Different Types of Metabolic Pathways a) Linear metabolic pathway: The product of each step is the substrate for the next step b) Cyclic pathway forms a closed loop: In the citric acid cycle, an acetyl group is metabolized (oxidized) via reactions that regenerate the intermediates of the cycle c) Spiral pathway: The same set of enzymes catalyze a progressive lengthening of the acyl chain
Different Types of Metabolic Pathways Linear pathway Cyclic pathway Spiral pathway Fatty acid synthesisConversion of 3-phospho- Kreb’sycle glycerate into serine Enz 1 Enz 2 Enz 3
Comparing Pathways• Anabolic & catabolic pathways involving the same product are not the same• Some steps may be common to both• Metabolic pathways are multi-step pathways• This also allows regulation mechanisms to turn on pathway and other turned off
Single-step versus Multi- step Pathways (capturing chemical energy)A • The uncontrolled combustion of glucose releases a largeC amount of energy all at onceE • A multi-step enzyme – catalyzed pathway – releases the same amount of energyF but conserves much of it in a manageable form
Single-step versus Multi- step Pathways (capturing chemical energy)A 1. Multiple points whereC metabolites can enter & leave 2. Multiple points for regulationE 3. Substrates & products can be involved in more than oneF metabolic reaction
Regulation of Metabolic Pathways Starting Substrate Final End Product Final End Product Starting Substrate
Regulation of Metabolic Pathways Irreversible Reactions
Regulation of Metabolic Pathways Feedback Inhibition• Feedback inhibition occurs when a product (usual the final product) of a pathway controls the rate of its own synthesis through inhibition of an early step, usually the first step.
Regulation of Metabolic Pathways Feed-forward Activation• Feed-forward activation occurs when a metabolite produced early in a pathway activates an enzyme that catalyzes a reaction further down the pathway.
Regulation of Metabolic Pathways Coordinated Regulation + • Can serve to deplete intermediates & accelerate their re-supply• The existence of multiple control points is to be expected
PH 6.4 – 6.9. Activated by Cl-, digests starch to Acidic PH of stomach (1-2) is dextrins, maltose & unsuitable for salivary isomaltose amylase 4. - (Activated by Cl ). 3. Brush border (Intestinal juice,Succus Entericus) containing:Maltase, Lactase & Sucrase
Carbohydrate Digestion in Small Intestine• Salivary amylase stops working in acidic stomach (if pH 4.5)• 50% of dietary starch digested before it reaches small intestine• Brush border enzymes act upon oligosaccharides, maltose, sucrose & lactose• lactose indigestible after age 4 in most humans (due to lack of lactase)
2. Absorption Monosaccharides Absorption• Sodium-glucose transport proteins (SGLT) in membrane help absorption of glucose & galactose• Fructose absorbed by facilitated diffusion then converted to glucose inside the cell
Pentoses absorbed by Passive Diffusion According concentration gradient Hexoses absorbed by Active Transport Against concentration gradientAbsorption of Pentoses & Hexoses
Lactose Intolerance• Some individuals [90% of certain races (adult blacks & Orientals)] have a defect in lactase enzyme• Undigested maltose (osmotically active compound) passes to the bowel (large intestine), acted upon by bacteria of large intestine producing short chain fatty acids & CO2 gas, leading to: 1. Abdominal distention (cramping) 2. Abdominal pain 3. Nausea 4. Bloating 5. Watery diarrhea
Fate of Absorbed Glucose• Glucose enter Liver & Brain by Passive diffusion• Glucose enter other tissues (skeletal muscle, adipose tissue, etc…) by Active transport• Active transport is enhanced by Insulin
Utilization of Glucose Anabolic Reactions Catabolic Reactions1) Storage in the form of Glycogen (Glycogenesis)2) Storage in the form of TAG 1) Glycolysis (Lipogenesis) 2) Kreb’s Cycle3) Synthesis of sugar alcohols and amino sugars 3) Pentose Shunt4) Interconversion between 4) Formation of monosaccharides Uronic acids
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