NST 103 MT 2 (Sul)

In fasting/starvation…

Low insulin & High glucagon and epinephrine in the blood

Protein synthesis is suppressed

Cholesterol synthesis is low

FA & TAG synthesis are negligible

while they last, glycogen supplies to maintain plasma glucose levels and used for energy

FFA are released from stored TAG, β-oxidized to acetyl CoA and (1) used directly for energy or (2) converted to KB (liver) which can be used by other tissues for energy

Protein catabolism releases AAs that can be used for energy either directly (via pyruvate and/or TCAC) or indirectly (converted to glucose in liver, utilized for energy in different tissues

cAMP and glucagon + epinephrine

glucagon & epinephrine bind to receptors on the membrane, which activates G protein

• GDP converted to GTP (active form)

GTP stimulates adenylyl cylcase (AC, enzyme in the membrane), which converts ATP into cAMP

cAMP stimulates PKA, which phosphorylates proteins, influencing their activity

substrates in GNG

• Lactate:

  • Produced by anaerobic glycolysis in muscles and red blood cells

  • converted into pyruvate in the liver

• Glycerol:

  • Released from the breakdown of triglycerides in adipose tissue

  • an be converted into DHAP (intermediate)

• Amino acids (especially alanine):

  • During periods of fasting or starvation, muscle proteins break down, releasing amino acids.

  • converted into intermediates such as pyruvate or oxaloacetate

• Propionate:

  • breakdown of certain fatty acids and can be converted into succinyl-CoA.

GNG vs glycolysis

	GNG	glycolysis
purpose	synthesize glucose from non-carbohydrate precursors	break down glycose into pyruvate to produce ATP
when	fasting	high energy demand
location	liver, kidneys	all cells, esp muscle and brain
starting material	lactate, glycerol, glucogenic amino acids	glucose
end products	glucose	pyruvate (lactate in anaerobic conditions)
energy requriment	consumes 4 ATP and 2 GTP	produces net gain of 2 ATP p
key regulatory enzymes	pyruvate carboxylase, PEP carboxykinase, fructose-1,6-biphosphate, glucose 6-phosphatase	hexokinase, PFK1, pyruvate kinase
hormonal regulation	glucagon, cortisol: promote glucose production during fasting	insulin: romotes glucose breakdown to provide energy or store as fat.

3 bypass steps in GNG

Gluconeogenesis involves bypassing the irreversible steps of glycolysis.

• Pyruvate → Oxaloacetate → Phosphoenolpyruvate (PEP):

  • Enzymes: Pyruvate carboxylase and PEP carboxykinase.

• Fructose-1,6-bisphosphate → Fructose-6-phosphate:

  • Enzyme: Fructose-1,6-bisphosphatase.

• Glucose-6-phosphate → Glucose:

  • Enzyme: Glucose-6-phosphatase.

Pyruvate to Phosphoenolpyruvate (PEP)

glycolysis: Pyruvate kinase (converts phosphoenolpyruvate to pyruvate)

bypass:

• Pyruvate carboxylase: Converts pyruvate to oxaloacetate in the mitochondria. This step requires ATP.

• Phosphoenolpyruvate carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP). This step requires GTP.

• This bypass requires energy (1 ATP and 1 GTP) and occurs in two steps, starting in the mitochondria and moving to the cytosol.

Fructose-1,6-bisphosphate to Fructose-6-phosphate

• Glycolysis Enzyme: Phosphofructokinase-1 (PFK-1) (converts fructose-6-phosphate to fructose-1,6-bisphosphate)

• Bypass Enzyme in Gluconeogenesis:

  • Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.

• This step is a simple hydrolysis reaction and does not require ATP.

Glucose-6-phosphate to Glucose

• Glycolysis Enzyme: Hexokinase/glucokinase (converts glucose to glucose-6-phosphate)

• Bypass Enzyme in Gluconeogenesis:

  • Glucose-6-phosphatase: Converts glucose-6-phosphate to glucose.

