Glucose metabolic Pathways

Factors affecting the fate of glucose

1. The orientation of the digestive system

2. The source of glucose in the diet (fiber or starch)

Auto-enzymatic Digesters: will absorb

glucose from the lumen of the small intestine, across the enterocyte, into general circulation toward the liver first and then for deposit at a one of the key target location

• Liver, Brain, Muscle or Adipose tissue.

Factors affecting the rate of glucose - Hind-gut fermenters (Forage diet)

• fiber will be broken down by bacterial enzymes and the glucose will be fermented by bacteria to derive volatile fatty acids (VFA)

• VFA will be absorbed from across the colonocytes of the large intestine into general circulation where they will be deposited at the liver for further processing

Factors affecting the rate of glucose - Hind-gut fermenter (High Starch diet)

• Starch will be largely digested in the small intestine by mammalian enzymes, and glucose will be absorbed across the enterocyte of the lumen, into general circulation and deposited at the target organs (liver, muscle, adipose)

• Any glucose from starch reaching the hindgut will be fermented as well as any glucose derived from forage.

Factors affecting the rate of glucose - Ruminant (ALL diets)

• Regardless of diet, starch and fiber are degraded in the rumen by bacterial enzymes and glucose fermented by the bacteria to derive VFA.

• Diet will affect the rate of fermentation and the combination of VFA.

• VFA are absorbed across the rumen wall, into general circulation, and will be deposited at the liver for further processing

Sodium-dependent GLUcose Transporter (SGLUT-1)

transports both glucose and sodium into the enterocyte via facilitated diffusion

Glucose is facilitated out of the cell by

• GLUT-2 following the concentration gradient

• The monosaccharide enters capillary blood inside the villus

Secondary active transport

The entire process of glucose being transported across the enterocyte from the lumen and into the blood capillary of the villus

Pancreatic Islet cells

• β-cells (account for about 65 – 80 % of the Islet cells) manufacture the peptide hormone insulin

• Insulin is released proportionally to the concentration of glucose

Insulin & glucose uptake

• insulin binds to receptors on the cell membrane of the liver, muscles and fat cells

• By doing so, a GLU transporter is brought to the cell membrane surface

• This increases glucose uptake by the cells

• Glucose is either used immediately for cellular energy or

• The liver and muscle cells convert glucose to glycogen for storage

• The fat cells (adipose) convert glucose into triglycerides

Type I diabetes vs Type II diabetes

Type I diabetes: autoimmune destruction of β-cells

Type II diabetes: low insulin production or insulin resistance of receptor cells

• Consequence: failure to produce sufficient insulin, target tissues fail to take up glucose from the blood, extreme high blood glucose, target tissue cellular starvation

How does the body turn glucose into energy?

Cellular respiration at the tissue level

• All cells of all bodily tissues require energy

Anabolism, Catabolism, Metabolism

Anabolism, Catabolism, Metabolism

Anabolism: the building of a substrate from subunits.

Catabolism : the breakdown of a substrate into useable subunits

Metabolism: the combination of anabolism and catabolism

Cellular Respiration of Glucose 3 major cascades:

1. Glycolysis

2. Krebs Cycle

3. Electron Transport Chain

Where does glucose catabolism happen?

cytoplasm/cytosol and the mitochondria

Glycolysis Goal

catabolic break down of glucose to form two pyruvates, produce 2 ATP

• who: all living cells on earth (aerobic/anaerobic)

• Where: the cytosol of the target cell liver/muscle/adipose (cytoplasm: the fluid part)

Glycolysis what’s happening

a 6 carbon molecule (glucose) is enzymatically broken down into two 3 carbon molecules (pyruvate) by 10 enzymes & there is a net of 2 ATPS

What is NADH

NADH =Byproduct of catabolic process “reduced form” (aka: reducing equivalents) of NAD

NAD

co-enzyme of NADH

• Nicotinamide adenine dinucleotide

What’s happening in the Krebs cycle?

