Biochemistry Lesson 1: Metabolism -1 (Finals) PDF

LESSON 1: METABOLISM -1 (FINALS) ENERGY PRODUCTION METABOLIC PATHWAY A metabolic pathway is a sequence of reactions used to produce one product or ac...

LESSON 1: METABOLISM -1 (FINALS) ENERGY PRODUCTION METABOLIC PATHWAY A metabolic pathway is a sequence of reactions used to produce one product or accomplish one process. o Each pathway consists of a series of chemical reactions that convert a starting material into an end product (e.g., the citric acid cycle and the electron transport chain). Fortunately, there are a great many similarities among the major metabolic pathways in all life forms. CATABOLISM OF FOOD CELLULAR RESPIRATION The catabolism of food is a three stage process. During cellular respiration, plants and animals Stage I: The digestion of large, complex combine energy-rich compounds with oxygen molecules into simpler ones. from the air, producing CO2 and releasing o The most common reaction in digestion is energy. hydrolysis: Cellular respiration can be represented as: A portion of the energy released during respiration is captured in the form of adenosine triphosphate (ATP), which stores energy for use in other processes. Stage II: The small molecules from digestion are o The remainder of the energy from respiration broken down into even simpler units, usually the is released as heat. two-carbon acetyl portion of acetyl coenzyme A (acetyl CoA): o Some energy is produced at this stage, but much more is produced during the oxidation of the acetyl units in Stage III. Stage III: This is referred to as the common catabolic pathway because the reactions are the same regardless of the type of food being degraded. o citric acid cycle METABOLISM o electron transport Metabolism is the sum of all reactions occurring o oxidative phosphorylation in an organism: Energy released in Stage III appears in the form o Catabolism: The reactions involved in the of energy-rich molecules of ATP. breakdown of biomolecules. The whole purpose of the catabolic pathway is to o Anabolism: The reactions involved in the convert the chemical energy in foods into synthesis of biomolecules. molecules of ATP, which carries energy to parts In general, energy is released during catabolism of the cell where energy is needed. and required during anabolism.  as an inorganic phosphate, Pi, or just phosphate. The transfer of a phosphoryl group from ATP to water is accompanied by a release of energy. Free energy, ΔG, is used as a measure of the energy change. o When energy is released, ΔG is negative. o When energy is absorbed, ΔG is positive. o When ΔG is measured under standard conditions, it is represented by ΔG o. o When ΔGo is measured under body conditions, it is represented by ΔGo’. The liberated free energy is available for use by the cell to carry out processes requiring an input of energy: ATP AS PRIMARY ENERGY CARRIER Other phosphate-containing compounds also Adenosine triphosphate, liberate energy on hydrolysis: ATP, consists of: o the heterocyclic base adenine o the sugar ribose o a triphosphate group At physiological pH, the protons on the triphosphate group are removed, giving ATP a charge of -4. In the cell, it is complexed with Mg2+ in a 1:1 ratio, giving it a net charge of -2. Compounds that liberate a large amount of free energy on hydrolysis are called high-energy compounds. The hydrolysis of ATP to ADP is the principal energy-releasing reaction for ATP. Some other hydrolysis reaction occur under some conditions, HYDROLYSIS OF ATP such as the hydrolysis of ATP to adenosine The triphosphate group is the part of the molecule monophosphate, AMP, and pyrophosphate, PPi: that is important in the transfer of biochemical energy. The key reaction is the transfer of a phosphoryl group, —PO32-, from ATP to another molecule. o During the hydrolysis of ATP in water, a phosphoryl group is transferred from ATP to a water molecule: This is usually followed by immediate hydrolysis of the pyrophosphate, which releases even more energy: o The products are adenosine diphosphate (ADP) and a phosphate ion, often referred to  often by acting as shuttle systems for the transfer The hydrolysis of of chemical groups (e.g., hydrogen transport). ATP and related Many important coenzymes are formed from compounds are vitamins. summarized Coenzyme A is a central compound in below: metabolism. It is part of acetyl coenzyme A (acetyl CoA), the substance formed from all foods as they pass through Stage II of catabolism. ATP-ADP CYCLE ATP functions as an immediate donor of free Coenzyme A energy rather Components of coenzyme A: than as an o vitamin B5 , pantothenic acid, in the center. energy storage o a phosphate derivative of ADP medium. o b-mercaptoethylamine, which puts a reactive o The sulfhydryl group (—SH) at the end of the turnover rate of ATP is very high: typically, molecule (CoA—SH). an ATP molecule is hydrolyzed within 1 minute after its formation. o At rest, a human body hydrolyzes ATP at the rate of about 40 kg every 24 hours. o During strenuous exertion, this rate may be as high as 0.5 kg per minute. ATP must be continuously regenerated from ADP if cellular work is to occur. The letter A is included in the name Coenzyme MITOCHONDRIA A to signify its participation in the transfer of The mitochondrion is the powerhouse of the cell. acetyl groups, but Coenzyme A transfers all acyl It is the organelle where many of the reactions of groups. This is important in fatty acid oxidation the common catabolic pathway occur. It consists and synthesis. of an outer membrane, which surrounds a inner membrane. o The folds of the inner membrane are called cristae. o The space that surrounds them is the matrix. Acyl groups are linked to coenzyme A through The enzymes for ATP synthesis (electron the sulfur atom in a thioester bond: transport and oxidative phosphorylation) are located on the cristae. The enzymes for the citric acid cycle are found within the matrix, near the surface of the inner membrane. ELECTRON TRANSPORTERS Most of energy for ATP synthesis is released when the oxygen we breathe is reduced. o Oxygen accepts electrons and H+, producing water. o The electrons come from the oxidation of fuel COENZYMES molecules, but are are not transferred directly Coenzymes are weakly-bound organic groups to the oxygen. that participate in enzyme-catalyzed reactions,  o These substrates first transfer the electrons to Biochemical reactions involving coenzymes are special coenzyme carriers. often written more concisely: The reduced forms of these coenzymes transfer the electrons to oxygen through the reactions of the electron transport chain. ATP is formed from ADP and Pi as a result of this flow of electrons. o This notation emphasizes the oxidation NICOTINAMIDE ADENINE DINUCLEOTIDE reaction and the involvement of the (NAD+) coenzyme. Nicotinamide adenine FLAVIN ADENINE DINUCLEOTIDE (FAD) dinucleotide Flavin adenine (NAD+), is an dinucleotide (FAD) electron transporter is another major which is a electron carrier. It is derivative of ADP a derivative of ADP and the vitamin and the vitamin nicotinamide (B3). riboflavin. The reactive site of NAD+ is in the nicotinamide The active site is located within the riboflavin portion. When oxidizing a substrate, the ring system. Unlike NAD+ , FAD accepts both nicotinamide ring accepts two electrons and one of the hydrogen atoms lost by the substrate, proton, forming the reduced coenzyme NADH: forming the reduced species FADH2. The substrates for reactions involving FAD are A typical cellular reaction in which NAD+ serves often those in which a —CH2—CH2— portion as an electron acceptor is the oxidation of an of the substrate is oxidized to a double bond: alcohol: In this reaction, one hydrogen atom of the alcohol substrate is directly transferred to NAD+, whereas the other appears in solution as H+. Both electrons lost by the alcohol have been transferred to the nicotinamide ring in NADH.