Transcript Ans 518_class 4

Carbohydrates as Energy
Sources
Na+/K+ pump is key to SGLT1 function; exchanges Na+ for /K+
to maintain the gradient; uses energy (i.e., ATPase); this allows
Na+ to drag the monosaccharide (glucose) along through the transport
channel and into the enterocyte.
Summary
• Complex carbs have been digested to
simple sugars
• Enterocyte has absorbed them and
delivered them to the blood
Key Pathways in Carbohydrate Utilization
•
Insulin-stimulated uptake
– Low circulating concentrations of insulin: GLUT4 is sequestered within cytosolic
vesicles in myocytes and adipocytes
– Accumulation of glucose in blood triggers insulin release from pancreatic ß-cells;
Insulin-receptor signaling induces the redistribution of GLUT4 from intracellular
storage sites to the plasma membrane; once incorporated into the cell
membrane, GLUT4 facilitates the passive diffusion of circulating glucose down its
concentration gradient
• The pathway in which GLUT4 is integrated into the plasma membrane begins with
insulin binding to its receptor dimer
• The receptor dimer autophosphorylates (intrinsic tyrosine kinase activity) and
subsequently activates insulin-responsive substrate-1 (IRS1), which initiates a series of
reactions to activate protein kinase B (PKB)
• Once phosphorylated, PKB is in its active form and phosphorylates other targets that
stimulate GLUT4 mobilization and trafficking to the plasma membrane
– Once inside cells, glucose is rapidly phosphorylated by hexokinase to form G6P;
it is this form of glucose that serves as the initial substrate for glycolysis
– G6P cannot diffuse back out of cells; thus G6P serves to maintain the
concentration gradient for glucose to passively enter cells.
Basal State
Insulin-Stimulated
From Oatey et al. Biochem. J. (1997) 327, 637±642
Converting Sugars to an Energy
Currency
• Key concepts
– Glycolysis
– TCA cycle
– Electron transport chain
– Oxidative phosphorylation
Sugar
ATP
Glycolysis
• G6P is trapped in cell cytosol; glycolysis
occurs in cytosol
1. ATP is consumed at RXN
1 and 3
2. 6C molecule split into two
3C molecules
3. 3C molecule drives
production of NADH
and ATP
4. No net ATP until PEP is
converted to pyruvate
5. Under aerobic conditions,
NADH will be further
utilized in mitochondria to
power the electron transport chain (generate ATP)
6. Under “anaerobic” conditions,
lactate is produced
Glycolysis Summary
• C6H12O6 + 2 NAD+ + 2 ADP + 2 P -----> 2
pyruvic acid, CH3(C=O)COOH + 2 ATP +
2 NADH + 2 H+
Steps 1 and 3 = - 2ATP
Steps 6 and 9 = + 4 ATP
Net "visible" ATP produced = 2
With insufficient oxygen, or in cells lacking mitochondria (RBC),
pyruvate will be reduced at the expense of NADH, to lactate by
lactate dehydrogenase.
TCA (Citric Acid) Cycle
• Now, we are shifting from the cytosol, to the
mitochondria
• Using C-containing molecules that originated
with dietary carbohydrate and metabolized to tricarboxylic acids , we are generating ATP and
reducing power that will flow into the electron
transport chain
• “anything containing C and H that can be
reduced to CO2 and H2O contains
energy”…..oxygen serves as the terminal eacceptor in the electron transport chain
Key Features, TCA Cycle
• The citric acid cycle begins with Acetyl-CoA (ACoA) transferring its
two-carbon acetyl group to the four-carbon acceptor compound
(oxaloacetate) to form a six-carbon compound (citrate)
• The citrate then goes through a series of chemical transformations,
losing first one, then a second carboxyl group as CO2. The carbons
lost as CO2 originate from what was oxaloacetate, not directly from
acetyl-CoA. The carbons donated by acetyl-CoA become part of the
oxaloacetate carbon backbone after the first turn of the citric acid
cycle. Loss of the ACoA-donated carbons as CO2 requires several
turns of the citric acid cycle. However, because of the role of the
citric acid cycle in anabolism, they may not be lost since many TCA
cycle intermediates are also used as precursors for the biosynthesis
of other molecules.