• This step occurs in the endoplasmic reticulum (ER) and allows free glucose to be released into the bloodstream.

regulation of glycolysis by glucagon and epinephrine

Glucagon and epinephrine trigger a cascade that leads to the phosphorylation of key enzymes

PFK-2 is phosphorylated and becomes inactive, which reduces levels of F-2,6-BP

• leads to decreased stimulation of PFK-1, reducing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.

Pyruvate kinase is phosphorylated and inactivated. This enzyme normally converts PEP (phosphoenolpyruvate) to pyruvate in glycolysis, and its inhibition further slows glycolysis.

regulation of glycogenolysis by glucagon and epinephrine

• Glycogen synthase (the enzyme that builds glycogen) is phosphorylated, which inactivates it.

• Glycogen phosphorylase, the enzyme responsible for breaking down glycogen into glucose-1-phosphate, is phosphorylated and activated.

  • promotes glycogenolysis, leading to the breakdown of glycogen and an increase in glucose availability.

regulation of gluconeogenesis by glucagon and insulin

Glucagon and epinephrine stimulate gluconeogenesis (GNG) when glucose is needed, such as during fasting.

Insulin inhibits GNG during the fed state, promoting glucose storage and usage instead of glucose production.

factors that influence pyruvate carboxylase

ATP required for conversion to oxaloacetate

Acetyl CoA is an allosteric activator, directing pyruvate into GNG

• levels rise during beta-oxidation (fasting)

PEPCK regulation by gene expression

• Glucagon (during fasting) increases the expression of PEPCK, promoting gluconeogenesis.

• Insulin reduces PEPCK expression, inhibiting gluconeogenesis in the fed state when glucose availability is high.

factors that regulate Fructose-1,6-bisphosphatase-1 (F16BPase-1)

• AMP acts as an allosteric inhibitor, reducing the activity of F16BPase-1, preventing gluconeogenesis when cellular energy levels are low.

• F-2,6-BP is also an allosteric inhibitor. During fasting, phosphorylation inhibits PFK-2, decreasing F-2,6-BP levels in the liver, which helps promote gluconeogenesis.

factors that influence Glucose-6-phosphatase

enzyme is located on ER and is influenced by substrate availability.

high Km: glucose-6-phosphate (G6P) must accumulate in higher concentrations for it to be converted to glucose.

Glucagon increases and insulin decreases the gene expression of glucose-6-phosphatase, similar to PEPCK, to regulate glucose output during fasting or feeding.

regulatory factors for gluconeogenesis

• Glucagon stimulates gluconeogenesis during fasting by increasing the expression of key enzymes like PEPCK and glucose-6-phosphatase.

• Insulin inhibits gluconeogenesis during feeding by reducing the expression of these enzymes.

• Allosteric regulators like AMP and F-2,6-BP inhibit gluconeogenesis when cellular energy or glucose is abundant, providing feedback to balance energy metabolism.

how glucagon and epinephrine affects the regulation of glycolysis and gluconeogenesis through the levels of fructose-2,6-bisphosphate (F-2,6-BP)

low F-2,6-BP levels due to gluycagon and epinephrine:

phosphorylation of PFK-2 redcues F-2,6-BP], removing its stimulatory effect on PFK-1, and inhibitory effect on F1,6-BPase-1

When F-2,6-BP is low (due to PFK-2 phosphorylation), F1,6-BPase-1 activity increases, promoting the conversion of fructose-1,6-bisphosphate (F-1,6-BP) back to fructose-6-phosphate (F-6-P) in gluconeogenesis. This favors the production of glucose.

the reduction in F-2,6-BP reduces the activity of PFK-1, slowing down glycolysis (

gluconeogenesis pathway

1. Pyruvate to Oxaloacetate (OAA):

• In the mitochondria, pyruvate is converted to oxaloacetate (OAA) by the enzyme pyruvate carboxylase. This reaction requires ATP and produces CO₂.

• Aspartate can also be converted to OAA through transamination reactions.

2. OAA to Phosphoenolpyruvate (PEP):

• OAA cannot cross the mitochondrial membrane, so it must be reduced to malate (or converted to aspartate) and transported to the cytosol.

