Biochem Flashcards
primary monosaccharides (3)
- glucose
- fructose
- galactose
how are primary monosaccharides/sugar monomers (glucose, fructose and galactose) absorbed across the brush border of enterocyte
via carrier mediated mechanisms (which demonstrate stereo specificity, saturation kinetics and can be specifically inhibited)
sodium dependent glucose transporter (2)
- SGLT1 (sodium-glucose linked transporter) also transports galactose, into the epithelial cell from the intestinal lumen
- GLUT2 uniporter transports monosaccharides into the circulation/blood (facilitated)
is SGLT1 an e.g. of primary or secondary active transport
secondary
why is SGLT1 describes as a symport
because membrane bound protein binds glucose and sodium at different sites
what drives the SGLT1
-conc gradient of sodium created by the sodium/potassium ATPase, which moves sodium out of the cell in exchange for potassium, using ATP for energy
role of sodium co-transporters
-transport sodium together with another molecule, eg. SGLT1 present on the enterocyte luminal membrane (enterocyte = cell of intestinal lining)
how is the sodium gradient (required for glucose symport by SGLT1) maintained
- by the sodium/potassium ATPase, which keeps the intracellular sodium concentration low (sodium enters blood/exits enterocyte, and potassium enters enterocyte)
- the sodium conc gradient set up by the ATPase pump is the driving force for sodium transport into the cell from the lumen via SGLT1, which also brings in glucose (or galactose)
clinical signif of the SGLT1 (2)
- dehydrated patients found to absorb sodium much better when glucose was also provided
- WHO oral solution saved lives of millions of children with severe diarrhoea
what mediates facilitative sugar transport
members of the GLU transporter family
role of GLUT 2 uniporter
- transports monosaccharides into the circulation across the basolateral enterocyte membrane
- eg. of facilitated diffusion
how do members of the GLUT transporter family allow the movement of sugars across the membrane
via formation of an aqueous pore across the membrane
how frequent are the GLUT family of glycosylated transmembrane proteins predicted to span the membrane
12 times with both amino- and carboxyl- termini located in the cytosol
subclasses of GLUT transporter family
-on basis of sequence homology and structural similarity, three subclasses of sugar transporters have been identified
definition of glycogenesis
synthesis of glycogen from glucose
definition of glycogenolysis
breakdown of glycogen to form glucose
definition of gluconeogenesis
synthesis of glucose within the body from non-carbohydrate precursors such as amino acids, lactic acid and glycerol
what is glycogen
= main storage form of glucose in liver and muscle cells
->polymer and storage form of glucose
when is liver glycogen broken down and released
-broken down between meals and released to maintain blood glucose levels for red blood cells and brain
role of muscle glycogen
- not available for maintenance of blood glucose levels
- provides energy via glycolysis and the TCA cycle during bursts of physical activity
when does blood glucose from diet peak
after meals
what determines the time and frequency of glycogenolysis
-fluctuates dependent upon meal times and snacking
when is gluconeogenesis at its peak
-overnight, where it is the primary source of glucose as hepatic glycogen is used up and decreases
structure of glycogen
- polymer consisting of glucose molecules joined by alpha 1-4 glycosidic links
- branches are introduced by alpha 1-6 glycosidic links
glycogenesis (step 1)
- step 1 = glucose -> glucose-6-phosphate (using hexokinase) -> can either go onto glycolysis (the breakdown of glucose by enzymes, releasing energy and pyruvic acid) or forms glucose-1-phosphate (using phosphoglucomutase) -> glycogen synthesis
glycogen synthesis requirements
-requires a glycogen ‘PRIMER’ containing at least 4 glucose residues
what is the glycogen primer (required for glycogen synthesis) made up by/ covalently attached to
an enzyme called glycogenin, which uses UDP- glucose aka ‘activated glucose’
formation of UDP-glucose (step 2 of glycogenesis)
- UTP + Glucose-1-phosphate -> UDP-Glucose + pyrophosphate (PPi)
- reversible reaction
