cmb chapter 3

8/18/2019 Cmb Chapter 3

1/4


2/4


3/4

Oxidation-reduction (redox) reactions involve a change in theelectronic state of reactants. –

When a substrate gains electrons, it is reduced. –

When a substrate loses electrons, it is oxidized. –

When one substrate gains or loses electrons, another substancemust donate or accept those electrons.• In a redox pair, the substrate that donates electrons is a

reducing agent.• The substrate that gains electrons is an oxidizing agent.

The Capture and Utilization of Energy• Reduced atoms can be oxidized, releasing energy to do

work.• The more a substance is reduced, the more energy that can

be released.• Glycolysis is the first stage in the catabolism of glucose,

and occurs in the soluble portion of the cytoplasm.• The tricarboxylic (TCA) cycle is the second stage and it

occurs in the mitochondria of eukaryotic cells.

Glycolysis and ATP Formation• Of the reactions of glycolysis, all but three are near

equilibrium ( Δ G ~ 0) under cellular conditions.• The driving forces of glycolysis are these three reactions.

•

Glucose is phosphorylated to glucose 6-phosphate by using ATP.• Glucose 6-phosphate is isomerized to fructose 6-phosphate.

• Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphophateusing another ATP.

• Fructose 1,6-bisphosphate is split into two three-carbonphosphorylated compounds.

• NAD+ is reduced to NADH when glyceraldehyde 3-

phosphate is converted to 1,3-bisphosphoglycerate.• Dehydrogenase enzymes oxidize and reduce

cofactors.• NAD+ is a non protein cofactor associated with

gluceraldehyde phosphate dehydrogenase.• NAD+ can undergo oxidation and reduction at different

places in the cell.• NADH donates electrons to the electron transport chain

in the mitochondria.

• ATP is formed when 1,3-bisphosphoglycerate is converted

to 3-phosphoglycerate by 3-phosphoglycerate kinase.• Kinase enzymes transfer phosphate groups.• Substrate-level phosphorylation occurs when

ATP is formed by a kinase enzyme.• ATP formation is only moderately endergonic compared with

other phosphate transfer in cells.• Transfer potential shown when molecules

higher on the scale have less affinity for the groupbeing transferred than are the ones lower on thescale.

• The less the affinity, the better the donor.

• 3-phosphoglycerate is converted to pyruvate via three

sequential reactions, in one of them a kinase phosphorylates ADP.

• Glycolysis can generate a net of 2 ATPs for each glucose.

• Glycolysis occurs in the absence of oxygen, it is an

anaerobic pathway.• The end product, pyruvate, can enter aerobic or

anaerobic catabolic pathways.

Anaerobic Oxidation of Pyruvate: The Fermentation Process

Fermentation restores NAD+ from NADH. –

Under anaerobic conditions, glycolysis depletes the supply of NAD+ byreducing it to NADH.

– In fermentation, NADH is oxidized to NAD+ by reducing pyruvate.

– In muscle and tumor cells pyruvate is reduced to lactate.

– In yeast and other microbes, pyruvate is reduced and converted to

ethanol. – Fermentation is inefficient with only about 8% of the energy of glucose

captured as ATP.

Reducing Power•

Anabolic pathways require a source of electrons to formlarger molecules.

• NADPH donates electrons to form large biomolecules.

– NADPH is a non protein cofactor similar to NADH.

–

The supply of NADPH represents the cell’sreducing power.

–

NADP

+

is formed by phosphate transfer from ATPto NAD+.• NADPH and NADH are interconvertible, but have different

metabolic roles.• NADPH is oxidized in anabolic pathways.

• NAD+ is reduced in catabolic pathways.• The enzyme transhydrogenase catalyzes the transfer of

hydrogen atoms from one cofactor to the other. –

NADPH is favored when energy is abundant. –

NADH is used to make ATP when energy is scarce

Metabolic Regulation• Cellular activity is regulated as needed.

• Regulation may involve controlling key enzymes of metabolic

pathways.• Enzymes are controlled by alteration in active sites.

–

Covalent modification of enzymes regulated byphosphorylation such as protein kinases.

–

Allosteric modulation by enzymes regulated bycompounds binding to allosteric sites.

• In feedback inhibition, the product of

the pathway allosterically inhibits one ofthe first enzymes of the pathway.

Separating Catabolic and Anabolic Pathways• Glycolysis and gluconeogenesis are the catabolic and

anabolic pathways of glucose metabolism. –

Synthesis of fructose 1,6-bisphosphate is coupledto hydrolysis of ATP.

– Breakdown of fructose 1,6-bisphosphate is via

hydrolysis by fructose 1,6-bisphosphatase ingluconeogenesis.

–

Phosphofructokinase is regulated by feedbackinhibition with ATP as the allosteric inhibitor.

– Fructose 1,6-bisphosphatase is regulated by

covalent modification using phosphate binding. –

ATP levels are highly regulated. • Anabolic pathways do not proceed via the same reactions as

the catabolic pathways even though they may have steps incommon.

– Some catabolic pathways are essentially

irreversible due to large ΔG°’ values. –

Irreversible steps in catabolic pathways arecatalyzed by different enzymes from those inanabolic pathways.


4/4

cmb chapter 3

Documents