• Once in the cytosol, malate is oxidized back to OAA.

• OAA is then converted to PEP by the enzyme phosphoenolpyruvate carboxykinase (PEPCK), which requires GTP.

3. Fructose-1,6-bisphosphate to Fructose-6-phosphate:

• The process continues through reversible steps of glycolysis until fructose-1,6-bisphosphate (F-1,6-BP) is converted to fructose-6-phosphate (F-6-P) by the enzyme fructose-1,6-bisphosphatase.

4. Glucose-6-phosphate to Glucose:

• In the final step, glucose-6-phosphate (G6P) is converted to free glucose by the enzyme glucose-6-phosphatase in the liver. This step occurs in the endoplasmic reticulum and allows glucose to be released into the bloodstream.

why Acetyl-CoA can’t be used for glucose synthesis

• Although intermediates in the TCA cycle (such as oxaloacetate and fumarate) can be used as substrates for gluconeogenesis, Acetyl-CoA itself cannot contribute to the net synthesis of glucose because its carbons are lost as CO₂ during the TCA cycle.

• Even though fatty acids break down to generate Acetyl-CoA, this breakdown does not contribute carbons for glucose synthesis since the carbons from Acetyl-CoA are lost in the cycle.

Acetyl-CoA and TCA cycle

• Acetyl-CoA (2 carbon atoms) enters the TCA cycle (also known as the citric acid or Krebs cycle) by combining with oxaloacetate (OAA) (4 carbon atoms) to form citrate (6 carbon atoms).

• As the cycle progresses, two carbon atoms are lost as CO₂ during the conversion of citrate to α-ketoglutarate and succinyl-CoA. This means that for every molecule of Acetyl-CoA that enters the cycle, two carbon atoms are lost, which prevents the net synthesis of carbon-containing molecules like glucose.

nutrient requring reactions in GNG

magnesiun: associated with ATP and GTP

Niacin: component of NAD/NADH

biotin

in foods:

• widely distributed

• present in free and bound forms

• avidin (protein): biotin inhibitor found in egg and can be inactivated by heating

in lumen

• bound biotin hydrolyxed via pancreatic biotinidase, releasing free biotin

absorprtion of free biotin

simple diffusion

na dependent secondary active transport

role of biotin as a coenzyme

in carboxylation reactions by serving as a carrier of a carboxyl group (from bicarbonate) to various substrates.

how is biotin used as a coenzyme

Biotin is attached to a lysine residue of an enzyme to form a biotinyl-lysine-enzyme complex.

In the presence of ATP, HCO₃⁻ is activated and attached to biotin, forming the carboxybiotinyl enzyme complex.

• involves the consumption of ATP, which is converted to ADP Pi

• The carboxyl group (COO⁻) is transferred from the biotinyl-enzyme complex to a receiving molecule, which completes the carboxylation reaction.

• Once the carboxyl group is transferred, biotin is regenerated in the enzyme complex, allowing it to participate in further reactions.

what determines our energy needs

basal metabolic rate

thermic effect of food

thermoregulation (thermogenesis)

physical activity (daily life and exercise)

energy expenditure measurments (BMR)

direct calorimetery: measure heat release from body by measuing changes in the temperature of the environment

indirect calorimetry: measure oxygen uptake and CO2 release as surrogate for heat production

• use doubly habelled H2O - based on difference between turnover rate of H and O of body H2O as a function of CO2 production (by product from cell respiration)

what determines BMR/RMR

lean body mass

hormone levels (thyroid, catecholamines)

body surface area

abnormal body temperature (fever)

age (decreased lean body mass)

pregnancy

caffiene & tobacco use

gender: males > females

Kleier’s equation

regression equation: log M = 0.756logW + 1.87

simplified metabolic body size (W^3/4): M = 70 x W^3/4

energy balance

= intake - expenditure

positive balance: weight gain

negative balance: weight loss

1 lb gain in body fat requires ~3500 kcal

if the gain is over 1 year, energy balance averages +10 kcal/day

energy from diet and body stores to meet daily needs

dietary macronutrients (CHO, fat, protein)