- however highly active pyrophosphatase in cells hydrolyses the PPi and drives reaction in favour of UDP-glucose synthesis (PPi + H20 -> 2 Pi)
what enzyme catalyses the formation of UDP-glucose
UDP-glucose pyrophosphorylase
step 3 of glycogenesis
- glycogen is synthesised from UDP-glucose
- > catalysed by glycogen synthase
what is the rate limiting enzyme of glycogenesis
= glycogen synthase (catalyses synthesis of glycogen from UDP-glucose)
role of glycogen synthase (step 3 glycogenesis)
- adds ONE glucose molecule to glycogen at a time forming alpha 1-4 glycosidic bonds
- it can only EXTEND chains of branches (cannot introduce branches or start new molecules as this requires a primer made by glycogenin)
what is the branching enzyme during glycogenesis (step 3)
= transglycosylase, which introduces alpha 1-6 glycosidic branches into glycogen (branches occur approximately every 8-10 glucose residues)
how often do alpha 1-6 glycosidic branches occur in glycogen
every 8-10 glucose residues
hormonal control of glycogenesis
-control is exerted on glycogen synthase activity (synthesises glycogen from UDP-glucose)
hormonal response to hyperglycaemia stimulus (include source and effect of hormone)
(hyperglycaemia = excess of glucose)
- hormone = insulin
- source = pancreatic beta cells
- effect = activation of glycogen synthase (which synthesises glycogen from UDP-glucose)
hormonal response to hypoglycaemia stimulus (include source and effect of hormone)
(hypoglycaemia = deficiency of glucose in bloodstream)
- hormone = glucagon
- source = pancreatic alpha cells
- effect = inactivation of glycogen synthase (which synthesises glycogen from UDP-glucose)
structure of cholesterol (5)
- almost entirely carbon and hydrogen (27 carbon atoms)
- one hydroxyl group which is often esterified to a wide range of fatty acids
- almost completely saturated (only one double bond)
- ring structure is almost planar therefore lies flat
role of cholesterol in mammalian cell membranes
regulator of membrane fluidity
3 important classes of biologically active compounds that cholesterol is a precursor of (3)
- bile acids
- steroid hormones
- vitamin D
importance of cholesterol metabolism
cholesterol =
- > aetiology/cause of cardiovascular disease
- > major component of gall stones
amount of cholesterol in typical western diet
- approx 500mg daily
- meat, eggs, dairy products
how much cholesterol don humans synthesise per day
- approx 1g
- depends on dietary intake
solubility of cholesterol
low solubility in water (cholesterol esters are less soluble than the 30% of circulating cholesterol that is in free form)
how much of circulating cholesterol is in the free form
-only 30% (the majority is esterified through the cholesterol hydroxyl (-OH) group and are called cholesterol esters (CE) and are less soluble in water
where are cholesterol esters stored
in lipid droplets in the ER
transport/storage of cholesterol
cholesterol is incorporated into lipoproteins where it is present as free cholesterol and cholesterol esters which are located in the core of the molecule
extracellular uptake of cholesterol when intracellular levels drop (5)
- lipoproteins constitute a pool of extracellular cholesterol
- LDL binds to a cell surface receptor which is internalised and digested by the lysosomes (e.g. of endocytosis)
- > free cholesterol is released and esterified
- > the receptor is recycled to the membrane
- free cholesterol is a negative feedback regulator of its own synthesis
what mediates regulation of cholesterol synthesis
-family of transcription factors (DNA- binding) called ‘sterol regulatory element-binding proteins’ (SREBS)
function of SREB’s (2)
- regulate the transcription of the genes encoding the enzymes involved in cholesterol synthesis (HMG CoA reductase and HMG CoA synthase)
- regulate a gene encoding a cell surface receptor (apoprotein receptor)
enzymes involved in cholesterol synthesis
- HMG CoA reductase
- HMG CoA synthase
effect of hepatic sterol depletion (3)
- increases SREBs, therefore:
- > increases cholesterol synthesis
- > increases expression of the cell surface apoprotein receptor (which can bind lipoprotein particles)
molecular weight of cholesterol
= 386Da
describe endocytosis and give e.g..