• A series of enzymatic reactions that occur inside all aerobic organisms (NOT bacteria)

• involves the oxidative metabolism of acetyl units and is the main generating source of reducing equivalents

oxidative phosphorylation

In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP

ATP synthase

embedded in the inner mitochondrial membrane is a protein pore complex

• turbine that is powered by the flow of H+ ions across the inner membrane down a gradient and into the mitochondrial matrix

• As the H+ ions traverse the complex, the shaft of the complex rotates

• This rotation enables other portions of ATP synthase to encourage ADP and Pi to create ATP

How many net total ATPs are produced by every glucose molecule?

Therefore, for every glucose molecule that enters aerobic respiration, a net total of ~36 ATPs are produced

Fermentation of glucose by bacteria

• In the rumen or caecum of allo-enzymatic digesters there exists a very large and diverse population of symbiotic bacteria which compete for energy substrates (like glucose)

• Bacteria may live in most sections of the GIT but constraints confine them to the larger organs which have relatively neutral pH ( 6 – 7).

• This pH range is optimal for bacterial enzyme secretion & fermentation

Bacteria and transporting glucose

• Bacteria will attach to polysaccharide particles and secrete their enzymes to cleave the bonds

• The bacteria will transport the glucose inside their cell wall/membrane and ferment it

Fermentation of glucose by bacteria goal

fermentation of glucose provides the bacterium with energy (ATP) and it’s end-products (volatile fatty acids or other intermediates) in turn provide the host animal with a source of energy

• Generally speaking, bacteria convert glucose to 2 pyruvate via glycolysis

• This nets them 2 ATP

Because bacteria lack a mitochondria, the energy yield of fermentation is

MUCH lower than cellular respiration

Pyruvate in fermentation by bacteria

• Pyruvate is the central intermediate molecule which can be converted to a variety of fermentation end-products.

• Disposal of reducing equivalents is the primary factor determining the fate of pyruvate.

Major fermentation end products

Ethanol

Lactate

Acetate

Propionate

Butyrate

• Acetate, propionate, butyrate are 3 primary VFAs

Glucose to pyruvate is

Glycolysis

NADH and bacteria

What is easy and preferred in fermentation?

• Formation of NADH is necessary for bacteria to reduce glucose BUT does not generate ATP from the pumping of H because bacteria LACK a mitochondria!

• Therefore, bacteria must cleave the H off NADH to form NAD+ to keep reducing new glucose that come in. IF too much NADH accumulates, pH of the bacteria drops and the bacteria may stop fermenting or die.

• They secrete dehydrogenase enzymes to cleave the H and help them get rid of excess H inside them.

• Understand that fermentation end-products that result in a NET=0 of reducing equivalents are easy and preferred.

• Accumulation of NADH can become a problem and limit the bacteria from producing that type of end-product, in favor of a different end-product with a more favorable balance.

What is ethanol

alcohol, 2 carbon

• Reducing equivalents are recycled (Net = 0)

• in the absence of oxygen, this is a simple and quick way to make energy and recycle reducing equivalents

• thanol can be toxic/lethal to bacteria = self limiting

• Ethanol can be absorbed into the blood stream via passive diffusion across the rumen or caecal wall

• target organ = liver

• ethanol is oxidized to acetate

Lactate

(3 Carbon)

• Reducing equivalents are recycled (Net=0)

• another simple/quick way to make energy

• Lactate is passively diffused into the blood to the target organ, the liver - This process is very slow and costly

Lactate limiting factors

• Typically only occurs when the pH of the fermentation is below 6

• pH of an environment can affect the enzymes being secreted & potentially change the fate of pyruvate!

• Lactate is usually only produced when the diet is high in readily available starch & sugar.

• Highly influenced by the rate of glycolysis

• Lactic acid is a strong acid and can reduce pH of the bacterial environment.

• In large quantities it can also reduce the pH of the host animals blood causing metabolic acidosis.

Acetate

• Acetate (2 C)

• Acetate is always the most abundant VFA produced regardless of diet

• Net ATP: 4

• Net reducing equivalents: 4 (FDH2)

• Limiting factor: Must be able to get rid of all the reducing equivalents.

• CO2 and H produced from acetate production is used by methanogens

Acetate in animal cells of the host

• In the animal cells of the host: Acetate is passively diffused across the rumen wall, into the bloodstream, carried to the liver first.