absorbed nutrients

metabolic fuels

stored energy in complex molecules

use of metabolic fuels

fuel needs are tissue specific

maintaining an adequate glucose supply to meet the needs of the brain is central

glucose sparing: utilization of non-glucose substrates for energy by tissues that are not obligatory glucose users

synergistic relationships among organs are essential

what organs use glucose as energetic substrates

brain

RBC & retina

adipose

skeletal muscle

intestine

kidney

liver

heart

what organs use ketone bodies as energetic substrates

brain

skeletal muscle

intestine

kidney

liver

heart

what organs use fatty acids as energetic substrates

adipose

skeletal muscle

kidney

liver

heart

what organs use glycogen as energetic substrates

skeletal muscle

liver

what organs use amino acids as energetic substrates

skeletal muscle (BCAA)

intestine (Gln)

Kidney (Gln)

Liver

what organs use Lac as energetic substrates

kidney

liver

heart

obligatorily glycolytic tissues

brain

RBC and retina

coordination between liver and other organs through metabolite cycles

• glucose-lactate (cory cycle)

• glucose-alanine (cahill cycle)

• glucose-fatty acid (randle cycle): inhibition of glc utilization when FA’s are present (heart, muscle)

• importance: spare glucose by peripheral tissues for utilization by brain

insulin, glucagon, epinephrine/norepinephrine receptors and their action

bind to cognate plasma membrane receptors and activate intracellular signaling (kinase/phosphatase, Ca++ signaling)

• peptide, water soluble hormones

glucocorticoids & thyroid hormone receptors and their action

bind to cognate receptors (GR and TR) and affect transcription via hormone/receptor binding to specific DNA element in nucleus

• lipid soluble hormones

major hormones that influence energy metabolism

anabolic

• insulin (muscle, adipose, other)

counter regulatory

• glucagon (liver)

• catecholamines (liver, adipose, muscle, and other)

• glucocorticoids (most tissues)

thyroid hormone (most tissues)

influence of starvation on blood levels of substrates and hormones

insulins role in glucose homeostasis: high plasma insulin concentration

minimize elevations in plasma glucose concentration

INCREASED

• glucose uptake by peripheral cells

• glucose metabolism

• glycogen and protein synthesis

to move glucose out of blood

DECREASED

• gluconeogenesis by liver

• glycogen breakdown

• lipolysis by adipose sugar

reduce glucose entry into blood

glucagons role in glucose homeostasis: high plasma glucagon concentration

minimize the decrease in plasma glucose concentration

INCREASED

• glycogen breakdown

• gluconeogenesis

• ketogenesis (reduces glucose utilization)

DECREASED

• glycogen synthesis

metabolic effects of catecholamines are similar to those of glucagon

catecholamines (epinephrine/norepinephrine from adrenal medulla and norepinephrine from nerve endings) are released in the fight or flight response to stress and stimulate

• release of glucose from glycogen in liver

• release of FAs and glycerol from TAG in adipose tissue

glucocorticoids

secreted by adrenal cortext in response to stress, including fasting and exercise

results in..

• increased GNG (from muscle protein breakdown)

• increased lypolysis (release FA for energy, spare glc)

• increased ketogenesis (used for energy, spare glc)

• decreased synthesis of protein & TAG

does thyroid hormone regulate plasma glucose levels

not directly

stimulatory effect on BMR: stimulates uncopuling and increases Na+/K+ ATPase activity

thyroid hormone effects

from thyroid glands

• increases glucose utilization

• increases protein synthesis, promoting growth,

• stimulates FA synthesis and esterification to TAG

enthalpy and its relationship to nutrition and metabolism

change in enthalpy (ΔH) = heat absorbed (q) - work done (w)

heat (q) warms the body, sparing energy to perform work (w)

burning food in a bomb calorimeter will provide a measure of ΔH because w = 0 under these conditions

Why is measuring ΔH not accurate for true energy value

factors cause the biological energy value to be less than the value predicted by measuring ΔH

