metabolism and insulin Flashcards
insulin: explain insulin synthesis, storage and secretion; list the physiological actions of insulin, explain the mechanism of action of insulin and homeostatic mechanisms of control
what cell in the islets of langerhans produce insulin
B-cells
how does glucose enter B-cells
via GLUT-2
what acts as the glucose sensor in b-cells
glucokinase (hexokinase IV)
what does glucokinase do
catalyses reaction of glucose to glucose-6-phosphate (rate limiting step)
structure of proinsulin
insulin (A-chain and B-chain) and C-peptide
structure of insulin
A-chain and B-chain
why is C-peptide important in diabetes diagnosis
as C-peptide is produced in equimolar amounts as insulin (cleaved off), it remains in blood so can measure levels of C-peptide to determine how well pancreas is functioning
how is insulin released from B-cell
glucokinase activated → glucose-6-phosphate and ATP formed → ATP closes voltage-gated K+ channel → voltage-gated Ca2+ channel opens → Ca2+ influx → vesicles containing insulin fuse with plasma membrane → release of insulin by exocytosis
structure of the insulin receptor
2 a-subunits protruding from membrane which contain receptors for insulin; 2 b-subunits within membrane and protruding into cell
what do the b-subunits in the insulin receptor contain
tyrosine kinase domains which cause phosphorylation of cell protein substrates
how does insulin decrease blood glucose
increases glycogenesis, glycolysis, glucose transport into cells (via GLUT4), amino acid transport and protein synthesis (metabolic and mitogenic effect); decreases lipolysis (therefore increases lipogenesis), proteolysis, gluconeogenesis and glycogenolysis
insulin effect on amino acids
oxidises amino acids; gluconeogenic amino acids are produced and go to liver in circulation, where used to make glucose in gluconeogenesis
what causes B-cells to function: increase insulin release
high blood glucose, glucagon, certain amino acids, certain gastrointestinal hormones, parasympathetic activity
what causes B-cells to function: decrease insulin release
sympathetic activity, somatostatin
what is first phase insulin release
stored insulin which is released immediately after eating
