CHAPTER 2: CARBONHYDRATE METABOLISM- GLYCOLYTIC ENZYMES ppt

In the process known as cellular respiration , cells extract the energy stored in glucose molecules.. - Not only are simple sugar molecules a major fuel for cellular work, but their carb

CHAPTER 2: CARBONHYDRATE METABOLISM- GLYCOLYTIC ENZYMES

INTERNATIONAL UNIVERSITY SCHOOL OF BIOTECHNOLOGY

BIOCHEMISTRY

Learning objectives

1 Review the carbonhydrates

2 Learn the names of the 10 enzymes of glycolysis

3 Learn the structures of the intermediates in the

glycolytic pathway

4 Explore the structures of the glycolytic enzymes

5 Understand the chemical mechanisms of the enzymes

of glycolysis

2.5 Triose phosphate Isomerase

2.6 Glyceraldehide-3- phosphate dehydrogenase

2.7 Phosphoglycerate kinase

2.8 Phosphoglycerate mutase

2.9 Enolase

2.10 Pyruvate kinase

Detailed Content

2.5 Triose phosphosphate Isomerase

Structure

Catalytic mechanism

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2.6 Glycealdehide-3- phosphate dehydrogenase

Structure

Catalytic mechanism

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1 CARBOHYDRATES:

* Sugars, the smallest carbohydrates , serve as

fuel and carbon sources

* Polysaccharides , the polymers of sugars, have storage and structural roles

Carbohydrates include both sugars and

their polymers The simplest carbohydrates are the monosaccharides ,

or single sugars, also known as simple sugars Disaccharides are double sugars, consisting of two monosaccharides joined

by condensation The carbohydrates that are macromolecules are polysaccharides , polymers of many sugars

1.1 Sugars, the smallest carbohydrates, serve as fuel and carbon sources

- Monosaccharides (from the Greek monos, single, and sacchar, sugar) generally

have molecular formulas that are some multiple of CH2O

- Glucose (C6H12O6), the most common monosaccharide , is of central importance in the chemistry of life In the structure of glucose, we can see the trademarks of a sugar:

- The molecule has a carbonyl group and multiple hydroxyl groups Depending on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a ketose

(ketone sugar)

- Glucose , for example, is an aldose ; fructose , a structural isomer of glucose, is a

ketose (Most names for sugars end in -ose )

- Monosaccharides , particularly glucose, are major nutrients for cells In the process known as cellular respiration , cells extract the energy stored in glucose molecules

- Not only are simple sugar molecules a major fuel for cellular work, but their carbon skeletons serve as raw material for the synthesis of other types of small organic molecules, such as amino acids and fatty acids

Fig 3.1 The structure and classification of some monosaccharides

Sugars may be aldoses (aldehyde sugars) or ketoses (ketone sugars), depending on the location of the carbonyl group (pink) Sugars are also classified according to the length of their carbon skeletons A third point of variation is the spatial arrangement around asymmetric carbons

(compare, for example, the gray portions of glucose and galactose)

Fig 3.2 Linear and ring forms of glucose

- A disaccharide consists of two monosaccharides joined by a

glycosidic linkage, a covalent bond formed between two

monosaccharides by a dehydration reaction For example, maltose is a disaccharide formed by the linking of two molecules of glucose

- Also known as malt sugar, maltose is an ingredient for brewing beer The most prevalent disaccharide is sucrose, which is table sugar Its two monomers are glucose and fructose

- Plants generally transport carbohydrates from leaves to roots and other nonphotosynthetic organs in the form of sucrose

- Lactose, the sugar present in milk, is another disaccharide, consisting

of a glucose molecule joined to a galactose molecule

Fig 3.3

Examples

of disaccharide

synthesis

1.2 Polysaccharides, the polymers of sugars,

have storage and structural roles

- Polysaccharides are macromolecules , polymers with a

few hundred to a few thousand monosaccharides joined

- Starch, a storage polysaccharide of plants , is a polymer consisting entirely of glucose monomers Most of these monomers are joined by 1-4 linkages (number 1 carbon to number 4 carbon), like the glucose units in maltose The angle of these bonds makes the polymer helical

- The simplest form of starch, amylose , is unbranched Amylopectin , a more

complex form of starch, is a branched polymer with 1-6 linkages at the branch points.

