Lecture 5: Cell MetabolismBiology 219

Dr. Adam Ross

Cellular Respiration

• Set of reactions that take place during the conversion of nutrients into ATP

• Intricate regulatory relationship between several “families” of reactions

• Goal for this class is to understand relationships and regulatory mechanisms, not to memorize intermediate substrates, names of enzymes, or steps of reactions.

Two general classes of reaction

• Aerobic• Requires oxygen

• Endurance exercise

• Anaerobic• Does not require oxygen

• Strength exercise

Courtesy of Richard Wheeler via visual.ly

Glycolysis to ETC

Blood

vessel

Carrier

protein

Glucose

Oxygen

Extracellular fluid

Plasma

membrane

Glycolysis

glucose pyruvate

Mitochondrion

Citric

Acid

Cycle

Electron

Transport

Chain

Transition

Reaction

Electrons

transferred

by NADHCytoplasm

Electrons

transferred

by NADH

and FADH2

Electrons

transferred

by NADH

+32 ATP 36 ATP+2 ATP +2 ATP

Glycolysis

• Occurs in the cytosol

• Breakdown of glucose (6-C) into two pyruvate (3-C) molecules

• Multi stage pathway:• Energy investment stage- adding ATP to isomerize glucose• Cleavage stage- split 6-C into two 3-C molecules• Energy harness stage- utilize free energy with 2NADH and 4ATP (2 net ATP)

• NADH is a carrier of high energy electrons which are used by the electron transport chain

• Pyruvate is the branch point between aerobic and anerobicmetabolism• Where the molecule is sent is determined by the oxygen status of the cells in

need of energy

Glycolysis

• Start with Glucose

• Add 2 ATP

• End with• 2 Pyruvate

• 4 ATP (2 net)

• 2 NADH

Glycolysis

Figure 3.23

Cytoplasm

2 ADP

Glucose

Energy-

investment

phase

Glycolysis (in cytoplasm)

2 NADH

2 NAD+

4 ATP

4 ADP

Energy-

yielding

phase

2 ATP

The two molecules of

pyruvate then diffuse

from the cytoplasm into

the inner compartment

of the mitochondrion,

where they pass through

a few preparatory steps

(the transition reaction)

before entering the citric

acid cycle.

During the remaining

steps, four molecules

of ATP are produced.

During the first steps,

two molecules of ATP are

consumed in preparing

glucose for splitting.

2 Pyruvate

Two molecules of nicotine

adenine dinucleotide

(NADH), a carrier of

high-energy electrons,

also are produced.

Pyruvate

• If sent to the aerobic pathway (cells have enough oxygen)• Goes to transition step and TCA cycle in mitochondria

• TCA cycle can be called Kreb’s cycle on Citric Acid cycle (in other classes)• In Bio 219 it is called the Tri-Carboxlic Acid cycle (TCA cycle)

• If sent to the anaerobic pathway (cells are oxygen poor)• Pyruvate is converted to lactate in the cytosol

• Via lactic acid pathway

Pyruvate

Transition reaction

• Before entering TCA cycle, pyruvate (3-C) must be converted

• pyruvate (3C) + CoA + NAD+ → acetyl CoA (2C) + CO2 + NADH (x 2 per glucose molecule)

yield: 1 NADH (x 2) – 1 for each Pyruvate

product: 1 CO2 (x 2)

• acetyl CoA is a key intermediate which transfers 2C (acyl) units to the Citric Acid Cycle

Transition reaction

• Start with:• 2 pyruvate (3 carbon molecules)

• 2 Coenzyme A

• End with:• 2 CO2

• 2 NADH

• 2 Acetyl CoA (2 carbon molecule)

NADH

(electron passes

to electron

transport chain)

NAD+

Pyruvate (from glycolysis)

Acetyl CoA

CoA

Coenzyme A

CO2

Transition Reaction (in mitochondrion)

Citric Acid Cycle

A molecule of NADH is

formed when NAD+

gains two electrons

and one proton.

One carbon (in the form

of CO2) is removed

from pyruvate.

The two-carbon

molecule, called

an acetyl group,

binds to

coenzyme A

(CoA), forming

acetyl CoA,

which enters the

citric acid cycle.