• completeness of absorption

• completeness of oxidation

• efficiency with which W can be captured during metabolism under physiological conditions

Atwater factors

Protein: 4 kcal/g

Fat: 9 Kcal/g

Carbohydrate: 4 kcal/g

values are roughly correct for…

1) difference in completeness of chemical vs biological oxidation

2) absorption efficiency

Gibbs Free Energy

predicts the direction of reaction / spontaneity

ΔG = ΔH - TΔS

if ΔG ~0: rxn is near equilibrium and direction can be easily reversed with a change of substrate or product concentration

if ΔG is ±: it is non equilibirum and proceeds in forward (-ΔG) or reverse (+ΔG) direction

How does the body use ΔH?

Energy in enthalpy must be packed into ATP that the body can utilize for metabolic purposes

2 reactions are linked: 70% of the chemical energy is converted to biological energy

hydrolysis of phosphate bonds

releases high energy

• energy released is not constant for all phosphate bonds

• influenced by resonance stability, charge repulsion, and ionization

ΔG^o for selected compounds

thioester

R-S-CO-R'

have large, negative ΔG^o

Pantothenic acid

component of ACP and CoA

essential nutreint

obtain from foods and intestinal microflora

UCP1

unique to brown adipose tissue with high mitochondria

permits reentry of H+ into mitochondrial matrix

generates heat rather than ATP for body temperature

coupling respiration from ATP production by UCP1 dissipates heat (thermogenesis)

Nutrients required for universal electron carriers

NAD+, NAPD+: Niacin

FMN, FAD: riboflavin

Nutrients required for electron transport chain

I NADH-dehydrogenase complex-Q: niacin, riboflavin, Fe-S

II Succinate dehydrogenase complex -Q: riboflavin, Fe-S

III cytochrome b-c1 complex cytochrome c: Fe (heme), Fe-(S)

IV cytochrome oxidase complex: Fe (heme), Cu

acquiring niacin

ingested as nicotinic acid and nicotinamide

widely distributed

bound to carbohydrates in corn, released with lime (CaOH2, alkali treatment)

may be synthesized from tryptophan (60 mg tryptophan = 1 NE)

transport of niacin

absorbed from lumen via 2 active transporters

transported to blood and cells

niacin deficiency

pellegra (rough skin)

symptoms: dermatitis, diarrhea, dementia (3Ds)

was endemic with corn from new world

now rare, associated with corn (low in tryptophan) without lime treatment to release niacin, or sorgym (low in niacin and tryptophan)

riboflavin

component of FMN and FAD

ingested as FMN and FAD from meat and green leafy vegeables

enzymatic hydrolysis releases free riboflavin in lumen

transported to blood, where it binds to proteins

riboflavin deficiency

ariboflavinosis

stunted growth

dermatitis

burning of mucous membranes

anemia

neuropathy

assessing riboflavin status

measure FAD-dependent glutathione reductase activity in erythrocytes

• GSSG + NADPH + H —→ 2GSH + NADP+

• catalyzed by glutathione reductase

with and without FAD for activity coefficient

high number = low endogenous FAD

anaplerotic function of the TCA cycle

TCA cycle intermediates can serve as precursors for synthesis of new compounds

entry of new carbon into the cycle is accompanied by exit via intermediates

any carbon that enters TCAC via an anaplerotic pathway will not result in net oxidation, but used in anotehr route

anaplerosis

reactions that allow the entry of carbon into the pools of TCAC intermediates (other than via acetyl-CoA)

process replenishes intermediates that are used for other biosynthetic purposes, allowing the TCA cycle to continue running efficiently.

example of anaplerosis

glutamine metabolism feeds into the TCA cycle via α-ketoglutarate, which can eventually replenish oxaloacetate, allowing for gluconeogenesis (the synthesis of glucose).

glutamate for TCA cycle in cancer cells

Glutamine in circulation is the highest among amino acids

Glutamine used for biosynthesis of purine & pyrimidine (DNA synthesis for cell proliferation)

Glutamine for FA synthesis (membrane phospholipid synthesis)

pyruvate dehydrogenase (PDH)

catalyzes the irreversible oxidative decarboxylation of pyruvate, converting it into acetyl-CoA

