TOPIC- CARBOHYDRATE METABOLISM Dr. Jagdish kaur P.G.G.C., Sector 11, Chandigarh

Carbohydrate Metabolism Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms.

Metabolic pathways Carbon fixation, or photosynthesis, in which CO2 is reduced to carbohydrate. Glycolysis - the oxidation metabolism of glucose molecules to obtain ATP and pyruvate. Pyruvate from glycolysis enters the Krebs cycle, also known as the citric acid cycle, in aerobic organisms after moving through pyruvate dehydrogenase complex. The pentose phosphate pathway, which acts in the conversion of hexoses into pentoses and in NADPH regeneration. NADPH is an essential antioxidant in cells which prevents oxidative damage and acts as precursor for production of many biomolecules.

Glycogenesis - the conversion of excess glucose into glycogen as a cellular storage mechanism; this prevents excessive osmotic pressure buildup inside the cell. Glycogenolysis - the breakdown of glycogen into glucose, which provides a glucose supply for glucose-dependent tissues. Gluconeogenesis - de novo synthesis of glucose molecules from simple organic compounds. An example in humans is the conversion of a few amino acids in cellular protein to glucose. Metabolic use of glucose is highly important as an energy source for muscle cells and in the brain, and red blood cells.

Metabolism Summary

Glycolysis

Some facts about Glycolysis:- It occurs in cytoplasm. Anaerobic process. Discovered by:-Gustav Embden , Otto Meyerhoff and J. Parnas. Also known as EMP pathway. Only process available in all organism. Occurs in two phases:- 1st is ENERGY SPENDING PHASE for activation of glucose. 2nd is ENERGY RELEASING PHASE.

Regulatory enzyme of glycolysis is PHOSPHOFRUCTOKINASE. All enzyme of glycolysis require Mg2+ cofactor. RBCs and muscles gets energy by glycolysis. It is a Ten step mediated process. End product is two molecule of 3-C PYRUVATE.

Metabolic fate of Pyruvate :- Depend on cell’s need:- It occurs in three Major ways:- Aerobic Respiration. Anaerobic respiration. Lactic acid fermentation.

Link Reaction:- It is a linking reaction between glycolysis and citric acid cycle. It is also known as transition reaction or Gateway step. It occurs in the presence of enzyme pyruvate dehydrogenase. Pyruvate+NAD+ + Co-A======Acetyl Co-A+NADH+H+ CO2.

Kreb’s Cycle or CITRIC ACID CYCLE:- If oxygen is available each 3-C pyruvate molecule enters in mitochondrial matrix. Now , oxidation will complete in two phases:- 1. Formation of Acetyl Co-A (Oxidative decarboxylation.) 2. Kreb’s cycle. It occurs in mitochondrion.

Definition:- The citric acid cycle – also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle – is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate(ATP).

The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate). The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2 requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they might not be lost, since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules.

Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced. Electrons are also transferred to the electron acceptor Q, forming QH2. At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle continues.

Oxidative Phosphorylation and the Electron Transport Chain

The electron transport chain (ETC) uses the NADH and FADH2 produced by the Krebs cycle to generate ATP. Electrons from NADH and FADH2 are transferred through protein complexes embedded in the inner mitochondrial membrane by a series of enzymatic reactions. The electron transport chain consists of a series of four enzyme complexes (Complex I – Complex IV) and two coenzymes (ubiquinone and Cytochrome c), which act as electron carriers and proton pumps used to transfer H+ ions into the space between the inner and outer mitochondrial membranes.

The ETC couples the transfer of electrons between a donor (like NADH) and an electron acceptor (like O2) with the transfer of protons (H+ ions) across the inner mitochondrial membrane, enabling the process of oxidative phosphorylation. In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP. The role of molecular oxygen, O2, is as the terminal electron acceptor for the ETC. This means that once the electrons have passed through the entire ETC, they must be passed to another, separate molecule. These electrons, O2, and H+ ions from the matrix combine to form new water molecules. This is the basis for your need to breathe in oxygen. Without oxygen, electron flow through the ETC ceases

ATP SYNTHASE Embedded in the inner mitochondrial membrane. It is a turbine that is powered by the flow of H+ ions across the inner membrane down a gradient and into the mitochondrial matrix. As the H+ ions traverse the complex, the shaft of the complex rotates. This rotation enables other portions of ATP synthase to encourage ADP and Pi to create ATP. 

SUMMARY

GLUCONEOGENESIS

GLYCOGENESIS Phenomenon of transformation excess of glucose of blood plasma into glycogen. Chiefly occurs in liver and muscles.

HMP SHUNT Hexose Monophosphate shunt.