Download Lecture 5: Cell Metabolism

Lecture 5: Cell Metabolism
Biology 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
Electrons
transferred
by NADH
Blood
vessel
Glucose
Cytoplasm
Electrons
transferred
by NADH
Electrons
transferred
by NADH
and FADH2
Plasma
membrane
Carrier
protein
Citric
Acid
Cycle
Transition
Reaction
Glycolysis
glucose
pyruvate
Electron
Transport
Chain
Oxygen
Mitochondrion
Extracellular fluid
+2 ATP
+2 ATP
+32 ATP

36 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 anerobic
metabolism
• 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 (in cytoplasm)
Glycolysis
Cytoplasm
During the first steps,
two molecules of ATP are
consumed in preparing
glucose for splitting.
Glucose
During the remaining
steps, four molecules
of ATP are produced.
2 ATP
Energyinvestment
phase
2 ADP
4 ADP
4 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.
2 NAD+
Energyyielding
phase
2 NADH
2 Pyruvate
Two molecules of nicotine
adenine dinucleotide
(NADH), a carrier of
high-energy electrons,
also are produced.
Figure 3.23
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)
Transition Reaction
Transition Reaction (in mitochondrion)
Pyruvate (from glycolysis)
One carbon (in the form
of CO2) is removed
from pyruvate.
A molecule of NADH is
formed when NAD+
gains two electrons
and one proton.
CO2
NAD+
Coenzyme A
NADH
(electron passes
to electron
transport chain)
CoA
Acetyl CoA
Citric Acid Cycle
The two-carbon
molecule, called
an acetyl group,
binds to
coenzyme A
(CoA), forming
acetyl CoA,
which enters the
citric acid cycle.
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, 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.
Acetyl CoA
CoA
CoA
Oxaloacetate
Citrate
NADH
CO2
leaves
cycle
NAD+
TCA Cycle
NAD+
Malate
NADH
FADH2
ATP
FAD
ADP + Pi
-Ketoglutarate
Succinate
NAD+
NADH
CO2 leaves cycle
The citric acid cycle yields
One ATP from each acetyl
CoA that enters the cycle,
for a net gain of two ATP.
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 (inner membrane of mitochondrion)
Electron Transport Chain
The molecules of NADH and
FADH2 produced by earlier phases
of cellular respiration pass their
electrons to a series of protein
molecules embedded in the inner
membrane of the mitochondrion.
High
NAD+
NADH
As the electrons are transferred
from one protein to the next,
energy is released and used to
make ATP.
Potential energy
2e–
FADH2
Membrane
proteins
2e–
FAD
2e–
2e–
Eventually, the
electrons are
passed to oxygen,
which combines
with two hydrogens
to form water.
2e–
Low
Energy released is used
for synthesis of ATP
1
2 H+ + 2 O 2
H2O
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