• 20% of acetate is converted to Acetyl-CoA in liver cells for use in cellular respiration (enters Kreb’s cycle contributes ~ 10 ATP on it’s own.)

• 80% escape to other organs/muscle/adipose tissue

• Enter the Kreb’s cycle

• Fatty acid synthesis—> Acetate is the primary precursor for body condition and milk fat synthesis

Propionate

3 Carbon

• Why: Maximize ATP AND dispose of reducing equivalents. ( 2 pathways)

• 5 enzymes & 4 intermediates

• Net ATP: 4

• Net NADH: -2 (production of Pro is actually a H+ sink!)

• Succinate Pathway Most common (> 2/3 use this pathway)

H sink

reduces H available for CH4 production) and does not sacrifice C to CO2

• From a feed efficiency perspective propionate production is favorable because of the H sink

What bacteria can ferment lactate to propionate in acrylate pathway?

Megasphaera elsdenii

Propionate is the __ produced in fermentation

Propionate is the 2nd most abundant VFA produced in fermentation

• produced when there is high hydrogen in the environment, propionate can make energy and not make hydrogen as well

Propionate in the animal cells of the host

• Propionate passively diffuses across the rumen wall, into the blood, to the liver.

• Gluconeogenesis

  • Enters the Kreb’s cycle via Succinyl-CoA (will condense with a CO2 to form Succinate)

  • Reversed back to lactate→ Pyruvate→ Glucose → where needed

  • This is pathway captures Carbon!

Do we like hydrogen in the cell?

No

Butyrate

• Net ATP: 3

• Net RE: 2

• Net 2 CO2

• 60% of Butyrate is formed by the condensation of Acetate

• Limiting factor: if acetate production is compromised butyrate will also be compromised.

Butyrate in the animal cells of the host

• Butyrate is passively diffused across the rumen or large intestine wall.

• 70 - 95% of butyrate is used as energy by the enterocytes/ colonocytes themselves!

• Through `Butyrate→2 Acetyl-CoA →Kreb’s cycle of enterocyte mitochondria (~27 ATP)

BHB

β-hydroxybutryrate

• BHB can be used by the brain cells for fuel if glucose is low.

• BHB is the dominant ketone body produced by the liver (78% of ketones)

• Testing for ketosis, means testing for bhb

Ketone

serve as energy as a safety net when body is in starvation

Total Useable Energy/ mole glucose (kcal) for Glucose

686.0

What is the primary lipogenic (fat forming) metabolic precursor

Acetate

• simplest end-product to produce by bacteria but yields the lowest energy efficiency to the host animal.

primary gluconeogenic (glucose forming) metabolic precursor

Propionate

• most energy efficient VFA end-product to the host anima

What is butyrate a primary fuel source for?

For enterocytes & colonocytes to produce their ATP and is necessary for gut health

Peak fermentation occurs

3 – 12 h after a meal

Acetate will always be the

greatest VFA produced regardless of diet, then Propionate, then Butyrate

As starch in the diet increases

As starch in the diet increases, propionate generally increases relative to acetate. (but the ratio will never be inverted)

What dictates the fate of glucose?

Digestive orientation and diet source

What does glucose require for transportation into enterocytes?

Glucose requires SGLUT-1 for transportation into enterocytes from the lumen and is dependent on sodium.

What happens to glucose absorbed from small intestine?

Cellular respiration derives _ and may be _ during times of _

• Glucose absorbed from the small intestine is delivered to target organs and tissue.

• Cellular respiration derives cellular energy & may be stored during times of high energy intake

Glucose may be fermented by

bacteria which produce VFA & other viable end-products for host benefit

Bacterial fermentation yields

lower energy than cellular respiration

Acetate is produced in the _

Acetate is the metabolic _

• highest quantity followed by propionate and butyrate

• precursor for body fat and milk fat synthesis (lipogenic)

Propionate is the metabolic _

Propionate has the _ conversion _ for _

• Propionate is the metabolic precursor for gluconeogenesis in the liver

• Propionate has the highest conversion efficiency for the animal host

What affects the proportion of VFA produced?

Diet