1. decarboxylation of pyruvate

2. oxidation of 2C fragment, NADH formed

3. acetyl coa formation: 2C + CoA

complex of 3 enzymes

requires 5 coenzynes

• CoA: panthanoic acid

• NAD+: niacin

• FAD: riboflavin

• lipoic acid

• thiamine pyrophosphate: Vit B1

regulated by phosphorylation and dephosphorylation

• PDH kinase inactivates PDH by phiosphoylation when energy is high

• PDH phosphatase activates PDH by dephosphorylation when ADP, pyruvate, or calcium is high

5 coenzymes required by PDH

• CoA: panthanoic acid

• NAD+: niacin

• FAD: riboflavin

• lipoic acid

• thiamine pyrophosphate: Vit B1

pyruvate dehydrogenase complex

E1: Pyruvate dehydrogenase (use thiamine) E2: Dihydrolipoamide transacetylase (use lipoic acid)

E3: Dihydrolipoyl dehydrogenase (use riboflavin & niacin)

E1: Pyruvate dehydrogenase

catalyzes the decarboxylation of pyruvate producing a 2-carbon hydroxyethyl group attached to TTP

thiamine pyrophosphate (TPP). TPP is derived from vitamin B1 (thiamine)

• stabilizes the reaction intermediate during the decarboxylation.

E2: Dihydrolipoamide transacetylase

transfers the hydroxyethyl group from E1 onto lipoic acid (a cofactor attached to E2), which gets reduced in the process.

then transfers the resulting acetyl group to Coenzyme A (CoA), forming acetyl-CoA.

E3: Dihydrolipoyl dehydrogenase

regenerates the oxidized form of lipoic acid, allowing the cycle to continue.

uses FAD to oxidize the reduced lipoic acid, and the FADH₂ generated is subsequently reoxidized by NAD⁺, producing NADH.

α-Ketoglutarate Dehydrogenase (αKGDH)

parallel enzyme system that operates similarly to the PDH complex but works on α-ketoglutarate in the TCA cycle.

uses similar cofactors and mechanisms to convert α-ketoglutarate into succinyl-CoA, generating NADH and CO₂ in the process.

Co Factors used in what stages of PDH Complex

• Thiamine (B1) for E1.

• Lipoic acid for E2.

• Riboflavin (B2) and Niacin (B3) for E3.

thiamine

Thiamine pyrophosphate (TPP) is essential in:

• Decarboxylation of a-keto acids (PDH, aKGDHand metabolism of BCAA’s)

• Transketolation reactions (pentose phosphate pathway)In foods (and cells) thiamine is phosphorylated and present as TPP, which must be dephosphorylated to cross membranes

Thiamine deficiency

Thiamine is readily available in whole grain cereals, mostly in the outer layers of the grain, but is removed with milling & refining.

If the “white” flour (or rice) is not fortified, thiamine deficiency can occur.

can also occur with alcoholism

deficiency disease: berberi

• nervous and cardiovascular disorders

• confusion

• anorexia

• weakness followed by loss of muscular functions

following thiamine treatment, rapid improvemnet occurs

How is thiamine absorbed

free thiamine can be absorbed in free form

jejunum lumen: TPP must be dephosphorylated to be absorbed in the body

• pyrophosphatase and phosphatase dephosphorylatte TPP, resulting in free thiamine

• Once free thiamine is formed, it can be absorbed into the enterocytes (intestinal cells) via both passive and active transport mechanisms

in enterocytes, thiamine is phosphorylated to form thiamine diphosphate (TDP, active form)

Ii tissues, thiamine can be dephosphorylated back into its free form to enter the bloodstream or be rephosphorylated depending on cellular needs

Thiamine typically circulates as free thiamine in the plasma, while its active diphosphate form (TDP) is predominant inside cells.

