Chapter 18: Metabolic Pathways and Energy Production

Metabolism

All the chemical reactions that provide energy and the substances required for continued cell growth.

Catabolic Reactions

These are complex molecules are broken down to simpler ones with an accompanying release of energy.

Anabolic Reactions

These utilize energy available in the cell to build large molecules from simple ones.

Adenosine Triphosphate (ATP)

A high-energy compound that stores energy in the cells. It consists of adenine, a ribose sugar, and three phosphate groups.

Cell membrane

It separates the contents of a cell from the external environment and contains structures that communicate with other cells.

Cytoplasm

It consists of the cellular contents between the cell membrane and nucleus.

Cytosol

It is the fluid part of the cytoplasm that contains enzymes for many of the cell’s chemical reactions.

Endoplasmic reticulum

It is the rough type processes proteins for secretion and synthesizes phospholipids; smooth type synthesizes fats and steroids.

Golgi complex

It modifies and secretes proteins from the endoplasmic reticulum and synthesizes cell membranes.

Lysosome

It contains hydrolytic enzymes that digest and recycle old cell structures.

Mitochondrion

It contains the structures for the synthesis of ATP from energy-producing reactions.

Nucleus

It contains genetic information for the replication of DNA and the synthesis of protein.

Ribosome

It is the site of protein synthesis using mRNA templates.

salivary glands

Enzymes produced in the _______ hydrolyze some of the 𝜶-glycosidic bods in amylose and amylopectin, producing maltose, glucose, and dextrins — which contain three to eight glucose units.

Glucose

It is the primary energy source for muscle contractions, red blood cells, and the brain.

Emulsification

A process where the bile salts break the fat globules into *micelles*.

pancreas

Enzymes from the ______ hydrolyze the triacylglycerols to yield monoacylglycerols and fatty acids, which are then absorbed into the intestinal lining where they recombine to form triacylglycerols.

Chylomicrons

The nonpolar compounds are then coated with proteins to form lipoproteins which are more polar and soluble in the aqueous environment of the lymph and bloodstream.

Oxidation

A reaction that involves the loss of hydrogen or electrons by a substance, or an increase in the number of bonds to oxygen.

Reduction

A reaction that involves the gain of hydrogen ions and electrons or a decrease in the number of bonds to oxygen.

Nicotinamide adenine dinucleotide (NAD+)

An important coenzyme in which the vitamin niacin provides the nicotinamide group, which is bonded to ribose and ADP.

Flavin adenine dinucleotide (FAD)

A coenzyme that contains ADP and riboflavin.

Riboflavin

Also known as Vitamin B2, consists of ribitol and flavin.

Coenzyme A

* Its function is to prepare small acyl groups for reactions with enzymes.
* The thiol group which bonds to a two-carbon acetyl group to produce the energ yrich thioester acetyl-CoA.

Glycolysis

* A pathway wherein the glucose in the bloodstream enters our cells where it undergoes degradation.
* It is an anaerobic process; no oxygen is required.

Energy-investing phase

The energy is obtained from the hydrolysis of two ATP, which is needed to form sugar phosphates; the first five reactions.

Energy-generating phase

The energy is obtained from the hydrolysis of the energy-rich phosphate compounds and used to synthesize four ATP; the last five reactions (6-10).

pyruvate

The _____ produced from glucose can now enter pathways that continue to extract energy.

Aerobic Conditions

* In glycolysis, two ATP were generated when one glucose molecule was converted to two pyruvates.
* Under these conditions, pyruvate moves from the cytoplasm into the mitochondria to be oxidized further.

Anaerobic Conditions

* When we engage in strenuous exercise, the oxygen stored in our muscle cells is quickly depleted.
* Under these conditions, pyruvate remains in the cytoplasm where it is reduced to lactate.

Citric Acid Cycle

* A series of reactions that connects the intermediate acetyl-CoA from the metabolic pathways in stages 1 and 2 with electron transport and the synthesis of ATP in stage 3.
* It is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle.

Citric Acid

A tricarboxylic acid, which forms in the first reaction.

electron transport

In _____, hydrogen ions and electrons from NADH and FADH2 are passed from one electron carrier to the next until they combine with oxygen to form H2O.

Oxidative phosphorylation

The energy released during electron transport is used to synthesize ATP from ADP and Pi.

Chemiosmotic model

Links the energy from electron transport to a H+ gradient that drives the synthesis of ATP.

ATP synthase

An enzyme complex that uses the energy released by H+ ions returning to the matrix to synthesize ATP from ADP and Pi .

Beta-oxidation

This is where fatty acids undergo the removal of two-carbon segments, one at a time, from the carboxyl end.

Fatty Acid Activation

It combines fatty acid with coenzyme A to yield fatty acyl-CoA. The energy for the activation is obtained from the hydrolysis of ATP to give AMP and two inorganic phosphates.

Ketone Bodies

The products of ketogenesis: are acetoacetate, 𝜷-hydroxybutyrate, and acetone.