- receptor mediated
- specific and saturable
- movement into the cell
- eg. binding of LDL - delivering cholesterol to a cell expressing the receptor
describe exocytosis and give e.g.
- secretory
- movement out of the cell
- eg. when chylomicrons carrying TAGs, cholesterol etc. leave the enterocytes and enter the lacteals
role of lipoproteins
- transport vehicles for lipids in the lymph and blood
- transport fat soluble vitamins too e.g. vitamin A and vitamin E
structure of lipoprotein molecule (2)
- core of hydrophobic lipids (cholesterol esters and triglycerides)
- surrounded by a shell (made up of polar lipids aka phospholipids, apoproteins and free cholesterol)
role of apoproteins within shell of lipoprotein (2)
- structural and metabolic as they interact with cellular receptors
- also regulate activity of enzymes involved in lipid transport and distribution
how are lipoproteins distinguished (2)
- size
- density as each contain diff amounts of lipids and proteins (the more lipid, the lower density and the more protein, the higher density)
what does VLDL stand for
very low density lipoprotein
what does IDL stand for
intermediate density lipoprotein
what does LDL stand for
low density lipoprotein
what does HDL stand for
high density lipoprotein
major lipid for LDL
cholesterol
role of LDL
delivers cholesterol from the liver to cells for:
- > cell membranes
- > hormone production
LDL receptor
= membrane bound protein which binds LDL (by binding to its apoprotein), causing it to be taken up by the cell and dismantled
role of LDL apoprotein
binds to a specific LDL receptor
major lipid of HDL
phospholipid
what synthesises HDL (2)
- liver
- intestine
role of HDL (2)
- circulates in blood to collect EXCESS cholesterol from cells
- important for reverse cholesterol transport
lipid-protein ratio of HDL
lowest out of all lipoproteins
steps involved in reverse cholesterol transport (HDL role) (4)
- HDL salvages excess cholesterol from cells
- cholesterol is esterified with fatty acids
- it is then transported back to the liver
- and excreted as bile salts via biliary system or faeces
what organ is capable of metabolising and excreting cholesterol
ONLY the liver
what human cells can synthesise cholesterol (biosynthesis)
virtually all human cells
location of cholesterol biosynthesis within cells
-all reactions occur in the cytoplasm but some of the enzymes are attached to the ER membrane
site of cholesterol synthesis
- main site =liver
- lesser contributions = intestine, adrenal cortex, gonads
requirements for the synthesis of 1 molecule of cholesterol (3)
- source of C atoms (18 molecules of Acetyl-CoA)
- a source of reducing power (16 moles of NADPH)
- significant amounts of energy (36 moles of ATP)
what is mevalonic acid (mevalonate)
-3 molecules of acetyl-CoA are converted into the 6 carbon mevalonic acid
enzymes which catalyse the conversion of 3 acetyl-CoA molecules to the 6 carbon mevalonic acid (3)
- acetoacetyl-CoA thiolase
- HMG-CoA synthase
- HMG-CoA reductase
- > the transcription of the genes encoding the last 2 are regulated by SREBs
enzyme involved in catalysing the rate limiting step of cholesterol synthesis
HMG-CoA reductase
structural name of HMG-CoA
=3-hydroxy-3-methylglutaryl-CoA
rate limiting step of cholesterol synthesis
- formation of mevalonic acid (which is irreversible)
- catalysed by HMG-CoA reductase
location of HMGR (HMG-CoA reductase)
embedded within the ER
how is HMGR (HMG CoA-reductase) controlled (4)
- feedback inhibition
- rate of its degradation
- phosphorylation (it is active when it is NOT phosphorylated)
- gene expression (synthesis is stimulated by fasting and inhibiting dietary cholesterol, as hepatic sterols deplete)
effect of hormones on HMGR (4)
- insulin and T3 (tri-iodothyronine) increase its activity
- glucagon and cortisol inhibit activity
