biochemistry quiz 2

rods are ____ abundant in most mammals

have quantum sensitivity

and have relatively _____ time resolution

more abundant, slow

what do cones provide?

color discrimination

fine focus

rapid motion detection

which is more sensitive: rods or cones?

rods are more sensitive than cones

what is rhodopsin?

rhodopsin is a prototype of the largest class of receptors in the human body, G-protein coupled receptors (GPCRs)

what class of receptors is rhodopsin?

G-protein coupled receptors (GPCRs)

what do Rhodopsin mutations cause?

blinding diseases

what are the 3 blinding diseases caused by rhodopsin mutations?

Autosomal dominant retinitis pigmentosa (ADRP)

congenital stationary night blindness (CSNB)

11-cis retinal (chromophore)

what happens to a rod cell in the dark? (dark current)

membrane potential at ~ -40 mV

voltage controls neurotransmitter (glutamate) release at the synapse

cyclic nucleotide gated channels OPEN

Na+/K+ ATPase PUMPING

what are the 4 major components of a circulating dark current of rod photoreceptor cells?

rod outer segment: CNG channel & Na/Ca-K exchanger (NCKX1)

rod inner segment: Potassium channel & Na+/K+ ATPase

what happens in a rod dark current?

channels open

more current

higher energy demand

higher glutamate neurotransmitter output

what happens in a rod light current?

channels closed

less current

lower energy demand

lower glutamate neurotransmitter output

what happens to a rod cell in the light?

G-protein cascade leads to lower [cGMP]

membrane potential at ~ -70 mV

-40 to -70 mV = hyperpolarization! (most neurons depolarize)

cyclic nucleotide-gated cation channels CLOSED

kinetic energy converted to light energy

Na+/K+ ATPase STILL PUMPING

lower glutamate = release signal!!!!!!

what is the range for hyperpolarization?

-40 to -70 mV

how long will signaling continue?

until GTP is hydrolyzed

what catalyzes GDP-GTP exchange on the G-protein?

the activated receptor (MII)

where is the light receptor, rhodopsin found?

embedded in the disk membranes

what are the G protein and the effector? what is their function?

G protein = cGMP phosphodiesterase

effector = PDE6

they are peripheral membrane proteins that interact with rhodopsin and one another on the membrane surface

what molecule acts near the diffusion limit to hydrolyze cGMP? what does this cause?

G-protein activated PDE6.

lowering [cGMP] closes cGMP-gated cation channels in the plasma membrane.

lowering [cGMP] ___ cGMP-gated cation channels in the plasma membrane.

closes

calcium control (3 things)

GCAP

Rho. Kinase

cGMP channel

explain the 2 step mechanism of the recovery phase (the inactivation of activated rhodopsin)

1st step: RK adds phosphate groups to serine amino acids near the C terminus

2nd step: then, protein arrestin caps that region by binding to it

in this form, activated rhodopsin cannot interact with transducin to activate it (G-protein binding is blocked)

what are the recovery phase components?

rhodopsin kinase = recessive CSNB, Oguchi type

arrestin = recessive RP, recessive Oguchi CSNB

the GTPase Accelerating Protein (GAP) complex

transducin G-protein alpha subunit

regulator of G-protein signaling 9-1

RGS9-1 (membrane) adaptor protein

G-protein beta 5

PDE 6 gamma

what is the function of the GAP complex?

to accelerate the intrinsic GTPase activity of transducin-G-protein-alpha-subunit to increase the rate of GTP hydrolysis to GDP

GTPases are enzymes that catalyze the hydrolysis of GTP → GDP + Pi. this is made possible by the GAP complex, because it activates the GTPase. Would hydrolysis occur without the complex?

yes! but it would take minutes. with the complex, the reaction occurs with a time constant of ~ 200 ms in mammals. this is the rate-limiting step in vision: this reaction set the speed of phototransduction!

what are the 3 proteins of the GAP complex?

RGS9-1 (Regulator of G-protein Signaling 9-1)

R9AP (RGS9-1 Anchoring Protein)

G beta 5 (similar to the G beta subunit of transducin)

what turns off the G protein-PDE6 complex?

GAP complex

RGS9

slowed cone recovery

adaptation-inability to see fast motion

“bradyopsia”

R9AP-RGS9 anchor protein

same disease as RGS9-deficiency

GCAP-1A

dominant cone dystrophy

cone-rod dystrophy

Guanylate cyclase (GC-1)

recessive Leber congenital amaurosis

cone-rod dystrophy

how is the cGMP level reset to its dark concentration? part 1 Ca++

Calcium ions provide a feedback signal to restore cyclic GMP levels

 when cG-gated channels close, Ca²⁺ cannot enter the cell but it continues to be pumped out by the Na⁺/Ca²⁺/K⁺ exchanger

in the light, cytoplasmic Ca²⁺concentration drops ~10-fold.

how is the cGMP level reset to its dark concentration? part 2 cGMP

when cytoplasmic calcium levels fall, Ca²⁺ is released from Guanylate Cyclase Activating Proteins (GCAPs)

causing them to activate Guanylate Cyclase (GC)

Guanylate Cyclase restores resting levels of cGMP

what does light do to rhodopsin?

causes rhodopsin to be modified to the excited state

what does light get absorbed by?

absorbed by 11-cis retinal (it causes all trans retinal to form)

what causes the formation of GTP-Gt alpha?

interaction with transducin

what activates PDE6?

Gt alpha diffuses to PDE6 and displaces its gamma subunits

PDE6 hydrolyzes……

3’, 5’ cGMP to 5’ GMP and lowers [cGMP]

what causes the blockage of the dark current (cation flow)?

cGMP dissociates from the Ca2+/Na+ CNG channel and causes the blockage

The photoreceptor _______ (becomes more negative) from ~-40 to ~-70 mV on the inside (outside becomes more positive).

hyper polarizes / becomes more negative on the inside

when the photoreceptor hyperpolarizes, glutamate release at the synapse slows and causes a _________ in the bipolar cells. This is the light signal that is relayed to area 17 of the cortex (e.g., visual cortex).         

postsynaptic discharge