lecture 4 biochemistry (week 1) Flashcards
what is a proto-oncogene?
a proto-oncogene is a normal cellular gene that regulates cell growth and differentiation.
its primary function is to promote cell proliferation, survival, and division, allowing cells to grow and develop properly
when mutated they can cause normal cells to become cancerous
what is c-Src?
it is the first proto-oncogene and a tyrosine kinase
what is a tyrosine kinase?
a tyrosine kinase is a type of enzyme that catalyzes the transfer of a phosphate group from ATP (adenosine triphosphate) to a tyrosine residue in a protein’s side chain
what are tyrosine kinases important in?
the processes of cell signalling, division, and metabolism
describe the insulin receptor
the insulin receptor is an a2b2 polypeptide, held together by disulphide bonds
what part of the insulin receptor does insulin bind to?
specifically binds to the a2 subunits
what does the binding of insulin to the receptor cause?
a signal being transmitted to the b2 subunits which activates an intrinsic tyrosine kinase within the cytosolic domain of the b subunit - the signal has crossed the membrane
describe insulin signalling in relation to the tyrosine kinase
the tyrosine kinase of the receptor phosphorylates specific Tyr residues within the receptor - autophosphorylation
these p-Tyr residues recruit signalling molecules to the receptor which are then phosphorylated
forms a signalling complex
what is dimerization?
dimerization is the process of joining two identical or similar molecular entities by bond
describe receptor activation after dimerization occurs
dimerization causes conformational changes bringing the tyrosine domains in close proximity - allows phosphorylation
describe the structure of the EGF receptor
in its unbound state the receptor contains two identical but separated monomeric units - both contain a binding site
when EGF binds dimerization occurs
what does ligand binding do in the context of this receptor?
ligand binding signal is transmitted across the membrane and the conformational change that results activates the intrinsic tyrosine kinase activity of the receptor.
consequence of ligand binding is the receptor gets auto phosphorylated
describe trans auto phosphorylation
activation of the tyrosine kinase domain of the receptor then promotes phosphorylation of specific tyrosine residues within the receptor
the dimeric receptor phosphorylates trans
Chain A phosphorylates tyrosine in chain B and vice-versa
what do photo-tyrosines do?
act as docking sites for signalling molecules
act as a magnet for the arrival of key signalling molecules
SH2 domains - an example of a protein domain found in many other types of protein - bind phospho-tyrosine residues
describe the activation of tyrosine kinase receptors in relation to the SH2 domain
auto-phosphorylation of specific tyrosine residues function to recruit SH2 domain-containing proteins to the receptor
these are then activated –> signal propagated
different receptors recruit different SH2 domain containing proteins
describe the characteristics of binding of the SH2 domains
all SH2 domains bind phospho-tyrosine
all SH2 domains have a pocket into which the P-Tyr residue fits
but not all bind all P-Tyr residues; there is a specificity dictated by the surrounding sequence in the target (receptor) polypeptide
two pronged plug concept
summarize the signalling complex
specific ligand binds to specific receptor
dimerisation leads to activation of the receptor
tyrosine kinase domain (movement of activation loop)
autophosphorylation of specific tyrosine residues
recruits SH2 domain-containing proteins to the receptor
different receptors recruit different SH2 domain-containing proteins
different SH2 domains recognise P-Tyr in different sequence contexts
give examples of a subset of SH2 domain containing proteins recruited by EGF receptors
PLC-gamma (SH3-SH2-kinase)
Ras-GAP (SH2-SH3-SH2–GTPase)
describe the ligand dependant assembly of signalling molecules
localised to the receptor and activated with precise spatial and temporal coordinates
different receptors assemble different protein complexes depending on which effectors are recruited which in turn is a function of which proteins are expressed in a given cell type.
hence receptor-x can exert two distinct functions in two different cell types because the downstream effectors are different
describe the steps after an increase in calcium is detected
signal molecule binds to GPCR –> GPCR subunit activates phospholipase C –> PhC breaks down PiP2 into and DAG and IP3 –> PKC binds to DAG –> activates PKC –> IP3 opens channel –> Ca binds to PKC
what are RTKs
receptor tyrosine kinases (RTKs) are a subclass of tyrosine kinases
play a crucial role in mediating cell-to-cell communication, cell growth, motility, differentiation, and metabolism.
describe the activation of PLCy by RTKs
PLC-γ recruited to receptor via specific SH2 domain
this brings PLC-γ into close proximity to the tyrosine kinase domain of the receptor.
PLC-γ is tyrosine phosphorylated and activated.
PLC-γ is now both active and near its substrate.
what is a G protein?
also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells
describe the MAPK cascade
Ras is a G-protein
Ras-GTP activates a signalling pathway
balance of Ras-GTP and Ras-GDP controlled by relative activity of Ras-GAP and SOS proteins
recruiting and activating these to receptors via SH2 domains is a key control step