regulation of cholesterol synthesis (as a result of high intracellular free cholesterol) (4)
- reduction in expression and activity of HMG-CoA reductase (limits synthesis) using drugs (statins)
- decreased regulation of LDL receptors (limits uptake)
- increase in efflux
- increase in rate of conversion to bile salts
effect of dietary restrictions on reduction of free cholesterol
dietary restrictions can only achieve a 15% reduction in free cholesterol
function of statins (drugs)
- inhibit cholesterol synthesis
- lowered intracellular cholesterol leads to increase in LDL expression
- promotes removal of LDL from blood
rate limiting enzyme in cholesterol biosynthesis
HMG-CoA reductase
what drugs inhibit HMG-CoA reductase activity
statins
main metabolic product from cholesterol
bile salts
how do bile salts differ from each other
differ in the no. and position of their hydroxyl groups (-OH)
what synthesises and secretes bile salts
the liver
formation of bile salts (2)
- prior to secretion, cholic acid and chenodeoxycholic acid are conjugated through the carboxyl group (-COOH) to glycine or taurine forming the four primary bile acids
- in physiological conditions they are ionised to form bile salts
where are bile salts stored
gall bladder
function of bile salts once released into the duodenum
act as detergents for emulsifying ingested fats
products from cholesterol
- bile salts
- vitamin D
- steroid hormones
function of vitamin D
-play a role in the regulation of calcium and phosphors metabolism
how is vitamin D a product of cholesterol
- vitamin D3 is synthesised in the skin by UV radiation of 7-dehydrocholesterol
- the active form of vitamin D3 is called calcitriol or 1,25(OH)2D which acts as a steroid hormone
function of active vitamin D3 (calcitriol/ 1,25(OH)2D)
acts as a steroid hormone
what are steroid hormones derived from
cholesterol
3 groups of steroid hormones (3)
- corticosteroids (21 carbon atoms)
- androgens (19 carbon atoms)
- estrogens (18 carbon atoms)
location of steroid hormone synthesis (conversion of cholesterol into steroid hormones) (3)
- adrenal cortex (corticosteroids)
- testis (androgens)
- ovary (oestrogen’s)
- > all 3 organs are capable of secreting small amounts of steroids from other groups
mechanism of action of steroid hormones (4, long card)
- all act by binding to specific receptors (nuclear or cytoplasmic)
- adjacent to the hormone binding domain in the receptor is a highly conserved DNA-binding domain, characterised by two zinc fingers which fit into the major groove of the DNA
- binding of the steroid facilitates translocation of the activated receptor
- the complex can then bind to a specific steroid response element (DNA) in a gene promoter region controlling gene expression
other members of the superfamily of steroid hormone receptors (3)
include:
- receptors for T3 (triiodothyronine)
- active forms of vitamin A and D
genetic variation of steroid receptors (2)
may be associated with hormone resistance and diverse clinical presentations
regulation of gene transcription by glucocorticoids (corticosteroid) (5)
- steroid binds to receptor in cytoplasm
- receptor dimerises
- exposes a nuclear localisation signal (NLS), complex enters the nucleus
- binds to DNA at a specific response element (SRE, GRE=glucocorticoid response element) aka hormone response element (HRE)
- activate promoter and switch transcription on (or off)
precursor molecule of bile acids
cholesterol
precursor molecule of steroid hormones
cholesterol
precursor molecule of vitamin D
cholesterol
transformation of cholesterol into bile salts and steroid hormones involve what type of reactions and are catalysed by what
involves hydroxylation reactions catalysed by cytochrome P450 monooxygenase
how do steroid hormones bring about their biological effects
by binding to steroid-specific hormone receptors which can affect gene transcription (increase, or decrease)
