Bio A Midterm 2 - 11/5

KRAS (the Death Star)

-a lethal, misshapen protein that jumpstarts a pathway that drives almost a quarter of all cancers

-causes sarcomas (cancers of bone, soft tissues, and connective tissues)

oncogenes

mutations in 3 separates RAS genes (KRAS, HRAS, and NRAS) that turned healthy human cells cancerous

G12C Mutation

-KRAS mutation where a glycine amino acid in the normal protein is replaced by a cysteine at the 12-position

-cysteine carries a sulfur atom, which served as a landing pad for a potential drug

what does a normal KRAS protein do?

it detects incoming signals and passes messages on to the next protein as part of a chain of biochemical reactions that control cell behaviours that malfunction in cancer (including growth and differentiation); acts as a central master switch

what do cancer-causing mutations do the KRAS protein?

they leave it stuck in the on position, which can cause uncontrolled cell growth

ion-channel-coupled receptors

-when activated, open to allow an influx of ions into the cell

-alternate name: ligand-gated ion channel

EX: release of calcium (used to amplify and spread the response throughout the cell) into the cytosol from the ER

G-protein-coupled receptors (GPCRs)

-activate G-proteins when activated by their ligand

-most common; heavily involved in animal sensory perception, especially smell

-almost always a single polypeptide chain with seven transmembrane domains that cross the membrane as alpha helices

-GTP is required to activate the G-proteins and allows a response to be generated

-each protein complex has 3 subunits that bind together in a trimeric complex and remain attached to the cell membrane

kinase

-add phosphate groups to other proteins

-fall into 2 major categories based on the amino acid R groups they modify:

1. tyrosine kinases ~ largest class of kinases; often receptors

2. serine/threonine kinases (Ser/Thr kinases)~ short hydrocarbon chains with an OH group; not associated with receptors; EX: PKA, PKC

phosphatases

enzymes that remove phosphate groups

enzyme-coupled receptors

-activate downstream proteins enzymatically once activated by binding to a signal

-when the ligand binds to the receptor, an enzyme known as a kinase is activated

-either have their own intrinsic phosphorylation capacity or have no intrinsic enzymatic activity, but will be closely associated with a kinase that is activated upon ligand binding

cyclic AMP

-produced when a plasma membrane-bound enzyme, adenylyl cyclase, is activated by the G alpha subunit of the activated G-protein

-can move throughout the cell and interact with enzymes that have binding sites for it

-short-lived molecule

what does phosphodiesterase do?

it breaks down cyclic AMP so that the cellular response is kept short

protein kinase A (PKA)

-cystolic protein/enzyme activated by cAMP

-when activated, it can have different effects:

-”fast” response where glycogen is broken down in the liver

-”slow” response where PKA can also enter the nucleus and phosphorylate transcription factors, which will activate these transcription factors, resulting in gene expression

PIP2

-phospholipid that gets split in two by an enzyme (phospholipase C) to produce two second messengers, DAG (phospholipid tail) and IP3 (phospholipid head)

-activated by a G beta gamma protein complex

receptor tyrose kinases (RTKs)

-where the kinase acts on tyrosine and is an intrinsic function of the activated receptor

-usually dimerize as they bond to the signal molecule

-enzymatic activity turns on once they dimerize and the ligand is bound → each protein in the dimer will phosphorylate the other one

-the added phosphate groups form the basis for docking sites that other proteins will bind to and be phosphorylated

Ras

-the first commonly activated molecule after receptor activation in RTKs

-molecular switch that has a superfamily of proteins that are all small monomers and GTPases (they convert GTP to GDP)

-have an “on” (GTP) and “off” (GDP) conformation

guanine-exchange factor (GEF)

in order for Ras to be activated, this second enzyme swaps out the GDP for GTP

GTPase-activating protein (GAP)

the GTP that is bound to the activated Ras will eventually be hydrolyzed with the help of another protein