FSHN 350 - Carbohydrates (Unit 2.1)

What are the pathways of carb metabolism

- glycogenesis
- glycogenolysis
- glycolysis
- gluconeogenesis

What is Glycogenesis

- making glycogen from glucose

What is Glycogenolysis

- break glycogen into glucose

What is Glycolysis

- breakdown of glucose into pyruvate

What is Gluconeogenesis

- produce glucose from a non-carb intermediate

Important tissues for glucose metabolism

- liver
- muscle

The liver

- takes up 20% of glucose
- uses GLUT 2 to transport to get into the cell

The muscles

- take up 80% of glucose
- uses GLUT 4 to transport to get into the cell

How is muscle selfish

- once muscle reaches its glucose limit, it stops taking up more -
- once muscle breaks down glycogen into glu, it does not release it into the blood

Process of making G6P

- glucose enters the cell
- phosphate attached to glucose (ATP --> ADP)
- forming G6P

GLUT transporters

- Glu enters the muscle cell via GLUT 4, phosphorylated by hexokinase -- making G6P
- Glu enters the liver cell via GLUT 2, phosphorylated by glucokinase -- making G6P

Enzymes that make G6P

- hexokinase in the muscle
- glucokinase in the liver
- both phosphorylate glucose into G6P

3 fates of G6P

- G6P --> glycogen (glycogenesis)
- G6P --> back to glucose (glycogenolysis)
- G6P --> glycolysis

What, where and when of Glycogenesis

- glucose --> glycogen
- in muscles and liver
- when in periods of energy excess
- anabolic
- consumes energy

Steps of Glycogenesis with Enzymes

Glucose --(a-b)----> G6P ---(c-e)----> Glycogen
a - hexokinase (muscle)
b - glucokinase (liver)
c - glycogenin
d- glycogen synthase
e - branching enzyme

What are the steps of Glycogenesis

1. glucose --> G6P
2. G6P --> G1P
3. G1P --> UDP-glucose
4. UDP-glucose + glycogenin = Glycogen (short chain of Glu)
5. Glycogen chains extended w/ glycogen synthase
6. Glycogen adds branching via branching enzyme

Glycogenin

- enzyme in glycogenesis that attaches to UDP-glucose to make a short glu chain
- attaches via α 1-4 bonds

Glycogen synthase

- enzyme that extends the short glu chain (glycogen chain)
- extends via α 1-4 bonds
- when G6P conc. increase, conc. of Glycogen synthase increase, meaning more glycogen is going to be made

Branching enzyme

- enzyme that creates branching points after glycogen has been created
- creates branches via α 1-6 bonds

Regulation of Glycogenesis

- G6P inhibits hexokinase in muscle (negative feedback)
- G6P does NOT inhibit glucokinase in liver
- G6P increases Glycogen synthesis
- Epi and glucagon inhibit Glycogen synthesis
- insulin directly regulates glucose uptake in muscle (via GLUT 4)
- insulin indirectly regulates glucose uptake in liver
- insulin increases activity of glucokinase
- insulin activates glycogen synthase

When would we be using Glycogenolysis

- when we need to break glycogen into glucose
- fasting, starving, working out

What, where and when of Glycogenolysis

- glycogen --> glucose units
- in liver and muscle
- when in periods of energy deficit
- catabolic
- Glycogen phosphorylase will use a phosphate to take Glu off, making G1P

Glycogenolysis (muscle)

- Glycogen --> G1P --> G6P --> Glycolysis
- muscle does not want to give up any Glu so it won't go back to Glu but will go to Glycolysis

Glycogenolysis (Liver)

- Glycogen --> G1P --> G6P --> Glycolysis
- Glycogen --> G1P --> G6P --> Glucose
- will make glucose when the body needs more
- liver is the master Glu regulator

Insulin and glucagon in the pathways

- insulin increases Glycogen synthase (make glycogen)
- glucagon and epi increase Glycogen phosphorylase (make G1P)