- Plants store starch as granules within cellular structures called plastids , including chloroplasts (see fig5-6a) By synthesizing starch, the plant can stockpile surplus glucose Because glucose is a major cellular fuel, starch represents stored energy The sugar can later be withdrawn from this carbohydrate bank by hydrolysis, which breaks the bonds between the glucose monomers

- Animals store a polysaccharide called glycogen, a polymer of glucose that is like amylopectin but more extensively branched Humans and other vertebrates store glycogen mainly in liver and muscle cells Hydrolysis of glycogen in these cells

1.2.1 Storage polysaccharides

Fig 3.4 Storage polysaccharides

These examples, starch and glycogen , are composed entirely of glucose monomers, abbreviated here as hexagons The polymer chains tend to spiral to form helices

- Organisms build strong materials from structural polysaccharides For example, the polysaccharide called cellulose is a major component of the tough walls that enclose plant cells On a global scale, plants produce almost

1011 (100 billion) tons of cellulose per year; it is the most abundant organic compound on Earth

- Like starch, cellulose is a polymer of glucose, but the glycosidic linkages in these two polymers differ The difference is based on the fact that there are actually two slightly different ring structures for glucose

- When glucose forms a ring, the hydroxyl group attached to the number 1 carbon is locked into one of two alternative positions: either below or above the plane of the ring These two ring forms for glucose are called alpha (a) and

beta (b), respectively

- In starch, all the glucose monomers are in the a configuration.

- In contrast, the glucose monomers of cellulose are all in the b configuration ,

1.2.2 Structural polysaccharides

Fig 3.5

Starch and

cellulose structures

- The differing glycosidic links in starch and cellulose give the two

molecules distinct three-dimensional shapes Whereas a starch molecule is mostly helical, a cellulose molecule is straight (and never branched), and its hydroxyl groups are free to hydrogen-bond with the hydroxyls of other cellulose molecules lying parallel to it

- In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils These cables are a strong building material for plants as well as for humans, who use wood, which is rich in cellulose, for lumber.

- Enzymes that digest starch by hydrolyzing its a linkages are unable

to hydrolyze the b linkages of cellulose In fact, few organisms possess enzymes that can digest cellulose Humans do not; the cellulose fibrils in our food pass through the digestive tract and are

Some microbes can digest cellulose, breaking it down to glucose monomers Another important structural

polysaccharide is chitin, the carbohydrate used by arthropods

(insects, spiders, crustaceans, and related animals) to build their exoskeletons An exoskeleton is a hard case that surrounds the soft parts of an animal Pure chitin is leathery, but it becomes hardened when encrusted with calcium carbonate, a salt

- Chitin is also found in many fungi, which use this polysaccharide rather than cellulose as the building material for their cell walls

- Chitin is similar to cellulose, except that the glucose monomer of chitin has a nitrogen-containing appendage:

Fig 3.7 Chitin, a structural polysaccharide

(a) Chitin forms the exoskeleton of arthropods This

cicada is molting, shedding its old exoskeleton and

emerging in adult form

(b) Chitin is used to make a strong and flexible

surgical thread that decomposes after the wound

or incision heals

- The molecule known as ATP, short for adenosine triphosphate, is the central character

in bioenergetics

- The triphosphate tail of ATP is the chemical equivalent of a loaded spring; the close

packing of the three negatively charged phosphate groups is an unstable, energy-storing arrangement The chemical "spring" tends to "relax" by losing the terminal phosphate

- The cell taps this energy source by using enzymes to transfer phosphate groups from

ATP to other compounds, which are then said to be phosphorylated Phosphorylation

primes a molecule to undergo some kind of change that performs work, and the molecule loses its phosphate group in the process

ATP = ADP + Pi

- For example, a working muscle cell, for example, recycles its ATP at a rate of about 10

million molecules per second To understand how cellular respiration regenerates ATP,

THE STRUCTURE OF ATP, NAD+

Fig 3.8 The structure and hydrolysis of ATP

The hydrolysis of ATP yields inorganic phosphate and ADP

In the cell, most hydroxyl groups of phosphates are ionized ( O-)

Fig 3.9 NAD+ as an electron shuttle

The full name for NAD+, nicotinamide adenine dinucleotide, describes its structure; the molecule consists of two nucleotides joined together (Nicotinamide is a nitrogenous base, although not one that is present in DNA or RNA.) The enzymatic transfer of two electrons and one proton from some organic substrate to NAD+ reduces the NAD+ to NADH Most of the electrons removed from food are

- Electrons lose very little of their potential energy when they are transferred from food to NAD+

- Each NADH molecule formed during respiration represents stored

energy that can be tapped to make ATP when the electrons complete their

"fall" from NADH to oxygen

- How do electrons extracted from food and stored by NADH finally reach

O2?

(1) The reaction between H2 and O2 to form H2O + gases = explosion +

release of energy

(2) Cellular respiration also brings H2 and O2 together to form H2O, but there

are two important differences First, in cellular respiration, the H2 that reacts with O2 is derived from organic molecules Second, respiration uses an

electron transport chain to break the fall of electrons to O2 into several energy-releasing steps instead of one explosive reaction

- The transport chain consists of a number of molecules, mostly

proteins, built into the inner membrane of a mitochondrion

- Electrons removed from food are shuttled by NADH to the

"top" end of the chain At the "bottom" end, O2 captures these electrons along with H2, forming water.

