Biochemsitry - Metabolism in Health and Disease part 2 Flashcards
What is the starting point of cholesterol synthesis?
What molecules have a positive regulatory effect on cholesterol synthesis?
What molecules have an inhibitory effect?
Acetyl CoA
Insulin - up regulates HMG-coA reductase
Glucagon - down regulates HMG-coA reductase
What is the substrate for the only control point in cholesterol synthesis?
What is this substrate converted to?
What enzyme catalyses this reaction?
Beta-Hydroxy-beta-methyl-glutaryl-coA
Mevalonate
HMG-coA-reductase
What other processes does cholestrol regulate?
What type of feedback process are these?
Stimulates the proteolysis of the controlling enzyme HMG-coA reductase
Decreases uptake of LDL particles by endocytosis
Negative
What happens to cholesterol to make it a cholestrol ester?
What effect does this have on cholestrol?
Fatty acid is esterfied at position 3
Makes cholestrol more hydrophobic
Why is de novo fatty acid synthesis important?
Conversion and storage of excess glucose - inefficient and abnormal fatty acid storage causes insulin resistance and diabetes
What is the initial substrate of de novo fatty acid synthesis?
What is the starting point for fatty acid synthesis?
Where does fatty acid synthesis occur?
Pyruvate
Acetyl coA
The cytosol
What are three ways in which pyruvate can be produced?
Which is the main pathway?
Glycolysis, alanine synthesis and lactic acid synthesis
Glycolysis
What is the control step in fatty acid synthesis?
What compounds have positive and inhibitory effects on the enzyme that controls this step?
Carboxylation of acetyl coA to malonyl coA, catalysed by acetyl coA carboxylase
Citrate - positive regulation
Long-chain fatty acyl coA
What effects do insulin, glucagon and adrenaline have on fatty acid synthesis? Why?
Insulin - feeding hormone wants to get rid of glucose so enhances fatty acid synthesis
Glucagon - fasting response, under fasting conditions metabolise to produce energy, fatty acid synthesis requires lots of energy so is inhibited
Adrenaline - fight/ flight response, need to generate ATP to help with this response - break down fatty acids not synthesise them so FA synthesis inhibited
Is acetyl coA carboxylase active or inactive when phosphorylated?
Active when not phosphorylated
What are the two essential fatty acids?
What type of double bond do they have?
Linolenic acid - three double bonds first is omega 3 double bond
Linoleic acid - two double bonds, first is am omega 6 double bond
Where does LDL deliver cholesterol?
What does HDL do with cholestrol?
Extrahepatic - non-liver tissues
Takes cholestrol from the cells
What are the two ways a cell can obtain cholestrol?
It can synthesise cholesterol
Or obtain cholesterol via LDL
Describe the process by which a cell takes up LDL?
Uses special LDL receptors
LDL particles bind to LDL receptor, several endosomes aggregate (endocytosis)
LDL particles are internalised and form endosome vesicles
In endosome LDL particle separates from receptor, receptor recycles back to plasma membrane
Part of the endosome containing LDL particle fuses with an intracellular lysosome
Lysosome hyrdrolyses lipids and proteins releasing cholesterol, fatty acids and amino acids to be used as needed
What is the lipoprotein(s) in the exogenous lipoprotein pathway?
What is the lipoprotein(s) in the endogenous lipoprotein pathway?
Chylomicrons
VLDL/ IDL, LDL and HDL
Why do acetyl coA and oxyaloacetate combine to form citrate at the start of fatty acid synthesis?
Acetyl coA is the starting point of fatty acid synthesis but there is no mitochondrial membrane transporter for acetyl coA but there is one for citrate.
How to insulin, glucagon and adrenaline affect the activity of acetyl coA carboxylase (ACC)?
High glucagon and adrenaline bind to receptors and increase levels of cyclic AMP cAMP activates a class of protein kinases which phosphorylate ACC and deactivate it.
Under feeding conditions insulin binds to receptors and through signal transduction mechanisms activates phosphatases which remove phosphates activating ACC and allowing acetyl coA to be converted to malonyl coA
What is the name of the enzyme that breaks fatty acids into their key components?
Hormone sensitive lipase
Describe these genetic disorders:
- Refsum disease
- X-linked adrenoleukodystrophy
- MACD deficiency
- Zellweger syndrome
Inability to breakdown phytanic acid, caused by lack alpha-hydroxylase enxyme, causes neurological damage
Defect in peroisomal activation of VLCFA, causes progressive brain damage
Most common deficiency in fatty acid oxidation, can oxidise FA up to about 14C long, causes severe hypoglycaemia
Peroxisome synthesis defect, causes enlarged liver
What are flippases, floppases and scramblases?