Lipoic acid

function

• energy production

• reduce oxidative damage to membrane lipids (antioxidant)

• can be synthesized

regulation of flux through PDH and TCA cycle

Energy status (most important)

NAD+/NADH

Substrate activation

Product inhibition

Phosphorylation

Allosteric regulation

Regulation of E1 subunit of PDH

• PDH Kinase phosphorylates and inactivates PDH in response to high energy levels (high NADH, ATP, acetyl-CoA).

• PDH Phosphatase dephosphorylates and activates PDH in response to signals like calcium (indicating muscle contraction) and insulin (indicating the fed state).

Inhibitory energy indicators

PDH

• NADH

• ATP

• GTP

CS

• NADH ATP

• ICDH

• NADH

ICDH:

• NADH

• ATP

alphaKDGH

• NADH

• ATP

energy indicators that activate..

PDH

• NAD

• ADP

• AMP

CS: N/A

ICDH:

• NAD+

• AMP

• ADP

alphaKDGH

• AMP

high mitochondrial [citrate] effect on TCA cycle flux

When citrate levels in the mitochondria are high, it acts as a feedback inhibitor to slow down the TCA cycle, preventing unnecessary energy production.

The citrate is exported to the cytosol, where it is used for fatty acid synthesis

avoidance of citrate inhibition of CS

Pyruvate is generated when glucose flux through glycolysis is high.

High [pyruvate] in mitochondria activates PDH (over-riding energy status signals), and acetyl-CoA is converted to citrate.

If flux through ICDH is saturated, citrate is transported out to cytosol.

High cytoplasmic [citrate] activates acetyl-CoA carboxylase (ACC) stimulating lipogenesis

energetic stresses that activate AMPK

• glucose deprivation

• oxygen deprivation

• ischemia

• metabolic posions

• exercise (muscle contraction)

activation of AMPK

AMP binds AMPK with very low Kd (ultrasensitive)

High [ATP] antagonizes AMP/AMPK binding

Ca activates CaMKKb which phosphorylates AMPK

PhosCr (high energy status of muscle) inactivates AMPK

metabolic effects of AMPK activation

AMPK and insulin sensitivituy

AMPK activation is independent of insulin action

Thus, AMPK activation stimulates uptake of glucose into cells in a manner that is insulin independent

This may be one mechanism by which metformin increases insulin sensitivity in type 2 diabetes.

ADPK and adipokines

White adipose tissue (WAT) secretes adipocyte hormones (adipokines) to regulate energy metabolism.

Leptin: increases as WAT expands.

• decreases food intake, Increases energy expenditure & thermogenesis.

• One of the leptin action is by inhibiting AMPK in hypothalamus to decrease food intake.

Adiponectin: Increases food intake by activating AMPK in hypothalamus.

How does NADH enter the mitochondria for ETC?

• Malate-Aspartate Shuttle: transfer of reducing equivalents from cytosolic NADH to mitochondrial NADH without loss of energy,

• Glycerophosphate Shuttle: Transfers reducing equivalents from NADH to FADH₂, which results in lower ATP yield

Net Energy Yield From Anaerobic Metabolism of Glucose (Glycolysis)

Net Energy Yield From Aerobic Metabolism of Glucose (TCAC)

key non-equilibrium reactions for TCA cycle

• Pyruvate Dehydrogenase (PDH):

  • Converts pyruvate to acetyl-CoA and generates NADH.

  • links glycolysis to the TCA cycle.

  • releases CO₂ and generates NADH,

• Citrate Synthase:

  • Catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate.

• Isocitrate Dehydrogenase (ICDH):

  • Converts isocitrate to α-ketoglutarate, generating NADH and releasing CO₂.

• α-Ketoglutarate Dehydrogenase (αKGDH):

  • Catalyzes the conversion of α-ketoglutarate to succinyl-CoA, generating NADH and releasing CO₂.

production of produce NADH, FADH₂, and ATP in TCA

• NADH is generated in the steps catalyzed by pyruvate dehydrogenase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase.

• FADH₂ is generated in the conversion of succinate to fumarate by succinate dehydrogenase.

• ATP (or GTP) is generated in the conversion of succinyl-CoA to succinate by succinyl-CoA synthetase.