Biochemistry Lec:1 Dr.Radhwan - PowerPoint Presentation

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Biochemistry

Lec:1

Dr.Radhwan

M.

Asal

Bsc

. Pharmacy

MSC ,PhD Clinical

Biochemistry

Slide2

Bioenergetics and Oxidative Phosphorylation

Bioenergetics describes the transfer and utilization of energy in biologic systems . The direction and extent to which a chemical reaction proceeds is determined by the degree to which two factors change during the

reaction, these

are

enthalpy

and

entropy.

Neither

of these thermodynamic quantities by itself is sufficient to determine whether a chemical reaction will proceed spontaneously in the direction it is written.

enthalpy

and entropy can be used to define a third quantity,

free energy (G),

which predicts the direction in which a reaction will spontaneously proceed.

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The change in free energy comes in two

forms , G and G°. The first, G is the more general because it predicts the change in free energy and, thus, the direction of a reaction at any specified concentration of products and reactants.

G

°

which

is the energy change when reactants and products are at a concentration of 1

mol/L

Slide5

Sign

of G predicts the direction of a reaction The change in free energy, AG, can be used to predict the direction of a reaction at constant temperature and pressure. Consider the reaction:

Negative AG:

AG

is a negative number, there is a net loss of energy, and the reaction goes spontaneously

Positive AG:

AG is a positive number, there is a net gain of energy, and the reaction does not go spontaneously AG is zero: AG = 0, the reactants are in equilibrium.

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G

depends on the concentration of reactants and products

G

of the reaction

A B depends on the concentration of the reactant and product. At constant temperature and pressure, the following relationship can be derived:where

AG° is the standard free energy change ,R is the gas constant 1.978 cal/mol

• degree). T is the absolute temperature (°K).

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Relationship between AG° and

Keq: in reaction A B a point of equilibrium is reached at which no further net chemical change

takes place ,

when A is being converted to B as fast as B is being

converted to A. state, the ratio of [B] to [A] is constant, regardless of the actual concentrations of the two compounds:

at equilibrium the overall free energy change (AG) is zero. Therefore,

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G

° of two consecutive reactions are additive:

The standard free energy changes

( G

°) are additive in any sequence of consecutive reactions, as are the free energy changes

( G).

Gs of a pathway are additive: This additive property of free energy changes is very important in biochemical pathways through which substrates must pass in a particular direction

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ATP consists of a molecule of adenosine (adenine + ribose) to which three phosphate groups are attached

,if one

phosphate is removed, adenosine diphosphate (ADP) is produced; if two phosphates are removed, adenosine monophosphate (AMP) results. The standard free energy of hydrolysis of ATP,

G

°, is approximately -7300

cal

/mol for each of the two terminal phosphate groups

.Because of this large, negative AG°,

ATP is called a high-energy phosphate compound

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ATP is able to act as a donor of high-energy

phosphate to compounds, ADP can accept high-energy phosphate to form ATP from those compounds. In effect, an ATP/ ADP

cycle

connects those processes

.

There are three major sources of ∼P taking part

in energy conservation or energy capture:(1) Oxidative phosphorylation: The greatest quantitative source of ∼P in aerobic organisms. Free energy comes from respiratory chain oxidation using molecular O2 within mitochondria .

(2) Glycolysis: A net formation of two ∼P results from the formation of lactate from one molecule of glucose.

(3) The citric acid cycle: One ∼P is generated directly in the cycle at the succinyl thiokinase

step .

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Phosphagens

act as storage forms of high-energy phosphate and include creatine phosphate, occurring in vertebrate skeletal muscle, heart, spermatozoa, and brain; and arginine phosphate, occurring in

invertebrate muscle

. When ATP is rapidly being utilized as

a source

of energy for muscular contraction,

phosphagens permit its concentrations to be maintained, but when the ATP/ADP ratio is high, their concentration can increase to act as a store of high-energy phosphate.ATP Allows the Coupling of Thermodynamically Unfavorable Reactions to Favorable OnesThe phosphorylation of glucose to glucose 6-phosphate, the first reaction

of glycolysis, is highly endergonic and cannot proceed under physiologic conditions.

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(1) Glucose+Pi → Glucose 6- phosphate+ H2O

( ΔG0′ = +13.8 kJ/ mol)To take place, the reaction must be coupled with

another—more

exergonic—reaction such as the

hydrolysis of

the terminal phosphate of ATP

.(2) ATP→ADP+Pi (ΔG0′ = −30.5 kJ /mol)When (1) and (2) are coupled in a reaction catalyzed by hexokinase, phosphorylation of glucose readily proceeds in a highly exergonic reaction that under physiologic conditions is irreversible.

Adenylyl Kinase (Myokinase) Interconverts Adenine NucleotidesThis enzyme is present in most cells. It catalyzes the

following reaction:ATP+AMP Myokinase 2ADP

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This allows:

(1) High-energy phosphate in ADP to be used in the synthesis of ATP.(2) AMP, formed as a consequence of several

activating reactions

involving ATP, to be recovered

by rephosphorylation

to ADP.

(3) AMP to increase in concentration when ATP becomes depleted and act as a metabolic (allosteric) signal to increase the rate of catabolic reactions, which in turn lead to the generation of more ATP.When ATP Forms AMP, Inorganic Pyrophosphate (PPi

) Is producedThis occurs, for example, in the activation of long chain fatty acids , this reaction is accompanied by loss of free

energy as heat, which ensures that the activation reaction will go to the right; and is further aided by the hydrolytic splitting of

PPi, catalyzed by inorganic pyrophosphatase,

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a reaction that itself has a large ΔG

0 combination of the above reactions makes it possible for phosphate to be recycled and the adenine nucleotides to interchange.