What, where and when of Glycolysis

- break down Glu into pyruvate
- cytoplasm
- when in periods of energy demand
- functions in anaerobic and aerobic conditions
- catabolic
- yields energy

Glycolysis steps (investment phase)

1. Glucose ---(ATP-->ADP)---> G6P
3. F6P ---(PFK)---> F1,6BisP
4. F1,6BisP--> 2 G3P carbon molecules
- 2 mol ATP invested to make 2-G3P

How is PFK regulated

- AMP + (positive feedback, high conc. of AMP, b/c ATP broken down into AMP, so we need more ATP)
- ATP - (negative feedback, high conc. of ATP, so stop making so much)

Glycolysis steps (payoff phase)

6. make ATP from NAD+ ---> NADH
7. make ATP via substrate level
10. making pyruvate + ATP via substrate level
- we use pyruvate kinase to make pyruvate

Product of Glycolysis

- 4 ATP
- 2 NADH
- 2 pyruvate

Regulation of Glycolysis

- AMP -- always increase
- Insulin - increase
- F1,6BisPhosphate -- increase
- ATP -- always decrease
- Citrate -- decrease
- Acetyl CoA -- decrease

How to get electrons into the mitochondria

- Malate-Aspartate shuttle
- Glycerol-Phosphate shuttle

Malate-Aspartate shuttle

- e- passed from NADH to oxaloacetate (OAA), forming malate
- malate easily enters mitochondria
- malate gives e- to new NAD and forms NADH (in the mitochondria)
- NADH goes to the ETC to make ATP

Glycerol-Phosphate shuttle

- e- passed from NADH to DHAP, forming glycerol-phosphate
- glycerol-phosphate easily enters mitochondria
- glycerol-phosphate gives e- to new FAD and forms FADH (in the mitochondria)
- FADH goes to the ETC to make ATP

Liver overview

- Glu enters -- via GLUT 2
- Glu phosphorylated -- via glucokinase (GK)
- insulin activates (GK)
- insulin activates glycogen synthase
- Glycogen phosphorylase -- break Glycogen into G1P
- Glu-6-Phosphatase -- breaks down G6P into Glu

Muscle overview

- Glu enters -- via GLUT 4 (insulin dependent)
- Glu phosphorylated -- via hexokinase (HK) inhibited by G6P (negative feedback)
- G6P has negative feedback on HK
- insulin activates (HK)
- insulin activates glycogen synthase
- Glycogen phosphorylase -- break Glycogen into G1P

Liver story (Glycogenesis)

- Glu enters the cell via GLUT 2
- Glu gets phosphorylated by glucokinase, making G6P
- G6P converted to G1P
- G1P ultimately uses Glycogenin to make a Glu chain
- Glycogen chain extended with Glycogen synthase
- Glycogen branches out with the branching enzyme
- the presence of insulin will activate GK and glycogen synthase

Liver story (Glycogenolysis)

- Glycogen broken down by glucose phosphorylase into G1P
- G1P --> G6P
- G6P --> Glu via Glucose 6 phosphatase
- glucagon receptors help with the release of Glu

what does high G6P mean?

- a lot of G6P means a lot of glycogen can be formed, so a lot of glycogen synthase is going to be needed

Liver story (Glycolysis)

- Glycogen broken down by glycogen phosphorylase into G1P
- G1P --> G6P
- G6P --> Glycolysis

Muscle story (Glycogenesis)

- Glu enters the cell via GLUT 4 (insulin dependent)
- Glu gets phosphorylated by hexokinase, making G6P
(G6P has negative feedback on HK)
- G6P converted to G1P
- G1P ultimately uses Glycogenin to make a Glu chain
- Glycogen chain extended with Glycogen synthase
- Glycogen branches out with the branching enzyme
- the presence of insulin will activate HK and glycogen synthase

Muscle story (Glycogenolysis)

- cannot make Glu because muscle has no Glucose 6 Phosphatase

Muscle story (Glycolysis)

