carbohydrate metabolism Flashcards
what does the pancreas do?
secretes hormone insulin into blood in response to an increase in blood glucose
why do we need to maintan blood glucose levels?
as glucose is a primary fuel source of brain cells via glycolysis and neurotransmitter synthesis
how much glucose is consumed in a normal diet per day?
300g
what happens when glucose enters the blood?
some taken up by liver via facilitated transport (uptake) stores 80-100g
insulin activates hexokinase/glucokinase to store glucose as glycogen (glycogen synthesis)
liver hexokinase reaction reversible so glucose can leave liver cell to maintain blood glucose (glycogenolysis and gluconeogenesis)
describe most remaining glucose being taken up by skeletal muscle?
glucose enters muscle via facilitated transport stores up to 400-700g
hexokinase (glucokinase in liver) phosphorylates glucose using ATP to maintain gradient
glucose-6-phosphate is formed and can’t leave cell so enters glycolysis or glycogen synthesis
what is glucose transported via in skeletal muscle intake?
GLUT4 (maintained hours after exercise)
which is translocated to plasma membrane from intracellular vesicles in response to insulin, muscle contraction and Ca2+ (main)
what occurs in glycogen synthesis?
4 steps
glucose-6-phosphate converted to glucose-1-phosphate (G1P) by phosphoglucomutase
uridine diphosphate (UDP) glucose is synthesised from G1P and uridine triphosphate
UDP glucose binds to pre-existing glycogen molecule or glycogenin &UDP released
glycogen synthase and branching enzymes form proglycogen which grows into macroglycogen
what is glycogenin?
enzyme involved in converting glucose to glycogen
situated at core of glycogen molecule
what stimulates glycogen synthesis?
insulin
what is the purpose of glycogen synthesis?
glucose molecules added to chain of glycogen for storage
=glycogenesis
what reduces glycogen synthesis?
muscle contraction/exercise
what is glycogenolysis and what inhibits it?
glycogen breakdown into glucose for utilisation (opposite of glycogenesis)
insulin
what stimulates glycogenolysis ?
(glycogen breakdown)
muscle contraction/exercise
which process is preferential during exercise?
glycolysis (breaking down glucose into usable ATP in liver and skeletal muscle)
what are the main signals stimulating glycogenolysis?
(breaking down of glycogen)
calcium and adrenaline (high rates during high contraction in exercise)
what reduces with exercise?
insulin
what is G6P derived from?
glycogenolysis and glucose uptake
where ensures blood glucose is maintained during postabsorptive period and during fasting?
liver as acts as a reserve
what has to happen when glycogen reserve is exhausted in liver?
glucose must be made from non-carbohydrate precursors (lactate, amino acids and glycerol) via gluconeogenesis
what increases during fasting?
circulating fatty acuds and glycerol
why can’t fatty acids be used as pre-cursors?
pyruvate dehydrogenase and pyruvate kinase reactions are irreversible (acetyl-CoA from fatty acids can’t form glucose)
what is gluconeogenesis?
conversion of pyruvate (made from non-carbohydrate precursors) to glucose
what important role does Acetyl-CoA play?
inhibiting pyruvate dehydrogenase and diverting pyruvate to oxaloacetate and malate
when is lactate produced at high rates?
during exercise in muscle and in red blood cells
what is pyruvate kinae inhibited by?
alanine (amino acid)
what can the liver dephosphorylate?
glucose-6-P
where is glycerol released from?
adipose tissue
what rate is the liver and gut releasing glucose at rest?
liver - 0.1g/min
gut - 1.5g/
what rate is glucose taken up from the blood in the brain?
0.1g/min (same rate as released from liver)
what is the relationship between exercise intensity and rate of glucose uptake?
the heavier the intensity, the higher the rate of glucose uptake by skeletal muscle e.g in leg (1g/min so 10x increase than normal)
what happens during heavy exercise to blood glucose release and uptake levels?
how is this resolved?
blood glucose levels will fall as release rate slower than uptake rate (needs 2g/min)
so rate of glycogenolysis increases as well as glucose output from liver and stores in skeletal muscles used (as adrenaline inhibits insulin production which normally inhibits glucose output)
why does higher exercise intensity lead to increased glycogen broken down?
higher rate of exercise, high turnover of ATP = high concentration of AMP = activates glycogen phosporylase = greater rate of glycogen broken down
when does carbohydrate oxidation decline and why?
how to solve this?
carbohydrate oxidation declines after prolonged period of exercise due to depleted glycogen sources
ingest carbohydrates to increase/maintain blood glucose levels and therefore carbohydrate oxidation
how can you delay muscle glycogen use during exercise and therefore prolong exercise and maintain glucose levels?
ingest carbohydrate electrolyte solution (any carb)
what is the maximum carbohydrate oxidation rate faciliatated through 1 type and several types of CHO ingestion?
can’t oxidise more than 1-1.2g/min
unless you ingest different types of CHO together
describe glycogen loading?
depleting muscle glycogen stores then
glycogen loading straight after, then lack of CHO, then loading a few days before competition can increase overall glycogen levels
due to increase in insulin sensitivity in muscles
when is glycogen resynthesis highest?
few hours post exercise
what is glycogenesis?
conversion of glucose to glycogen to be stored in the liver
= glycogen synthesis
what effect would an increase in glycogenolysis have on glucose uptake and why?
uptake would decrease as large amount of glucose-6-phsophate present from glycogenolysis (breakdown of glycogen to glucose)
increased levels inhibit hexokinase so less glucose uptake in cell
what are the effects of the drug antimycin A?
inhibitor of cytochrome C
causes ATP synthesis and oxidative phosphorylation to be inhibited