Transition Reaction

Tricarboxylic Acid Cycle

• Multi-step series of Redox reactions

• Occurs in the matrix of the mitochondria

• 2-C from acetyl CoA is broken down to C02• 1st step: Ac-CoA + oxaloacetate (4-C) > citrate (6-C) + CoA

• Subsequent steps: citrate is converted back to OAA and e- are captured• 2-C atoms in citrate are fully oxidized to form CO2

• Electrons are transferred to NADH and FADH2

• Yield: 3 NADH + 1 FADH2 + 1 ATP (x2)

• Product: 2 CO2 (x2)

Tricarboxylic Acid Cycle

Acetyl CoA

CoA

Citrate

CO2

leaves

cycle

NAD+

NADH

-Ketoglutarate

CO2 leaves cycle

NADH

FAD

Succinate

FADH2

Malate

NAD+

ATP ADP

TCA Cycle

NADH

Oxaloacetate

NAD+

Pi+

Acetyl CoA, the

two-carbon compound

formed during the

transition reaction,

enters the citric acid

cycle.The citric acid cycle also

yields several molecules of

FADH2 and NADH, carriers of

high-energy electrons that

enter the electron transport

chain.

The citric acid cycle yields

One ATP from each acetyl

CoA that enters the cycle,

for a net gain of two ATP.

CoA

Lactic acid pathway

• Pyruvate is sent here when cells are using anaerobic resporation

• 2 pyruvic acid + 2 NADH > 2 lactate and 2 NAD+• Generates 2 Lactate and NAD+ is regenerated for use in glycolysis

Electron transport chain

• Electron carrier proteins are in the inner membrane of the mitochondria• 3 major complexes (I, III, IV)

• Cytochromes are iron containing electron carrier proteins in ETC

• NADH and FADH2 donate electrons to ETC

• Electrons move downhill (to lower energy stations) in ETC• Oxygen is terminal electron acceptor

Electron Transport Chain The molecules of NADH andFADH2 produced by earlier phasesof cellular respiration pass their electrons to a series of protein molecules embedded in the inner membrane of the mitochondrion.

As the electrons are transferred from one protein to the next, energy is released and used to make ATP.2e–

2e–

2e–

2e–

2e–

NAD+

High

Low

Membrane

proteins

H2O

2 H+ + O2Energy released is used

for synthesis of ATP

FADH2

NADH

Electron Transport Chain (inner membrane of mitochondrion)

FAD

12

Po

ten

tial en

erg

y

Eventually, the electrons are passed to oxygen, which combines with two hydrogens to form water.

ATP Synthesis

• Chemiosmotic coupling mechanism• ETC complexes act as H+ ion pumps

• Create H+ gradient, intermembrane space has high [H+]

• H+ moves down its concentration gradient through ATP-synthase• This provides energy to phosphorylate ADP, creating ATP

• 3 H+ ions = 1 ATP

Complex Carbohydrates must first be broken down into glucose before entering glycolysis

Fats and proteins enter the process at different steps

Summary

Glycogen metabolism

• Glycogen synthesis (glycogenesis)• Glucose + ATP → G-6-P → glycogen

• Glycogen is stored mainly in liver and skeletal muscle cells

• Insulin stimulates glucose uptake into cells and glycogen synthesis• Secreted when blood [glucose] is high

• Glycogen breakdown (glycogenolysis)• Glycogen + Pi → G-6-P → glucose

• In liver glycogen → glucose → released to blood

• Glucagon stimulates glycogenolysis in liver when blood [glucose] is low

• In skeletal muscle: glycogen → G-6-P → metabolized by active muscle

Protein metabolism

• Protein catabolism (proteolysis)• Hydrolysis of polypeptides into A.A.

• Deamination of amino acids• Amino acids → keto acids + NH3

• Keto acids can enter TCA cycle

• NH3 converted to urea, and excreted via kidneys in urine

• Trans-amination: NH2 group can be transferred to another A.A.

