Lecture 03 Cellular Homeostasis Flashcards
What is cellular homeostasis
- maintenance of metabolic equilibrium within cell/organism
- regulation of molecular/cellular pathways
- compensation for dynamic changes
- maintenance of cell functionality and stability
Metabolite homeostasis
- small molecules
- ex) glucose–> provides energy, uptake into cell PRN, specialized transporters facilitate movement across PM
General info about glucose transporters (GLUTs)
-multiple transmembrane proteins located in PM
-function= transport glucose and other simple sugars across PM
-transport down a concentration gradient using facilitated diffusion
(Transport depends on concentration gradient of sugar)
GLUT1
- all cells
- unregulated (driven by concentration gradient)
- Km=1mM
GLUT2
- liver and pancreas
- unregulated
- Km= 10mM
GLUT3
- neurons
- unregulated
- Km=1mM
GLUT4
- skeletal muscle, adipose and heart
- regulated by insulin!!!
- Km=5mM
Km
- affinity of enzyme for substrate
- substrate concentration at 1/2 max velocity
- Inversely proportional to affinity
- *Smaller Km= higher affinity
Aerobic vs Anaerobic glycolysis
- aerobic= glucose–>pyruvate–>TCA
- anaerobic= glucose–>lactate (no O2, no mito, ex=RBC)
Where is the glucose supply?
- diet (glucose, fructose, galactose)
- Glycogenolysis= breakdown of glycogen (liver/muscle)
- Glucgoneogenesis= De novo synthesis (liver) from non-carb sources
What is GLUT2 role in glucose sensing?
- location on pancreatic Beta cells
- allow bidirectional transport of glucose depending on concentration
- transport via facilitated diffusion
- GLUT2 Km=10–> high capacity but LOW affinity
- causes release of insulin
What is the mechanism of glucose sensing?
- GLUT2 mediates uptake of glucose
- Glucose metabolized to form ATP
- ATP closes ATP-dependent K+ channels
- Depolarizes PM
- Opening of voltage gated Ca2+ channels and influx Ca2+
- Relapse of insulin stored in vesicles (FAST)
- Ca2+ activates CaM kinase which causes sysnthesis of insulin and allows 2nd phase of insulin release (SLOW)
What is Fanconi-Bickel Syndrome
- cause= mutation in GLUT2 in liver, pancreatic Beta cells, enterocytes and PCT
- effect= stunted growth and enlarged liver (hepatomegaly)
- consequences= defect in uptake of simple sugars–> hyperglycemia after meal, hypoglycemia between meals, glucose not released from liver, hepatomegaly due to accumulation of glycogen in liver, defective insulin secretion
- treatment= eat small meals frequently, vitamin D supplementation
Regulation of GLUT4 by insulin
- GLUT4 transporter molecules present in microvesicles in cytoplasm (muscle/adipose)
- increase sugar in blood causes release of insulin
- insulin binds IR causing AUTOphosphorylation and activation of IR
- insulin signal cascade causes trafficking and fusion of vesicles with GLUT4 on PM
- increased GLUT4 facilitates glucose uptake into cells
Glucose tolerance test
- used to test for type 2 diabetes
- known amounts of glucose given to patient followed by 8-12 hour fasting period
- glucose levels measured before and again at defined intervals
- helps determine how quickly glucose levels fall and homeostasis is regained
- *normal fasting less than 6.1, diabetes greater than 10
What aspects of protein homeostasis are critical for cell function?
- synthesis
- folding
- post translational modification
- sorting/trafficking
- clearance/degradation
What regulates protein synthesis at transcriptional level?
- TF= trans-acting proteins that regulate transcription of genes
- have DNA binding domain
- either promote or repress the transcription of target genes which eventually regulates translation
What is involved in transcriptional regulation of PEPCK (phosphoenolpyruvate carboxykinase)
- cortisol and thyroxine are hormones that bind R and translocate to nucleus where they bind glucocorticoid response element (GRE) and thyroxine response element (TRE) on PEPCK gene
- transcription of PEPCK increases with increase in thyroxine/cortisol
- PEPCK= key enzyme in Gluconeogenesis in liver/kidney
Protein folding
- small proteins self fold into 3D structure
- large proteins need chaperones to help folding and protect against aggregation/proteolysis
Cystic fibrosis
- inherited disorder causes damage to lungs and digestive system
- thick/sticky mucus secretions
- defects in gene encoding for CFTR (CF transmembrane conductance regulator)
- result= defective CFTR protein which does not fold properly and is degraded
PTM (post translational modifications)
- occur at specific sites on certain a.a.
- crucial to function, regulation, subcellular localization, interaction with other molecules, degradation
What is acetylation
- attachment of Acetyl group
- ex) acetylation of histone loosen DNA to make available for transcription
What is Acylation? PTM
- attachment of acyl
- small G-protein affects attachment to subcellular membranes
What is glycation and glycosylation? PTM
-glycation- nonenzymatic attachment of glucose
-glycosylation- enzymatic attachment of sugars
Ex) high levels of glycated Hb in pts with poorly managed diabetes
What is phosphorylation? PTM
-attachment of phosphate group
-activate or inhibit function
(Added by kinases)
What is ubiquination?
- attachment of ubiquitin
- tags proteins for degradation by proteasome
IC protein degradation
- Lysosomes= acid hydrolases in vesicle
2. Proteasomes= trash can chew up ubiquinated proteins
EC protein degradation
- proteins degraded by EC proteolytic enzymes as needed
- often secreted as inactive precursors (ZYMOGEN)
What is the job of the mitochondria?
- energy production/balance
- mitochondrial matrix site of:
1. TCA
2. B oxidation of FA
3. Urea cycle
4. Storage of Ca2+
Mitochondrial Biogenesis
- process by which number of mito is increased in individual cells (more= healthier)
- aerobic exercise increases mito–> protective
- protein are encoded by mito and nuclear genome
Fusion
- damaged mito fuse with healthy mito to try and heal/salvage bad mito and cure defects
- helps mitigate stress by mixing of contents or partially damaged mito as a form of complementation
Fission
- damaged mito breaks into 2, one with healthy parts other with bad parts and one with damage gets degraded
- create new mitochondria/remove damage
- facilitate apoptosis
3 fates of mitochondria under stress
- Degrade misfolded or damaged proteins–>get healthy mito
- Segregation of damaged mito through fission–> get healthy mito
- Elimination of damaged mito by mitophagy–>lysosome
What is autophagy?
- basic catabolic mechanism involving degradation of dysfunctional organelles and other cellular components
- ensures recycling or cellular components
- surround organelles with DOUBLE membrane to make autophagosome which fuses with lysosome to break down organelle