Ketosis

A condition of the accumulation of ketone bodies; which occurs in severe diabetes, diets high in fat and low in carbohydrates, alcoholism, and starvation.

Transamination

* An 𝜶-amino group is transferred from an amino acid to an a-keto acid, usually a-ketoglutarate.
* A new amino acid and a new 𝜶-keto acid.

Oxidative Deamination

The ammonium group in glutamate is removed as an ammonium ion.

Urea Cycle

* A series of reaction that detoxifies ammonium ion by forming urea.
* The ammonium ion, which is the end product of amino acid degradation, is toxic if it is allowed to accumulate.

Reaction 1: Phosphorylation

In the initial reaction, a phosphate group from ATP is added to glucose to form glucose6-phosphate and ADP.

Reaction 2: Isomerization

The glucose-6-phosphate, the aldose from reaction 1, undergoes isomerization to fructose6-phosphate, which is a ketose.

Reaction 3: Phosphorylation

The hydrolysis of another ATP provides a second phosphate group, which converts fructose-6-phosphate to fructose-1,6-bisphosphate.

Reaction 4: Cleavage

Fructose-1,6-bisphosphate is split into two three-carbon phosphate isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

Reaction 5: Isomerization

Because dihydroxyacetone phosphate is a ketone, it cannot react further. However, it undergoes isomerization to provide a second molecule of glyceraldehyde-3-phosphate, which can be oxidized.

Reaction 6: Oxidation and Phosphorylation

* The aldehyde group of each glyceraldehyde-3-phosphate is oxidized to a carboxyl group by the coenzyme NAD+, which is reduced to NADH and H+.
* A phosphate group adds to each of the new carboxyl groups to form two molecules of the high-energy compound, 1,3-bisphosphoglycerate.

Reaction 7: Phosphate Transfer

* Phosphorylation transfers a phosphate group from each 1,3-bisphosphoglycerate to ADP to produce two molecules of the high-energy compound ATP.
* At this point in glycolysis, two ATP are produced, which balance the two ATP consumed in reactions 1 and 3.

Reaction 8: Isomerization

Two 3-phosphoglycerate molecules undergo _______, which moves the phosphate group from carbon 3 to carbon 2 yielding two molecules of 2-phosphoglycerate.

Reaction 9: Dehydration

Each of the phosphoglycerate molecules undergoes _______ (loss of water) to give two high-energy molecules of phosphoenolpyruvate.

Reaction 10: Phosphate Transfer

In a second direct phosphorylation, phosphate groups from two phosphoenolpyruvate are transferred to two ADP to form two pyruvate and two ATP.

Reaction 1: Formation of Citrate

In the first reaction of the citric acid cycle, the acetyl group from acetyl-CoA bonds with oxaloacetate to yield citrate.

Reaction 2: Isomerization

* The citrate produced in reaction 1 contains a tertiary alcohol group that cannot be oxidized further
* The citrate undergoes _______to yield its isomer isocitrate, which provides a secondary alcohol group that can be oxidized in the next reaction.

Reaction 3: Oxidation and Decarboxylation

* The secondary alcohol group in isocitrate is oxidized to a ketone.
* A decarboxylation converts a carboxylate group to a CO2 molecule producing 𝜶-ketoglutarate.
* The oxidation reaction also produces hydrogen ions and electrons that reduce NAD+ to NADH and H+.
* This reduced coenzyme NADH will be important in the energy-producing reactions we will discuss in electron transport

Reaction 4: Oxidation and Decarboxylation

𝜶-ketoglutarate undergoes oxidation and decarboxylation to produce a four-carbon group that combines with CoA to form succinyl-CoA

Reaction 5: Hydrolysis

Succinyl-CoA undergoes _____to succinate and CoA. The energy released is used to add a phosphate group to GDP which yields GTP.

Reaction 6: Oxidation

Hydrogen is removed from each of two carbon atoms in succinate, which produces fumarate, a compound with a trans double bond.

Reaction 7: Hydration

_______ adds water to the double bond of fumarate to yield malate, which is a secondary alcohol.

Reaction 8: Oxidation

The last step of the citric acid cycle, the secondary alcohol group in malate is oxidized to oxaloacetate, which has a ketone group.

Complex I

* Electron transport begins when NADH transfers hydrogen ions and electrons to complex I and forms the oxidized coenzyme NAD+.
* The hydrogen ions and electrons are transferred to the mobile electron carrier coenzyme Q 1CoQ2, which carries electrons to complex II.

Complex II

Q also obtains hydrogen ions and electrons from FADH2, generated by the conversion of succinate to fumarate in the citric acid cycle, which yields QH2 and the oxidized coenzyme FAD.

Complex III

Two electrons are transferred from the mobile carrier QH2 to cytochromes and to cytochrome c.

Complex IV

Four electrons from four cytochrome c are passed to other electron carriers until the electrons combine with hydrogen ions and oxygen O2 to form two molecules of water.