- Glycogen broken down by glycogen phosphorylase into G1P
- G1P --> G6P
- G6P --> Glycolysis

3 fates of pyruvate

- make Acetyl CoA
- make lactate
- make ethanol

Pyruvate --> Acetyl CoA

- CO2 released
- NADH created (NAD+ --> NADH)
- via PDH enzyme
- inhibited by ATP, NADH and Acetyl CoA

Pyruvate --> Lactate

- NADH --> NAD+
- via LDH enzyme

When do we make lactate

- when the rate of pyruvate formation, exceeds the rate of Acetyl CoA formation

What pathways need NAD to function

- Glycolysis
- TCA
- pyruvate --> Acetyl CoA

Importance of NAD+ in forming lactate

- we need NAD for the TCA
- the pyruvate --> lactate reaction allows for other reactions to occur because NAD+ is produced in this reaction (NADH --> NAD+)
- other reactions: the ones mentioned above

Lactate transportation

- intracellular lactate shuttle
- intercellular lactate shuttle

Intracellular lactate shuttle

- stays in the cell that made it
- lactate goes to mitochondria to undergo oxidation to pyruvate via mitochondrial LDH

Intercellular lactate shuttle

- lactate can enter the circulation and go to other tissues
- via the Cori Cycle

Lactate misconceptions

- it is a waste product produced by metabolism
- is formed only under anaerobic conditions
- is the cause for muscle soreness
- all are wrong

Cori Cycle

- intercellular shuttle
- if rate of lactate production in cell is too high, it ships it out

Cori Cycle story

- lactate accumulates in skeletal muscle and skeletal muscle lacks glucose 6 phosphatase
- it ships the lactate to the liver so the lactate can be converted into Glu via gluconeogenesis

MCT

- intracellular
- lactate enters the mitochondria via the MCT
- lactate then forms pyruvate
- then go to the TCA and make ATP

What, where, when of Gluconeogenesis

- making Glu from non-carb
- liver, kidney, intestine
- when low blood Glu
- consume energy
- yield NADH

When does Gluconeogenesis happen

- when we are fasting

Gluconeogenesis story

- say you ate at 7:00 and then went to bed
- immediately after - the body will break down that food for Glu
- 4 hours after - glycogen stored in the liver will be broken down
- 12-18 hours after - glycogen stores are empty so body goes through gluconeogenesis to increase blood Glu

What enters Gluconeogenesis

- pyruvate
- lactate
- glycerol from mono, di and triglycerides
- glycogenic amino acids

Steps of Gluconeogenesis

- pyruvate --(1+2)--> phosphoenolpyruvate --> reverse to glycolysis --> Fructose 1,6 BisPhosphate --(3)--> Fructose 6 phosphate --> G6P --(4)--> Glucose

Gluconeogenesis enzymes

1. pyruvate carboxylase
2. PEP carboxykinase
3. fructose 1,6 bisphosphatase
4. glucose 6 phosphatase

Regulation of Gluconeogenesis

- opposite of glycolysis
- AMP -
- ATP +
- Citrate +

Pentose phosphate pathway

- detour off glycolysis
- NO ATP made
- produce NADPH and R5P
- high after a carb high diet

NADH in pentose phosphate pathway

- used for lipid synthesis

R5P in pentose phosphate pathway

- used for synthesis of nucleotides and nucleic acids
- can lead to increased levels of uric acid

Glycogen storage diseases

1. McArdle's Disease (GSD-V)
2. Von Gierke's Disease (GSD 1)

McArdle's Disease (GSD-V)

- exercise intolerance
- low lactate levels after intense exercise
- happens in skeletal muscle
- no glycogen phosphorylase (so glycogen cannot be broken down when exercising)

Von Gierke's Disease (GSD 1)

- most common glycogen storage disease
- severe hypoglycemia (low blood Glu)
- high glycogen in the liver and kidney
- can lead to an enlarged liver -- hepatomegaly
- can't get rid of glycogen once its formed
- glucose 6 phosphatase is inhibited
- eat lower carb diets to solve issue

