3 introduction to metabolism-pdf

Three Stages of Metabolism

3 Stages of Metabolism

•First stage:

• It is the breakdown & degradation of

polysaccharides, proteins & lipids into

their simplest units either through

intracellular catabolism or during

digestion

Carbohydrates Proteins Lipids

Glucose Amino acids Free Fatty acids

1st Stage of Metabolism

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1

2

3

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Digestive juices

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Disaccharidase

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DipeptidaseDipeptide

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CoA.CoA

of Kreb’s cycle.

& glycerol.

Carbohydrates Proteins Lipids

Glucose Amino acids Free Fatty acids

Pyruvate Acetyl CoA

ATP

(substrate level phosphorylation)

1st

Stage

2nd

Stage

1st and 2nd Stages of Metabolism

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H2O ++

Carbohydrates Proteins Lipids

Glucose Amino acids Free Fatty acids

Pyruvate Acetyl CoA

ATP

(Substrate level phosphorylation)

NADH/FADH2

Oxidative phosphorylation (CO2 + H2O) + ATP

1st

Stage

2nd

Stage

Stages of Metabolism

3rd

Stage

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The Sun is Energy for Life

• Phototrophs use light to drive synthesis

of organic molecules (C6H12O6)

• Heterotrophs use these organic

molecules as building blocks

• CO2, O2, and H2O are recycled

Metabolism

Metabolism includes:

1. Digestion

2. Absorption

3. Catabolism

4. Anabolism

5. Excretion

Metabolism

• Catabolism: degradative

pathways, Usually energy-

yielding!

• Anabolism: biosynthetic

pathways, energy-requiring!

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

Kreb’sycle Fatty acid synthesis

Conversion of 3-phospho-

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)

• The uncontrolled combustion

of glucose releases a large

amount of energy all at once

A

C

E

F

• A multi-step enzyme –

catalyzed pathway – releases

the same amount of energy

but conserves much of it in a

manageable form

1. Multiple points where

metabolites can enter & leave

2. Multiple points for regulation

3. Substrates & products can be

involved in more than one

metabolic reaction

A

C

E

F

Single-step versus Multi-

step Pathways

(capturing chemical energy)

Starting Substrate

Final End ProductStarting Substrate

Final End Product

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

Regulation of Metabolic Pathways

Prof. Dr. Mamdouh M. El-Shishtawy

• It is the breakdown &

degradation of polysaccharides &

oligosaccharides into their

simplest units (Monosaccharides)

Digestion of Carbohydrates

1. Digestion

Digestible

Monosaccharides

Do not need

digestion

Starch

Hexoses

Carbohydrates

Glycogen

Lactose

Sucrose

Pentoses

Non-Digestible

Cellulose

The Digestive System

PH 6.4 – 6.9. Activated

by Cl-, digests starch to

dextrins, maltose &

isomaltose

Acidic PH of stomach (1-2) is

unsuitable for salivary

amylase

3. Brush border (Intestinal juice,

Succus Entericus) containing:

Maltase, Lactase & Sucrase

(Activated by Cl-).

4.

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)

Carbohydrate Digestion

in Small Intestine

& Isomaltose

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

2. Absorption

Absorption of Pentoses & Hexoses

Pentoses absorbed

by Passive Diffusion

Hexoses absorbed

by Active Transport

According

concentration

gradient

Against

concentration

gradient

Types of Transport Carriers

Monosaccharides Absorption

Liver

Digestion of Lactose

(Lactase)

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

Lactase deficiency

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

Reactions

1) Storage in the form of

Glycogen (Glycogenesis)

2) Storage in the form of TAG

(Lipogenesis)

3) Synthesis of sugar alcohols

and amino sugars

4) Interconversion between

monosaccharides

1) Glycolysis

2) Kreb’s Cycle

3) Pentose Shunt

4) Formation of

Uronic acids