Biochemistry Lec 1 Dr Radhwan M Asal Bsc

Biochemistry Lec: 1 Dr. Radhwan M. Asal Bsc. Pharmacy MSC , Ph. D Clinical Biochemistry

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.

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

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.

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).

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,

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

ATP consists of a molecule of adenosine (adenine + ribose) to which three phosphate groups are attached , if one adenosine phosphate is diphosphate removed, (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 Because of this large, negative AG°, ATP is called a highenergy phosphate compound of the two terminal phosphate groups.

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 O 2 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.

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 Ones The phosphorylation of glucose to glucose 6 -phosphate, the first reaction of glycolysis, is highly endergonic and cannot proceed under physiologic conditions.

(1) Glucose+Pi → Glucose 6 - phosphate+ H 2 O ( ΔG 0′ = +13. 8 k. J/ 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 (ΔG 0′ = − 30. 5 k. J /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 Nucleotides This enzyme is present in most cells. It catalyzes the following reaction: ATP+AMP Myokinase 2 ADP

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 produced This 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,

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 Phosphate By 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.

Biologic Oxidation Free energy change can be expressed in term of redox potential in addition to expressing free energy change in terms of ΔG 0′ , 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 p. H 0. 0). However, for biologic systems, the redox potential (E′ 0)is normally expressed at p. H 7. 0, at which p. H the electrode potential of the hydrogen electrode is − 0. 42 volts.

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 acceptor Oxidases 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.

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 a 3 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 Fe 3+ and Fe 2+ during oxidation and reduction. Furthermore, two atoms of Cu are present, each associated with a heme unit.

Dehydrogenase Cannot use Oxygen as a hydrogen acceptor There 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

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 electron transport 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

Other dehydrogenases depend on riboflavin The 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 dehydrogenases The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe 3+ and Fe 2+ during oxidation and reduction. Except for cytochrome oxidase (previously described), they are classified as dehydrogenases. In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other hand.

Hydroperoxidases use hydrogen Peroxide or an organic peroxide as substrate Two 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 Acceptors Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism.

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 H 2 O 2 and lipid hydroperoxides by reduced glutathione, protecting membrane lipids and hemoglobin against oxidation by Peroxides

Catalase Uses Hydrogen Peroxide as Electron Donor & Electron Acceptor Catalase is a hemoprotein containing four heme groups. In addition to possessing peroxidase activity, it is able to use one molecule of H 2 O 2 as a substrate electron donor and another molecule of H 2 O 2 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.

Peroxisomes are found in many tissues, including liver. They are rich in oxidases and in catalase, Thus, the enzymes that produce H 2 O 2 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 H 2 O 2.