4) bonds of starch
what is the digestion of carbs in the stomach
no digestion of carbs in the stomach
what is the digestion of carbs in the duodenum
- same as mouth
- pancreatic amylase hydrolyses (a1-4) bonds of starch
what is the digestion of carbs in the jejunum
final digestion by mucosal cell surface enzymes
- isomaltase hydrolyses (a1-6) bonds
- glucoamylase removes glucose from non-reducing ends
- sucrase hydrolyses sucrose
- lactase hydrolyses lactose
what are the main products of digestion
- glucose
- galactose
- fructose
how is glucose absorbed
- via Na+ - glucose symport
- ATP driven and maintains low cellular sodium concentration
how is galactose absorbed
utilises gradients to facilitate its transport
how is fructose absorbed
- binds to GLUT 5 and moves down conc. gradient into blood
what is the fate of absorbed glucose
- diffuses through intestine wall into blood then onto liver
- immediately phosphorylated to glucose 6-phosphate
why is glucose phosphorylated into G-6-P
traps glucose inside cells because GLUT transporter doesn’t recognise it in this form
what enzymes catalyses the reaction of glucose to G-6-P
- glucokinase (in liver)
- hexokinase (in other tissues)
how are glucokinase and hexokinase able to distribute glucose equally to liver and other tissues
hexokinase (other tissues)
- low Km = high affinity, so even at low conc. able to get glucose
- low Vmax so tissues are easily satisfied and don’t need much glucose
glucokinase (liver)
- high Km = low affinity
- but high Vmax so able to grab glucose quickly, so most glc in liver
what are the steps in the synthesis of glucose
- glycogenin covalently binds to glucose (from UDP-glucose)
- glycogen synthase extends glucose chains
- chains formed by glycogen synthase broken by glycogen branching enzyme and re-attached via (a1-6) bonds to give branch points
what is the degradation of glycogen
- glucose monomers removed one at a time from reducing ends as G-1-P via glycogen phosphatase
- de-branching enzyme removes set of 3 glucose residues and attaches them to nearest non-reducing end via (a1-4) bonds
- final glucose then removed by breaking (a1-6) linkage to release free glucose
this leaves unbranched chain
what is the fate of G-6-P
- goes to liver, turns into glucose and then moves into blood
- goes to skeletal muscle, goes through glycolysis and lactate and ATP produced
what is the order of digestion organs in digestion of carbs
mouth > stomach > duodenum > jejunum
what molecule does glycolysis start with
glucose
what is the end result of glycolysis
pyruvate + 2ATP
what are the first three steps of glycolysis
- phosphorylation of glucose to glucose 6-phostphate (IRREVERSIBLE), via hexokinase, uses 1ATP
- G-6-P > F-6-P, via phospho-hexo-isomerase
- phosphorylation of F-6-P > F1, 6-biP (IRREVERSIBLE), Bia phospho-fructo-kinase-1, 1ATP used
what is the result of the first three steps of glycolysis
F-1, 6 biP
what are the 4th and 5th steps of glycoslysis
- F-1, 6-biP turns into G-3-P via aldolase
- F-1, 6-biP may turns into DHAP but that then turns into G-3-P, via triose iso-phosphate isomerase
so we know have 2x G3P to start second half of glycolysis
what are the 6th, 7th, 8th and 9th steps of glycolysis
- oxidation of G-3-P > 1, 3 bisPG, via glyceraldehyde 3 phosphate dehydrogenase, 2x NADH produced
- 1, 3 bisPG + ADP > 3PG + ATP, 2x ATP produced, via phosphoglycerate kinase
- 3PG > 2PG, via phosphoglycerate mutase
- dehydration of 2PG > PEP, via enolase
what is the 10th step of glycolysis
- transfer of P from PEP > ADP,
2x ATP produced
pyruvate produced, via pyruvate kinase
what is the fate of pyruvate
- fermentation to ethanol and CO2 in yeast
- in aerobic conditions citric acid cycle then terminal respiration
- in anaerobic conditions we get lactate in muscle (NAD+ also produced which is needed for glycolysis)
what is the enzyme which catalyses the reaction of pyruvate to lactate
pyruvate dehydrogenase
- NAD+ regenerated which is need for glycolysis
what is the cori cycle
- (in liver) lactate>pyruvate>glucose (then move to blood) (then move to muscle) glucose>pyruvate>lactate (then move to blood then back to liver)
- so gluconeogenesis in liver, glucose moves through blood to muscle, then glycolysis and fermentation in muscle
- this repays oxygen debt run up by muscles in low O2
what are the steps in glycolysis which are irreversible
1, 3, 10
- G-6-P cannot directly be converted back to glucose
- F-1, 6biP cannot be directly converted back into F-6-P
- pyruvate cannot be directly converted back into PEP
what is gluconeogenesis
- reverse of glycolysis (pyruvate back to glucose)
what are the bypass reactions A and B which turn pyruvate back into PEP in gluconeogenesis
- pyruvate moves into mitochondria
- pyruvate converts into oxaloacetate
- oxaloacetate converts into malate
- malate moves out of mitochondria
- malate turns back into oxaloacetate
- oxaloacetate turns into PEP
what is the bypass reaction C which converts F-1, 6biP back into F-6-P
F-1, 6-biP + water > F-6-P + Pi
- via fructose 1, 6-biphosphatase
what is reaction D that converts G-6-P back into glucose
G-6-P + water > glucose
- dephosphorylation via glucose 6 phosphatase
what is the enzyme which converts pyruvate into acetyl coA
- pyruvate dehydrogenase complex (E1, E2, E3) (respiratory chain)
- pyruvate oxidised in mitochondria
at what points can galactose enter glycolysis
- enter at G-6-P
at what point can fructose enter glycolysis
- at F-6-P (adipose tissue)
- DHAP (liver)
- G3P (liver)