Flippases - used to flip PL from outer to inner bilayer
From inner to outer bilayer
Bidirectional, flip PL both ways, flip two at the same time
What is the only phospholipid with four fatty acids?
Cardiolipin
What do these terms mean:
- endocrine
- paracrine
- autocrine
Hormones released from gland into bloodstream, travels through circulation
Adjacent cells, on cell secretory the other has specific receptors for secretory molecules
Same cell acts as secretory and target cell
What type of molecule can pass through the phospholipid bilayer, hydrophobic or hydrophilic?
Hydrophobic
What are the three amino acids that are targets for post translational modifications?
Serine, threonine and tyrosine
What are ways of terminating the signal transduction pathway?
Removal of extracellular signal
Receptor sequestration
Receptor down regulation
Inactivation of receptor protein
Inactivation of signalling protein
Production of a inhibitory protein
What are the three components to a G protein coupled receptor?
Receptor, trimeric G-protein, effector protein
What happens when a hormone binds to a G protein coupled receptor?
Hormone binding induces conformational change in receptor
Activated receptor binds to G-alpha subunit
Receptor causes conformation change in G-alpha triggering dissociation of GDP
Binding of GTP to G-alpha triggers dissociation of G-alpha from receptor and G-beta-gamma
Hormone dissociates from receptor, G-alpha binds to effector activating it
Hydrolysis of GTP to GDP causes G-alpha to dissociate from effector and re associate with G-beta-gamma
What is the function of adenylate cyclase?
To hydrolyse ATP in a cell into cAMP
Which lipoproteins are rich in triacylglycerols?
VLDL and chylomicrons
What are membrane rafts contain high levels of?
Sphingolipids and cholesterol
What are the substrates for gluconeogenesis?
Glycerol
Acetyl coA
Lactate
Amino acids
Which lipoprotein enters the cells by endocytosis?
LDL
What are the characteristics of the enzyme ATPase?
It regenerates ATP from ADP and Pi or it hydrolyses ATP to generate energy for the active transport of protons (H+) across membranes
What do statins do?
Inhibit HMG coA reductase
What is the main source of carbons for fatty acid synthesis?
Malonyl coA
Why is de novo fatty acid synthesis important?
The reduction of blood glucose levels
In fatty acid synthesis reducing equivalents are provided by?
NADPH
Gluconeogenesis starts how long after a meal?
4 hours
In the process of fatty acid beta-oxidation the enzyme carnitine pamlitoyl transferase 1 plays an important role, what is it?
The conversion of fatty acetyl coA to fatty acetyl carnitine
What is the point of having pyruvate kinase as the final control point in gluconeogenesis?
Prevents a futile metabolic cycle that converts pyruvate > oxaloacetate > phosphenolpyruvate (PEP) > pyruvate
What is the co factor required for fatty acid synthesis?
NADPH
What are protein kinases, what do they do?
What are protein phosphatases?
A class of enzymes that use ATP to phosphorylate proteins within the cell by transferring a phosphate from ATP to a serine, threonine or tyrosine hydroxyl group in a protein
A group of enzymes that catalyse the removal of the phosphate by hydrolysis
Give examples of serine/ threonine kinases?
Give examples of tyrosine kinases?
PKA, PCK, MAPK, CaMK2
Receptor tyrosine kinase which targets PI3
Give examples of serine/ threonine protein phosphates.
Give examples of tyrosine phosphatases.
Ca2+ independent PP1 and PP2A
Protein tyrosine phosphatase (PTP)
What are the three main families of Ga proteins?
G-alpha-s: activates adenylyl cyclase
G-alpha-i: inhibits adenylyl cyclase
G-alpha-q: phospholipase C-beta
What factors modulate cAMP levels in the cells?
Activation of G-alpha-s and G-alpha-i (which is regulated by the ligand and GPCR activated pathways
Phosphodiesterase enzymes break down cAMP into inactive (5’-AMP)
Forskolin activates AC directly independent of GPCR pathway
What are the downstream targets of cAMP?
EPAC (exchange protein activated by cAMP)
Cyclic nucleotide-gated ion channels (involved in olfactory and visual signals)
cAMP dependent protein kinase aka protein kinase A (PKA)
What are some of the physiological processes regulated by cAMP?
Metabolism including glycogenolysis and lipolysis
Cardiac and smooth muscle contraction
Secretory processes
Ion channel conductance
Memory and cognitive functions
Cell growth differentiation and apoptosis
Inflammatory and immunological responses
What does EPAC do?