Other Nucleoside

Triphosphates Participate

in the Transfer

of High-Energy

PhosphateBy means of the enzyme nucleoside diphosphate kinase, UTP, GTP, and CTP can be synthesized from their diphosphates. All of these triphosphates take part in phosphorylations in the cell. Similarly, specific nucleoside monophosphate kinases catalyze the formation of nucleoside diphosphates

from the corresponding monophosphates.

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Biologic

OxidationFree energy change can be expressed in term of redox potential in addition to expressing free energy change in terms of ΔG0′

,it

is possible, in an analogous manner, to express

it numerically

as an

oxidation-reduction or redox potential (E′0). The redox potential of a system (E′0) is usually compared with the potential of the hydrogen electrode (0.0 volts at pH 0.0). However, for biologic systems, the redox potential (E′0)is normally expressed at pH 7.0, at which pH the electrode potential of

the hydrogen electrode is −0.42 volts.

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Enzymes involved in oxidation and reduction are called

oxidoreductases and are classified into fourgroups:

oxidases, dehydrogenases,

hydroperoxidases

,

and

oxygenases.Oxidases use oxygen as a hydrogen acceptorOxidases catalyze the removal of hydrogen from a substrate using oxygen as a hydrogen acceptor. They form water or hydrogen peroxide as a reaction product.

Some Oxidases Contain Copper :

Cytochrome oxidase is a hemoprotein

widely distributed in many tissues, having the typical heme prosthetic group present in myoglobin, hemoglobin, and other cytochromes .

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The

enzyme is poisoned by carbon

monoxide, cyanide, and hydrogen sulfide. It

has also

been termed cytochrome

a

3. It is now known that cytochromes a and

a3 are combined in a single protein, and

the complex is known as cytochrome aa

3. It contains two

molecules of heme, each having one Fe

atom that

oscillates between Fe3+ and Fe2+ during

oxidation and

reduction. Furthermore, two atoms of Cu are

present, each

associated with a heme unit.

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Dehydrogenase Cannot use Oxygen as a hydrogen acceptor

There are a large number of enzymes in this class. They perform two main functions:(1)

Transfer of hydrogen from one substrate to

another in

a coupled oxidation-reduction reaction

.These

dehydrogenases are specific for their substrates but often utilize common coenzymes or hydrogen carriers, eg, NAD+. This type of reaction, which enables one substrate to be oxidized at the expense of another, is particularly useful in

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enabling oxidative processes to occur in the absence of oxygen, such as during the anaerobic phase of glycolysis .

(2) As components in the respiratory chain of electrontransport from substrate to oxygen

.

Many dehydrogenases depend on

nicotinamide

Coenzymes

These dehydrogenases use nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+)—or both—and are formed in the body from the vitamin niacin ,The coenzymes are reduced by the specific substrate of the dehydrogenase and reoxidized by a suitable electron acceptor

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Other

dehydrogenases depend on riboflavinThe flavin groups associated with these dehydrogenases are

similar to FMN and FAD occurring in

oxidases. They

are generally more tightly bound to their

apoenzymes than

are the nicotinamide coenzymes.Cytochromes may also be regarded as dehydrogenasesThe cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe3+ and Fe2+ during oxidation and reduction. Except for cytochrome oxidase

(previously described), they are classified as dehydrogenases. In the respiratory chain, they are involvedas carriers of electrons from flavoproteins on

the one hand to cytochrome oxidase on the other hand.

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Hydroperoxidases use

hydrogen Peroxide or an organic peroxide as substrateTwo type of enzymes found both in animals and plants fall into this category:

peroxidases

and

catalase.

Hydroperoxidases protect the body against

harmful peroxides. Accumulation of peroxides can lead to generation of free radicals, which in turn can disrupt membranes and perhaps cause cancer and atherosclerosis.Peroxidases Reduce Peroxides Using Various Electron AcceptorsPeroxidases are found in milk and in leukocytes, platelets

, and other tissues involved in eicosanoid metabolism .

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The prosthetic group is

protoheme

. In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as

ascorbate

,

quinones

, and cytochrome c. The reaction catalyzed by peroxidase is complex, but the overall reaction is as follows:

In erythrocytes and other tissues, the enzyme glutathione peroxidase, containing selenium as a prosthetic group, catalyzes the destruction of H2O2 and lipid

hydroperoxides by reduced glutathione, protecting membrane lipids and hemoglobin against oxidation by Peroxides

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Catalase

Uses Hydrogen Peroxide as Electron Donor & Electron AcceptorCatalase is a hemoprotein containing four heme groups. In addition to possessing peroxidase activity, it is

able to

use one molecule of H2O2 as a substrate

electron donor

and another molecule of H2O2 as an oxidant

or electron acceptor. Under most conditions in vivo, the peroxidase activity of catalase seems to be favored. Catalase is found in blood, bone marrow, mucous membranes, kidney, and liver. Its function is assumed to be the destruction of hydrogen peroxide formed by the action of oxidases.

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Peroxisomes

are found in many tissues, including liver. They are rich in oxidases and in catalase, Thus, the enzymes that produce H2O2 are grouped with the enzyme that destroys it. However, mitochondrial and

microsomal electron

transport systems as well as xanthine

oxidase must

be considered as additional sources of H2O2.