Fructose metabolism

- absorption into the cell via GLUT 2
- not insulin dependent
- liver takes 20% Glu and 75% fructose
- fructose --fructokinase--> fructose 1 phosphate

Where does fructose join glycolysis

- fructose enters at the 2-G3P step
- so after the investment phase
- it bypasses the rate limiting step (which has PFK)

Once fat is created the liver can

- burn it
- store it
- package into VLDL
- VLDL carries and ships triglycerides

3 ways fructose increases risk of disease

- increase fatty liver
- increase risk heart disease b/c of VLDL
- increase levels of uric acid

Increase of fructose meaning

- means ATP depletion
- which makes AMP
- AMP creates uric acid

What is uric acid

- risk factor for heart disease and gout
- gout -- painful inflammation
- linked to diabetes

Glucose vs Fructose metabolism

- both use GLUT 2 to enter cell
- both phosphorylated by glucokinase and fructokinase
- fructose dangerous b/c its a fast pathway (b/c it bypasses PFK)

How fructose increase risk of heart disease

- increases gout b/c ATP depletion
- increase VLDL

What does fructose lead to

- it bypasses PFK and increases Acetyl CoA conc.
- this is converted to fat and can lead to:
- obesity, fatty liver and increased VLDL

What is Type 1 diabetes

- the body can not make insulin

What is Type 2 diabetes

- the body can make insulin, it just cannot utilize it

Type 1 effect

- no insulin means:
- GLUT 4 transporters do not work - so tissues starve for Glu
- Glu stuck outside the cells
- people must inject Glu
- lack of insulin = increase break down of fat
- high fate loss = high ketone production (dangerous)

Type 2 effect

- cannot use insulin means:
- Glu cannot get into the cell

What leads to Type 2

- over nutrition leads to type 2
- obesity is the biggest risk factor
- most common type (95% of people get this one)

4 areas impacted by lack of insulin

- fat
- liver
- muscle
- blood

Fat

- increase lipolysis -- breakdown of fat

Liver

- increase hepatic glucose output -- increased Glycogenolysis, so Glu
- increase VLDL -- bad cholesterol

Muscle

- decreased Glu uptake
- decreased glycogen synthesis

Blood

- decreased vasodilation

First step towards diabetes

- insulin intolerance

Insulin intolerance effects

- less Glu taken by insulin
- so body makes more insulin to compensate
- increased intolerance leads to increased production of insulin

Insulin intolerance story

- insulin production increases as insulin resistance/tolerance goes up
- Glu will stay normal (during this time) b/c of increased insulin production
- pancreatic beta cells reach Beta Cell Failure as resistance goes up and production cannot meet that
- this is when diabetes occurs

Diagnosing diabetes (3 tests)

- check A1C
- being over 126 mg/dL (fasting)
- being over 200 mg/dL (75g Glu)

A1C test

- the percent of hemoglobin in blood, that is bound to Glu
- over 6.5% = you have diabetes

Fasting test

- test bloog Glu after 8 hrs of fasting
- over 126 mg/dL = you have diabetes

75g Glu test

- take a 75g sugary drink in water, wait 2 hours
- over 200 mg/dL = you have diabetes

Limitations of these tests

- you could have fasting blood Glu and be on your way to diabetes
- there are different reactions for those that drink 75g (different body sizes)
- A1C cannot be used in pregnant moms
- b/c has a wide snapshot range, detects for 3 month range, not for right this second

Glucagon and diabetes

- increases gluconeogenesis
- increases glycogenolysis

Types 1 and 2

- both have increased glucagon
- associated with hyperglucagonemia

What does high glucagon mean for diabetes

- glucagon means more Glu from non carbs and more Glycogen broken into Glu
- all this Glu makes diabetes worse because insulin already can't do it's job

Gestational diabetes

- women that are not diabetes, end up having high blood Glu during pregnancy