BIO98 LECTURE 12

Regulation of Glucose Metabolism + Polysaccharides as Storage Molecules

Understand the difference between feed-forward and feed-back regulation

-Feedback inhibition: “We have enough products, let’s stop”

ex. hexokinase regulation

-Feed-forward activation: “We have enough substrates, let’s go”

ex. Fructose-2,6-bisphophate, Pyruvate Kinase (PK) Regulation

Regulation of Glycolysis

-involve irreversible steps

-Feedback inhibition & feed-forward activation

STEP1 Hexokinase regulation: hexokinase is inhibited by its product, which is?

classic feedback inhibition

STEP1 Hexokinase regulation: What does hexokinase do?

2-Deoxyglucose (2-DG) can be phosphorylated by hexokinase to make 2-DG-phosphate (2-DGp)

-Glucose turns into Glucose-6-phosphate (G6P) bc of hexokinase

STEP 2 Hexokinase regulation: What happens with 2-DGp?

-2-DGp CANNOT be CONVERTED by PHOSPHOGLUCOSE ISOMERASE (competitive inhibitor)

-Excess 2-DG blocks more glucose from being phosphorylated, starving cells

STEP 2 Hexokinase regulation: What happens to G6P?

-G6P turns into F6P (Fructose-6-phosphate) bc of phosphoglucose isomerase

Mechanisms of PFK Regulation

-AMP= adenosine monophosphate

• cells are low on ATP, need more energy, so do more glycolysis!

-Super high levels of ATP

• don’t need more energy - block glycolysis

-H+ build up = too much lactate

• build up of product - shut down

-Citrate: part of aerobic metabolism, also a product, same idea

Most PFK regulators work __

allosterically

• F-2,6-P • What is it and where does it come from?

Fructose-2,6-bisphophate = F-2,6-bisphosphate

• Strong PFK allosteric activator

• Produced by another isoform of PFK called PFK-2 (Phosphofructokinase-2)

• lots of F-6-P leads to production of F-2,6-BP = activates PFK to increase rate

• Feed-forward activation

PFK-2 Regulation

• AMP-activated protein kinase (AMPK) senses high levels of AMP, turns on the kinase activity of PFK-2

• PFK-2 then makes F-2,6-BP from F-6-P

• Ramps up glycolysis!

• Protein kinase A (PKA) inactivates PFK-2 kinase and activates PFK-2 phosphatase function • F-2,6-BP -> F-1,6-bP

• Loss of F-2,6-BP slows glycolysis

Pyruvate Kinase (PK) Regulation

• F-1,6-BP ; There are more reagents ready to go, Feed forward!

• ATP = high energy state

• We don’t need to keep going- shut down energy production!

• Acetyl-CoA and alanine are downstream products, if they are building up we don’t want to make more

Why regulate multiple enzymes?

-Glucose is primary point of entry for carbon at most cells

-this carbon is used in almost everything!

-glycolysis is common to practically all life on earth (evolving early on), lots of chances to build in regulatory elements

Glucose Storage

-Glucose: simple sugar that’s easily broken down

-Plants & animals store glucose in more stable polymers

-Glycose & other sugars on their own = monosaccharides

-Sugars are stored as polysaccharides

• Know the different types of glucose-derived polysaccharides

1) Plants: make starch and cellulose

2) Animals: glycogen

Glycogen (basic facts)

-a branching polymer

• ~75% found in muscle tissues

• 25% in liver

• MOST tissues cannot create glycogen

• glycogen can’t be transported (must be metabolized)

• muscle never shares liberated glucose, but the liver does !

Glycogen Synthesis Pathway

• Glucose-6-P (G6P) is converted to Glucose-1-P (G1P) by phosphoglucomutase (not phosphoglucose isomerase from glycolysis)

• G1P is attached to a UDP to facilitate addition to growing chain

• UDP is lost during chain addition

Glycogen Metabolism: Breakdown

1) Glycogen phosphorylase (enzyme; ase= breakdown): breaks down glycogen ; liver metabolic degradation of glycogen→ generates Glucose-1-phosphate

2) G-1-P is converted back to Glucose-6-phosphate by phosphoglucose mutase (PGM) ; usable form for glycolysis in liver.

• it can also be for muscles; the G6P is converted back into glucose, which will circulate back into blood. by glucose-6-phosphatease (G6Pase)

Glycogen Metabolism: Reciprocal Control

Glycogen Storage & Breakdown Regulation via Phosphorylation/De-phosphorylation

• coordinated effort of kinases and phosphatases that are activated or inactivated by hormones

• opposing activities are never activated at the same time

1) Glucagon stimulates kinase (uses ATP)→ Both glycogen synthase and glycogen phosphorylase get phosphorylated; glycogen phosphorylase activates while glycogen synthase inactivates. (BREAKDOWN OF GLYCOGEN)

2) Insulin triggers inactivation of glycogen phosphorylase (stops glycogen breakdown) & activation of glycogen synthase → Phosphatase activity = dephosphorylation of the enzymes/kinases (STORAGE)

Protein Phosphorylation

Proteins are phosphorylated primarily at serine, threonine, or tyrosine residues, but other amino acids can also be phosphorylated or dephosphorylated.

-It's just a hydroxyl going into a phosphate.Water is involved.

Glycogen Synthesis & Metabolism: the organs that store/consume?

liver & pancreas

-Hormone Signaling regulates glucose

-Dysregulation drives disease

• Type 1 diabetes: lack of insulin production drives elevated blood glucose

The influence of insulin

Move glucose from blood to cell: blood glucose level goes down.

-Insulin receptors recognize insulin and trigger the following:

-Protein phosphorylation:

1) Active transport

2) Glycolysis

3) Glycogen synthesis

4) Lipid synthesis (turn sugar into fat)

5) Lipid breakdown (turn sugar into protein) X (don’t wanna do this, we’d rather store sugar)

6) protein synthesis

7) Gluconeogenesis X (we don’t wanna make more glucose; we have too much)

-Insulin: “I want to get rid of blood glucose”

All of the following is true about glucose EXCEPT:
-The blood glucose level is maintained to keep glucose from precipitating in blood
-Decreased [Glucose] in blood can lead to coma

-Hormones like insulin and glucagon can regulate blood glucose levels

-The main source of energy for the brain is glucose
-Blood glucose levels are typically ~4-5mM

-The blood glucose level is maintained to keep glucose from precipitating in blood

The main reason why [AMP] is a key regulator of many metabolic reactions (rather than [ATP]) is:
-DeltaG of ATP -> AMP hydrolysis is higher than DeltaG of ATP -> ADP hydrolysis
-The relative change in [AMP] is usually much greater than the relative change in [ATP]
-[ATP] is typically greater than [AMP], ATP is the form present in nucleic acids
-AMP is less charged than ATP

-The relative change in [AMP] is usually much greater than the relative change in [ATP]

PFK-1 is regulated by all of the metabolites below EXCEPT:

-Activation by ADP or AMP
-Inhibition by ATP
-Inhibition by citrate
-Activation by glucose
-Activation by F-2,6-BP

-Activation by glucose

Glycogen formation. What is true?

-Glycogen synthesis requires CDP
-Glycogen synthesis is catalyzed by glycogen synthase

-Glycogen breakdown is catalyzed by glycogen synthase
-Glycogen breakdown is catalyzed by hexokinase

-Glycogen synthesis is catalyzed by glycogen synthase

Which statement is FALSE? When blood glucose level is high,

-Insulin goes up
-Glycogen breakdown goes up
-Glycogen synthesis goes up
-Glycolysis goes up

-Glycogen breakdown goes up