Tissue utilization of proteins

• In every cell there is some baseline level of protein turnover• Lost proteins are replaced by newly synthesized ones

• Protein metabolism is increased during starvation, injury, exercise and high protein diets

• Essential amino acids• AA that the body cannot synthesize fast enough to survive

• Must eat to stay alive

• Non essential AA can be synthesized from essential AA• There are a number of mechanisms by which AA are synthesized

Lipid metabolism

• Lipolysis- sign of an energy poor cell or organism• Catabolism of lipids

• Hydrolysis of triglycerides• Triglyeride + H2O = 3 fatty acids + glycerol

• Beta-oxidation• Step wise oxidation of fatty acids

• Fats are broken down into 2-C units

• Then converted to acetyl CoA and enter the TCA cycle

• Occurs in mitochondria

• >100 ATP per fatty acid

• >2x more energy per gram than CH2O

Lipid synthesis

• Fas are synthesized from 2-C units of acetyl CoA

• Combined with glycerol to make triglycerides and phospholipids

• Most lipid synthesis occurs in smooth ER

• Insulin stimulates glucose uptake and triglyceride synthesis in adipose cells

• Lipid synthesis is a marker of an energy rich cell or organism

Tissue utilization of fatty acid

• Triglycerides stored mostly in adipose tissue

• Lipids are transported in the blood by lipoproteins (HDL and LDL)

• Liver, heart, and resting skeletal muscle use mainly fatty acids for energy

• Only broken down aerobically

• Excessive fat metabolism causes formation of ketone bodies• Metabolic acidosis

• Type 1 diabetes mellitus

Gluconeogenesis

• Formation of glucose from non-carbohydrate sources• Occurs mostly in the liver

• Important during fasting or glucose depletion (exercise, stress, low carb diet)

• Stimulated by cortisol and glucagon

• Pyruvate, lactate, glucogenic AA, and glycerol can be used

Interconversion of substrates

• Glucose → acetyl CoA → fatty acids (lipid synthesis)

• Amino acids → keto acids → glucose (gluconeogenesis)

• Glucose → keto acids → amino acids (transamination)

• *Note: Fatty acids can NOT be converted into glucose or amino acids

Regulation of blood glucose

• Insulin• Made in the pancreas

• Released in response to an increase in blood [Glucose]

• Insulin binds to insulin receptor

• Causes cells to make more GLT transporters, and also causes the transporters to become more active

• Also causes fat to be stored rather than metabolized

Regulation of blood glucose

• Glucose is a very important molecule for survival• Need it to make energy to survive

• Can be toxic if large concentrations are in blood

• Must have ways to prevent blood [glucose] from getting to high or low

• Insulin and glucagon• There are others, but for the purpose to this class we will simplify the situation

Regulation of blood glucose

• Insulin is released from the β-cells of the pancreas• In response to increased blood [Glucose]

• Causes cells in body to uptake glucose via GLT4 transporter

• Glycogenesis and fatty acid synthesis also occur

Regulation of blood glucose

• Glucagon produced by the α cells of the pancreas• In response to low blood sugar

• Causes the inverse response of insulin• Breakdown of glycogen into glucose in liver

• Stimulates gluconeogenesis from non CH2O precursors

• Eventually blood [Glucose] will rise, and the glucagon response will turn off

Regulation of appetite

• Hunger/ satiety is determined by hypothalamic activity in response to the nutritive state of the body

• Satiety center- in ventromedial nucleus (VMN)• Artificial electrical stimulation elicits feelings of fullness

• Hyperphagia caused by a lesion on this center

• Anorexigenic molecules cause satiety

• Hunger center- in lateral hypothalamus (LHA)• Artificial electrical stimulation elicits large appetite

• Ablation causes aphagia

• Orexigenic molecules stimulate hunger

Boron & Boulpaep 2005

Essential minerals• Calcium

• Bones and intracellular signaling

• Copper• Enzyme cofactor

• Magnesium• Complexes with ATP

• Zinc• Antioxidant, enzyme cofactor

• Phosphorous• Bone, covalent modifications, ATP

Daily requirements

• None for carb or fat intake

• Protein • 0.8g/kg body weight

• Higher in pregnant women, athletes, and postsurgical patients

Summary

• Use of various pathways is determined by nutritive state and presence or absence of different metabolic fuels

• Hormones help to regulate metabolic pathways• Hormones are the response to a change in some homeostatic variable

• They help to induce the changes that will return the body to homeostasis