- Thus, electrons removed from food by NAD+ fall down the electron transport chain to a far more stable location in the

electronegative O2 atom

Food  NADH  electron transport chain 8n oxygen

Fig 3.10 An introduction to electron transport chains

(a) The uncontrolled exergonic reaction of H2 with O2 to form H2O releases a large

amount of energy in the form of heat and light: an explosion

(b) In cellular respiration, the same reaction occurs in stages: An electron transport

chain breaks the "fall" of electrons in this reaction into a series of smaller steps and stores some of the released energy in a form that can be used to make ATP

(The rest of the energy is released as heat.)

THE PROCESS OF CELLULAR RESPIRATION

* Respiration involves glycolysis, the Krebs cycle, and electron transport:

* Glycolysis harvests chemical energy by oxidizing glucose to pyruvate:

* The Krebs cycle completes the energy-yielding oxidation of organic molecules:

* The inner mitochondrial membrane couples electron transport to ATP synthesis:

* Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes:

Respiration involves glycolysis, the Krebs cycle,

and electron transport

- In a eukaryotic cell, glycolysis occurs outside the mitochondria in the cytosol

The Krebs cycle and the electron transport chains are located inside the mitochondria

- During glycolysis, each glucose molecule is broken down into two molecules of the compound pyruvate

- The pyruvate crosses the double membrane of the mitochondrion to enter the matrix, where the Krebs cycle decomposes it to carbon dioxide

- NADH or FADH2 transfers electrons from molecules undergoing glycolysis and the Krebs cycle to electron transport chains, which are built into the inner mitochondrial membrane

The electron transport chain converts the chemical energy to a form that can be used to drive oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration

- A smaller amount of ATP is formed directly during glycolysis and the Krebs cycle by substrate-level phosphorylation

(1) Glycolysis (color-coded teal throughout the chapter)

(2) The Krebs cycle (color-coded salmon)

(3) The electron transport chain and oxidative phosphorylation (color-coded violet)

33 Fig.3.11: Overview of the cellular respiration

Respiration is a cumulative function of 3 metabolic stages

(1) Glycolysis (color-coded teal throughout the chapter) - cytosol

Glycolysis, begins the degradation by breaking: glucose = two molecules pyruvate

(2) The Krebs cycle (color-coded salmon) - mitochondrial matrix

Decomposing a derivative of pyruvate to CO2

(3) The electron transport chain and oxidative phosphorylation (color-coded violet)

the electron transport chain accepts electrons from the breakdown products of the first two stages (usually via NADH) and passes these electrons from one molecule to

another

 The energy released at each step of the chain is stored in a form the

mitochondrion can use to make ATP

 - This mode of ATP synthesis is called oxidative phosphorylation because it

is powered by the redox reactions that transfer electrons from food to O2.

 - Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration

 - A smaller amount of ATP is formed by substrate-level phosphorylation when an enzyme transfers a phosphate group from a substrate molecule to

ADP "Substrate molecule" here refers to an organic molecule generated during the catabolism of glucose.

 cell respiration

 glucose = CO 2 + H 2 O + 38 molecules of ATP

Fig 3.12 Substrate-level phosphorylation

Some ATP is made by direct enzymatic transfer of a phosphate group from a substrate to

2 INTRODUCTION

Glycolysis is an almost universal pathway for extraction of the energy available from carbohydrates, shared among prokaryotes

glycolysis is the only significant source of energy from carbohydrates In aerobic organisms, considerably more energy

cycle Glycolysis produces energy in the form of ATP and NADH

The glycolytic pathway consists of 10 enzyme-catalyzed steps During glycolysis, glucose, a six-carbon carbohydrate, is oxidized

to form two molecules of pyruvate, a three-carbon molecule For each glucose molecule metabolized, the pathway produces two molecules of ATP and two molecules of NADH

2 INTRODUCTION

molecules besides glucose can enter at a few points along the

breakdown glucose-6-phosphate, can enter the glycolytic pathway at the second step Glyceraldehyde-3-phosphate, which is produced by photosynthesis, is also a glycolytic

into glycolysis when energy is needed Additionally, intermediates can be drawn out of the glycolytic pathway when energy levels are high, for use in biosynthetic pathways For instance, during active energy production pyruvate, the product

needed pyruvate serves as a substrate in amino acid

Fig 3.13: Reactions

of Glycolysis

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

- Glucose, a six-carbon sugar, is split into two three-carbon sugars

These smaller sugars are then oxidized and their remaining atoms

rearranged to form two molecules of pyruvate (Pyruvate is the ionized form

of a three-carbon acid, pyruvic acid.)

- The pathway of glycolysis consists of ten steps, each catalyzed by a specific enzyme We can divide these ten steps into two phases: The

energy investment phase includes the first five steps, and the energy payoff phase includes the next five steps

- During the energy investment phase, the cell actually spends ATP to

phosphorylate the fuel molecules and NAD+ is reduced to NADH by

oxidation of the food

glycolysis

2 GLYCOLYSIS