Carbohydrate Metabolism Flashcards
where does glycogen synthesis primarily occur?
in the liver and muscle
what happens if [blood glucose] drops?
- hypoglyceamia
- muscle weakness, loss of coordination, sweating, hypoglycaemic coma, death
what happens if [blood glucose] too high?
- hyperglycaemia
- non enzymatic modification of proteins, hyperosmolar coma
what happens when there is excess glucose in the body?
- glycogenesis
- pentose phosphate pathway
- fatty acid synthesis
what happens if there is a lack of glucose in the body?
- gluconeogenesis
- glycogenlysis
what happens to glucose in the liver?
- glycokinase phosphorylates glucose to G6P
- could be converted back to glycogen
- can be converted to ribose-5-phosphate
- can be converted to pyruvate
what are the steps in glycogenesis
- G6P is converted to G1P by phosphoglucomutase
- G1P is converted to UDP-Glucose by UDP-glucose-pyrophophorylase using UTP
- UDP-glucose is combined with glycogenic to initiate glycogenesis as it acts as a primer needed by glycogen synthase (GS) to attach to glucose molecules
- GS facilitates the addition of additional glucose monomers via 1,4-glycosidic bonds
how are the branches in glycogen formed?
for every 11 glucose monomers added, some are transferred by branching enzymes via 1,6-glycosidic bonds
why is glycogen used as a store and not glucose?
- glucose is osmotically active
- glycogen is more compact
- the branching structure allows for glycogen to rapidly be mobilised
what is glycogenolysis?
the breakdown of glycogen the [BG] is low
produces G1P nad glucose
what are the enzymes required to break down glycogen?
- phosphorylase: breaks α 1-4 bonds
- translocase: transports G6P to ER for further modification
- debranching enzyme: debranches glycogen by acting on the 1,6-glycosidic bonds
- phosphoglucomutase: converts G1P to G6P
what are the enzymes required to form glucose?
- phosphorylase
- transferase
- debranching enzyme
- phosphoglucomutase
- glucose-6-phophatase: converts G6P to glucose in liver and kidney (NOT muscle)
how is glycogen broken down?
step 1: break α 1,4 bonds of glycogen using phosphorylase to form G1P until there are only 4 residues attached from the α 1,6 bond
step 2: removal of α 1,6 bonds using the debranching enzyme. transferase moves 3 residues to the end of the non-reducing chain
step 3: phosphoglucomutase converts G1P to G6P
step 4: G6P is converted to glucose by glucose-6-phosphatase
how does the enzyme glycogen phosphorylase work?
- forms G1P
- it is a large multi subunit enzyme
- it is regulated by allosteric interactions that signal the cell’s energy state
- also regulated by reversible phosphorylation
- there are 2 forms, active (A) and inactive (B) and is activated by phosphorylase β kinase
how is phosphorylase β kinase activated?
- when [BG] high insulin is released into the bloodstream
- insulin activates cAMP, a secondary messenger
- cAMP activates kinase which is able to activate phosphorylase
how is glycogen breakdown inhibited in liver?
-the presence of glucose inhibits glycogenolysis by phosphorylase
how is phosphorylase B activated in muscle?
by 5’ AMP and NOT phosphorylation
-G6P blocks 5’ AMP activation
how is phosphorylase kinase regulated?
- phosphorylase Kinase is under dual regulation via two different receptor types.
- the most important is through the elevation of cAMP and the activation of PKA.
- the other is calcium-mediated through the alpha adrenergic/IP3 pathway.
how is glycogenesis and glycogenolysis reciprocally regulated?
- by glycogen synthase and glycogen phosphorylase
- glycogen synthase is activated when [glucose] high
- glycogen phosphorylase activated when [glucose] low
what is the significance of the pentose-phosphate pathway?
- metabolic pathway parallel to glycolysis
- produces ribose-5-phosphate sugar used in making RNA and DNA and NADPH
- no ATP is used or formed
what is the significance go gluconeogenesis?
- the body’s preferred fuel for the brain is glucose
- the body uses more glucose than stored in reserves so constantly needing to make more
- synthesis from non-carbohydrate source
wha are the steps in gluconeogenesis?
- pyruvate carboxylase converts pyruvate to oxaloacetic acid
- oxaloacetic acid is converted phosphoenol pyruvate by phosphoenol pyruvate carboxykinase
- phosphoenol pyruvate is converted to C3 molecule GAP
- GAP converted to fructose-1,6-bisphosphate
- fructose-1,6-bisphosphate converted to G6P by fructose bisphosphatase
- glucose-6-phosphatase converts G6P to glucose
what are gluconeogenic aa?
- aa that can be converted back to glucose
- can be fed in at diff parts of the process
how does pyruvate get converted to oxaloacetate in the liver?
- pyruvate brought into the mito (of liver) by pyruvate carrier
- pyruvate converted to oxaloacetate by pyruvate carboxylase
- in order to leave the liver oxaloacetate is converted to malate
- malate is brought out of the mito where it is converted back to oxaloacetate
- then the process of continues
what are the most important substrates in glyconeogenesis?
-alanine, lactate, glycerol
what does glyconeogenesis do and where does it take place?
- covnerts pyruvate to glucose
- takes place mainly in the liver and little in kidney but hone starvation, kidney gluconeogenesis increases by 40%
what is the fate of G6P in the liver and muscle?
-depends on the tissue
- for muscle: can be used for ATP synthesis for its own use (muscle use)
- muscle cannot control [bg] as it does not contain the enzymes
-in liver: has enzymes to convert G6P to glucose so controls [bg]