• Most of the energy made available by the oxidative steps of the
cycle is transferred as energy-rich electrons to NAD+, forming
NADH. For each acetyl group that enters the citric acid cycle, three
molecules of NADH are produced
Key Features, TCA Cycle
• Electrons are also transferred to the electron acceptor Q, forming
QH2.
• At the end of each cycle, the four-carbon oxaloacetate has been
regenerated, and the cycle continues
• Mitochondria in animals including humans possess two succinylCoA synthetases, one that produces GTP from GDP, and another
that produces ATP from ADP
• Products of the first turn of the cycle are: one GTP (or ATP), three
NADH, one QH2, two CO2
• Because two ACoA molecules are produced from each glucose
molecule, two cycles are required per glucose molecule. Therefore,
at the end of both cycles, the products are: two GTP (or ATP), six
NADH, two QH2, and four CO2
The Respiratory Chain
• We have generated reducing power: per turn
– (1) FADH2
– (3) NADH
– (1) QH2
But…… the energy currency we are
after is ATP
• The respiratory (e-) transport chain allows us to
finish extracting energy from what was our
dietary carbs by oxidation of the reducing power,
coupled to phosphorylation of ADP
• “Oxidative Phosphorylation”
ATP
What are the e- transporters?
ATP Yield
• Oxidation of NADH to NAD+ pumps 3
protons which charges the
electrochemical gradient with enough
potential to generate 3 ATP
• Oxidation of FADH2 to FAD+ pumps 2
protons which charges the
electrochemical gradient with enough
potential to generate 2 ATP.
Respiratory Chain
•
Protons are translocated across the membrane, from the matrix to
the intermembrane space
•
Electrons are transported along the membrane, through a series
of protein carriers
•
Oxygen is the terminal electron acceptor, combining with e- and
H+ ions to produce water
•
As NADH and FADH2deliver more H+ and electrons into the ETC,
the proton gradient increases, with H+ building up outside the
inner mitochondrial membrane, and OH- inside the membrane
•
It is the energy derived from this proton (electrochemical) gradient
that is used to drive phosphorylation of ADP (synthesis of ATP)
NADH and FADH2 carry protons (H+) and e- to the electron transport chain located in the
membrane. The energy from the transfer of electrons along the chain transports protons
across the membrane and creates an electrochemical gradient. As the accumulating
protons follow the electrochemical gradient back across the membrane through an ATP
synthase complex, the energy of the gradient is transferred by the ATP synthase system
from the gradient to ADP in the synthesis of ATP. At the end of the electron transport chain,
two protons, two electrons, and half of an oxygen molecule combine to form water. Since
oxygen is the final electron acceptor, the process is called aerobic respiration.
From Cell Nutrition, 3rd ed.
Excess Energy from CH2O can be
stored as fat
• De novo lipogenesis
– Acetyl CoA is the key
Lipogenesis: Key Points
• Occurs in the cytosol; encompasses
fatty acid synthesis and triacylglycerol
synthesis
• Catalyzed by acetyl CoA carboxylase
• Requires biotin for carboxylation rxn
• Product is malonyl CoA
This is the rate limiting step…….
Full activation requires excess energy as
is reflected in (1) adequate cellular ATP
and low AMP, (2) an accumulation of citrate
in the TCA cycle that ultimately exits the
mitochondria via an elaborate shuttle to
return to the cytosol
Lipogenesis: Key Points
• Sequential additions of 2C units (acetyl CoA)
results in the formation of palmitate
(C16:0)
8 acetyl CoA + 7 ATP + 14 NADPH + 6 H --->
palmitate + 14 NADP + 8 CoA + 7 ADP + 7 Pi +
6 H2O
• Catalyzed by Fatty Acid Synthase
– an enzyme complex with 7 catalytic activities
Lipogenesis: Key Points
• Palmitate is esterified on a glycerol
backbone to form “triacyl” glycerol
Lipogenesis: Key Points
• Avians and humans: Liver (hepatocyte) is
primary site of de novo lipogenesis
• Pig: about 100% in the adipocyte itself
• Beef: adipocyte
• Rodents: adipocyte and hepatocyte, about
50:50
• Dog/Cat: Mixed, but probably favors
hepatocyte
Overall Summary
• Digested, absorbed and metabolized
CH2O to convert the energy contained in
the hydrocarbons to a usable currency,
ATP
• Captured excess energy from these CH2O
molecules in an energy dense form,
suitable for storage (triacylglycerol), which
can be mobilized to meet deficits that
might arise later