Lecture 15: Signal Transduction/Lecture 16: Carbohydrates Flashcards
Classifications of cell signalling (3)
- Nature and origin of extracellular signal
- Type of receptor involved
- Physiological system
Ligands
Extracellular messenger molecules, bind to specific receptors, mainly bind membrane receptor proteins
Agonist
Ligand that initiates a biological response
Antagonist
Binds to receptor and causes no response, prevents other signalling molecules to bind and cause response
Kd
Dissociation constant, concentration of the ligand when the receptor is half-saturated, low values for Kd indicate high affinity
Kd= [R][L]/[R x L]
Second messengers
Propagate signals in cell, transient, kept at low intracellular levels until needed then are generated from abundant cellular precursors
Second messenger: cAMP
Derived from ATP by activation of adenylate cyclase
Removed by phosphodiesterase
Second messenger: phospholipids
Hydrolysis gives variety of second messengers
Ie. DAG, IP3
Second messenger: Ca
Cytosolic free Ca rises 100x from extracellular. and organelles sourced with ion activation
Skeletal contraction
Protein phosphorylation
Protein kinases transer PO4 from ATP to substrate proteins at Ser, Thr, and Tyr
Conserved catalytic and regulatory domains
Reversed by protein phosphatases
May modify activity or provide docking sites for signalling proteins
Target is usually protein kinases to amplify signal
Slower than Ca signalling
G-proteins
When bound to GTP, bind and activate downstream proteins
Intrinsic GTPase activity causes them to act as timers
1. Heterotrimeric G-proteins
2. Monomeric G-protiens
Heterotrimeric G-proteins
Directly activated by GPCRs
Monomeric G-proteins
Involved in signal transduction, small GTPases, Ras family
GPCR
7 transmembrane domains as alpha helices
Ligand binding to the outside causes conformational change that activates G-protein on the inside
GPCR activation of Protein Kinase A
- Hormone binds to GCPR
- Activation of Gs protein, alpha subunit dissociates and activated adenylyl cyclase
- Adenylyl cyclase makes cAMP, which acts on the regulatory subunits on PKA
- PKA activated when regulatory subunits interact with cAMP and dissociate from catalytic subunits
Receptor Tyrosine Kinase
Single transmembrane domain, respond to ligand binding by dimerizing, activates internal kinase domain and auto/cross-phosphorylate Tyr resides
Tyr (phosphorylated) act as docking sites for other proteins, uses modular protein domains to recruit signalling molecules and cause downstream responses
Ras
Monomeric G protein, involved in many RTK signalling pathways leading to cell proliferation
Interacts with multiple downstream signalling proteins when bound to GTP
Mutations near active site impair its GTPase activity leaving it always on
GlyH12
Active site of Ras
Mutations near site leave protein constitutively active
Ras mutations are present in 30% of humans cancers
Oncogenes
Cancer causing genes
Accelerates cell growth, reduced apoptosis, mitosis before DNA damage corrected, reduced/abnormal DNA repair, abnormal gene expression
(Also caused by loss of function of tumour suppressor genes)
Carbohydrates
Hydrates of carbon, general formula (CH2O)n, where n is greater than/equal to 3
Major roles: energy from diet, structural support, molecular recognition
Monosaccharide or polymers
Linear or branched
Glycosidic bonds
Attached carbohydrate monomers to form oligo/polysaccharides
Between hydroxyl groups to cause loss of H2O
Monosaccharides
Classified by:
- Nature of carbonyl group: aldoses, ketoses
- Number of carbons (numbered from carbonyl end)
- Ring size
- Stereochemistry
Aldoses
Monosaccharide that contained an aldehyde
Ketoses
Monosaccharide that contains a ketone
Furanose
Monosaccharide ring with 5 carbons and one oxygen
Pyranose
Monosaccharide ring with 6 carbons and one oxygen
Glyceraldehyde
Simplest sugar
One chiral centre
D and L form are enantiomers
Carbohydrate stereochemistry
Chiral centres
D/L destination reserved for chiral carbon furthest from carbonyl group
Most in D configuration (hydroxyl to the right)
Epimers
Two monosaccharide isomers with different orientation about any one carbon except anomeric carbon
Anomeric carbon
Carbon farthest from carbonyl group in monosaccharide, D/L configuration
Attack by OH group produces alpha and beta configurations
If OH group is unattached, alpha and beta forms can freely interconvert with beta predominating
Haworth projection
Fischer projection tilted 90 degrees right, anomeric hydroxyl group below (alpha) or above (beta) the plane of the ring
Linear carbohydrate chains
1 to 4 linkages using glycosidic bonds
Branched carbohydrate chains
1 to 6 linkages using glycosidic bonds
Carbohydrates with multiple hydroxyl groups
Aldohexoses
Glucose, galactose, mannose
Usually form 6-membered pyranose rings
Glycogen
Main storage form of glucose in mammals (liver, muscle)
alpha 1,4 linkages with alpha 1,5 branches every 12 residues
Starch
Energy storage in plants
Consists of glucose in alpha 1,4 linear (amylose) or alpha 1,6 branched (amylopectin) chains
Cellulose
Provides structural rigidity to plant cell walls
Undigestible beta 1,4 linked glucose chains
Heavily hydrogen bonded into fibres
Glycoproteins
Attachment of oligosaccharides to proteins: provides variety in molecular recognition
N-linked or O-linked
Carbohydrate portion of glycoproteins face exterior/lumenal membrane side
Functions in protein targeting and stability
N-linked
to Asn
Added during translation into ER as preformed 14-mer from dolichol
Trimmed and rebuilt to prices in the ER
O-linked
to Ser, Thr
Added one until at a time in the Golgi
Can be quite large
Proteoglycans
Consist of large glycosaminoglycans attached to peptides
Provide spongy hydrates coat to mammalian cells
Chitin
Makes exoskeleton of insects and crustaceans
ABO blood group
Sugars attached to cell surface protein or lipid
Lipopolysaccharides
Provide outer membrane barrier for Gram-negative bacteria