Mediates integrin-mediated cellular adhesion
E.g. Epac moderates vascular endothelial cell barrier - initiates a rigorous program of protective anti-inflammatory responses in vascular endothelial cells
What does PKA do in the cell?
Role in glycogenolysis - adrenaline mediates the breakdown of glycogen, cascade of events starts with hormone binding leading to activation of AC and then PKA which:
- Increases production of glucose-1-p from glycogen by activating phosphorylase kinase which activates glycogen phosphorylase
- Inhibits glycogenesis by directly phosphorylating and inhibiting glycogen synthase
- Net increase in glucose-1-P levels
Cell function
- in cardiac muscle activates contraction
- in smooth muscle activates relaxation
Activates CREB - control of gene expression (transcription)
- PKA translocates from cytoplasm to nucleus (where CREB resides), phosphorylate and activates CREB
- CREB is a transcription factor that regulates cellular responses
How many classes of PLC enzymes exist?
What are they all dependent on?
PLC-beta: activated by GPCR via G-alpha-q subunits
PLC-gamma: activated via tyrosine kinase receptors
Ca2+
How does the GPCR/PLC pathway work?
PLC hydrolyses phosphatidyl inositol 4, 5- bisphosphate (PIP2) and generates two second messengers diacylglycerol (DAG) and inositol 1, 4, 5-triphosphate IP3
DAG activates protein kinase C (PKC) which phosphorylates a target protein causing cellular effects
IP3 causes opening of IP3 gated Ca2+ channels and Ca2+ release from the ER/SR
The increase in intracellular Ca2+ concentration activates protein kinase C
What is the resting level of calcium in the cell?
What is the resting level of calcium outside the cell?
How much calcium is stored in the cell?
100nm
1.3 - 1.8mM
1mM in the ER/SR
What are the two pathways that increase intracellular Ca2+ increase?
How does calcium exit the cell?
Calcium channels - entry of small amount of calcium into cell leads to huge release if calcium (caused by calcium binding to RyR receptors
Initiation of release of IP3, released by phospholipase C - IP3 binds to calcium gated channels causing calcium release
Pumped out by calcium sodium exchange
When are store operated calcium channels (SOC/ CRAC) opened?
What is Orai1?
What happens when ER Ca2+ stores are depleted?
When ER/SR content is depleted
A protein calcium sensor that communicates with the store operated channel
STIM1 binds to Orai1 leading to channel opening, binding only occurs when calcium in store not bound to STIM1
What activates calmodulin?
What does calmodulin do?
What are the targets of calmodulin?
Calcium
Has roles in many Ca2+ regulated processes! relays to protein kinases and protein phosphatases
CaM dependent protein kinase (CaMK2) and CaM dependent protein phosphatase (calcineurin) (CaN)
What does CaM do?
What does CaN do?
Acts as a memory device
Is a major target for immunosuppressants, one of its main targets is the NFAT
What are the six families of enzyme linked receptors?
Receptors tyrosine kinases
Tyrosine-kinase associated receptors
Receptor serine/ threonine kinases
Receptor guanylyl cyclases
Death receptors
What are the three essential components of receptor tyrosine kinases?
Extracellular domain containing ligand-binding site
Single hydrophilic transmembrane alpha helix
Cytosolic domain that include a region with PTK
What are the three factors that interact with receptor tyrosine kinases?
Phospholipase C gamma
Phosphotidylinositol (PI-3-K)
GTPase activating protein - activates MAPK
What does PLC gamma do?
Cleaves PIP2 into DAG and IP3
DAG activates PKC
IP3 leads to activation of intracellular Ca2+
Describe the receptor tyrosine kinase PI3 kinase signalling pathway?
- Growth factor binds to RTK and activates it
- Activated PI3 kinase phosphorylates PIP2 into PIP3
- PDK1 and AKt (PKB) dock to PIP3 in the membrane
- PDK1 phosphorylates AKt and activates it
- Activated AKt dissociates from PIP3 in the membrane and initiates several cell responses
What is the ligand for receptor linker guanylyl cyclase?
What is the ligand for soluble guanylyl cyclase?
Natriuretic peptide
Nitric oxide gas
What are natriuretic peptides?
What are the types?
Where are they expressed?
A family of peptide hormones
ANP and BNP
ANP is mainly expressed in the atria of the heart
BNP is synthesised in the the ventricular myocardium
What are some of the functions of soluble guanylyl cyclase?
Relaxation of vascular
Regulation of synaptic signal transmission
Inhibition of platelet aggregation
Immunonmodulatory effects
What are the common structural features of nuclear receptors?
Unique N-terminal region that acts as a transcription activation domain
Central DNA-binding domain (DBD)
C-terminal hormone (ligand) binding domain
