BIOCH 310 Quiz 1 - Midterm

Glucose uptake by most cells requires ____ transporters

Passive

What does GLUT1 do? Where is it?

• Basal uptake of glucose

• In all RBC

• Kt = 1 mM

What does GLUT2 do? Where is it?

• Liver - removal of excess glucose for storage & pancreas - insulin regulation

• In liver, pancreas, intestine

• Kt = 15-20 mM

What does GLUT3 do? Where is it?

• High affinity basal uptake

• In brain

• Kt = lowest

What does GLUT4 do? Where is it?

• Regulated by insulin

  • tells glut4 more or less glucose

• In muscle, fat, heart

• Kt = 5mM

What does lowest Kt mean?

Can always uptake glucose

• increased transport

Fasting blood glucose levels are what?

~5 mM

In the intestine and kidney, there are also ______ transporters

Na-dependent active glucose transporters

Glucose uptake by muscles is impaired in ______

Type II diabetes

What kind of population has increased levels of GLUT4?

Endurance athletes

GLUT4 activity is increased by _____

Insulin

What are the catabolic fates of pyruvate?

Acetyl coA

Lactate

Ethanol

Fermentation is the _____ breakdown of nutrients without net _______

Anaerobic

Oxidation

What is the role of phosphorylation?

• Keep glucose conc low in cell

• activate glucose

• No plasma membrane for phosphorylated sugars

Why do we phosphorylate glucose?

Because its very stable

What enzyme performs the first ATP investment?

Hexokinase

What happens in the first ATP investment?

• Phosphate transfer

• Coupled

• Regulated by product inhibition

• Glucose → G6P

• Irreversible/regulated

What would inhibit hexokinase?

Increased G6P

What is Mg considered for Glucose?

True substrate

• neutralizes negative charges

Why does HK not bind ATP unless glucose is present?

Due to induced fit

• Squeezes out H2O so ATP can bind

If water was present, what would happen to glucose?

HK would break PA bond instead of glucose

What is a competitive inhibitor with glucose?

• Xylose

What are isozymes? Example?

Different proteins that catalyze the same rxn

• HK and glucokinase

HK: location, specificity, inhibition, Km

vs GK

HK: Ubiqutous, less specific, product inhibited, Low Km

GK: Liver/pancreas, more specific, regulated by compartmentation/regulatory protein, high Km

What does a high vs low Km mean?

Low: always saturated

High: Not at max activity

• most active w/ increased BS

Which is hyperbolic and sigmoidal - HK vs GK

Hyperbolic: HK

• high affinity

Sig: GK (due to binding of reg protein)

_____ is the primary fuel that regulates pancreatic islet-beta cell function

Glucose

What would need to bind to make GK inactive?

GK regulatory protein

What catalyzes G6P → F6P? What does it do?

Phosphoglucose isomerase (PGI)

• Promotes closure of 5-membered ring

What enzyme performs the 2nd ATP investment?

PFK-1

Describe the second ATP investment rxn

• Irreversible/tightly regulated

• P transfer

• Coupled

• F6P → F1,6P

What is the delta G of the first energy investment?

~-15kj/mol

• -30 to break PA bond and + 15 to make PE bond

What is the delta G of the second energy investment?

-14.2 kj/mol

What catalyzes the committed step in glycolysis?

PFK-1

What does aldolase do?

F1,6BP → DHAP + GAP

• Cleavage to two triose phosphates

What converts DHAP to GAP? What is the delta G?

Triose Phosphate Isomerase

+7.5kj/mol

What is the balanced equation for energy investment phase of glycolysis?

Glucose + 2ATP → 2GAP + 2ADP + 2H+

Which phase happens twice for every glucose?

Energy recovery

Which enzyme generates the first energy-rich compound?

GAPDH

Describe the generation of the first energy-rich compound

GAP + NAD+ + Pi → 1,3BPG + NADH + H+

What kind of molecule is 1,3BPG?

Acyl phosphate

What enzyme performs the first ATP generation?

Phosphoglycerate kinase

Describe the first ATP generation

1,3 BPG + ADP → 3PG + ATP

• SLP

• delta G - -18.5 kj/mol

Consumption of _____ pulls GAPDH rxn forward

1,3BPG

What performs the synthesis of 2-phosphoglycerate?

Phosphoglycerate mutase

What performs synthesis of PEP?

Enolase

• second energy rich intermediate = PEP

Describe the generation of the second energy-rich intermediate

2PG → PEP + H2O

• dehydration

• delta G = +7.5 kj/mol

Is PEP a mixed anhydride?

No

What enzyme performs the second ATP generation?

Pyruvate kinase

Describe the second ATP generation

PEP + H2O → Pyruvate (enol = unstable) = -16kj/mol

• hydrolysis

Pyruvate (enol) → Pyruvate = -46kj/mol

• tautomerization

• 2ATP is net

Entry of fructose carbon atoms into the glycolytic pathway in ______

Hepatocytes

What is the reaction for the first step in preparing fructose for catabolism in hepatocytes?

Fructose + ATP → Fructose-1-phosphate + ADP + H+

• done by fructokinase

Galactose in converted to _______

Glucose-1-phosphate + UDP-glucose

• via galactokinase

• Uses ATP → ADP

PFK-1 is an ______ enzyme

Allosteric

PFK-1 has 2 ATP binding sites

• substrate site

• allosteric site

The allosteric ATP site is only present in the __ state

T state

• Low affinity for ATP

High AMP and ADP overcome _______

ATP inhibition

• Shifts towards R state

For F6P, the substrate binds to the __ state

R

Which state is active or inactive: R or T

T

Would a high PEP indicate a shift in favour of gluconeogenesis or glycolysis?

Gluconeogenesis

• glycolysis starting to slow down

• pyruvate kinase slows down

What encourages F6P → F16BP?

• High AMP/ADP

• F26BP

What inhibits F6P → F16BP?

ATP

Citrate

PEP

What is a bigger indicator of energy in the cell: AMP or ATP?

AMP

If there were an ATP depletion, AMP would be the one that goes down the most

• ADP stays constant

F26BP is a ____ of PFK1

Regulator

What are the effects of insulin and glucagon on PFK2 in liver?

Insulin: signal for high BS = increase F26BP in liver

Glucagon: signal for low BS= slowing down glycolysis = decrease F26BP

What positively regulates pyruvate kinase?

F16BP

What negatively regulates pyruvate kinase?

Acetyl coA

ATP

Glucagon (in liver)

In starvation, would PK-L be increase or decrease?

Decrease

• To slow down glycolysis

After a high carb meal, would PK-L be increase or decrease?

Increase

• To store carbs = increase glycolysis

  • for fat metabolism

High levels of AMP activate _____

AMP-activated protein kinase

What happens once AMPK is activated?

AMPK-mediated phosphorylation events switch cells from active ATP consumption to active ATP production

AMP allosterically activates ______

Cytosolic enzyme AMPK

Would low energy mean high or low AMP levels? What generates it?

AMP needs to rise

• Adenylate KINASE generates AMP when ATP is low

What is the “eleventh” step of glycolysis?

Lactate production from pyruvate

Pyruvate + NADH → L-Lactate + NAD+

Lactate is a __________ product in skeletal muscle during anaerobic conditions

Dead end

• No use for it in cell