Microbiology 201 - Chapter

Overview of metabolism

Enzymes

Anaerobic metabolism

Respiration (aerobic metabolism)

Metabolism (Fig. 8.1)

All chemical reactions occurring in a cell that sustain life and reproduction

Two types of metabolic reactions

1. Anabolism Reactions involved in biosynthesis of cellular components

Ex. Proteins, nucleic acids, lipids, carbohydrates

o Needed for growth, reproduction and repair of cell

o Require energy, organic building blocks and inorganic nutrients

2. Catabolism Metabolic reactions that break down organic molecules

Ex. oxidation of glucose to CO2

o Produce energy and building blocks for anabolic reactions, movement and transport of substance across membranes

B. Enzymes

• Cells must perform thousands of different metabolic reactions
• Chemical reactions are too slow to sustain life processes

Enzymes are catalysts that speed up metabolic reactions in which chemical bonds are broken and formed

o Proteins composed of ~100s to1000s of amino acids

• Each metabolic reaction has it own enzyme
• Each enzyme is specific for:

1.) chemical reaction it catalyzes

2.) substrates and products of the reaction

• Reactants (enzyme substrates) are converted to products by the chemical reaction
  

Ex.
                      hydrolysis reaction
X -Y + H-OH ----------------------->X -H + Y -OH

Reactants                                             Products
(Enzyme substrates)

• The enzyme is not changed by the reaction and catalyzes the same reaction repeatedly
• Some enzymes are composed only of protein, others contain additional cofactors that aid catalysis

o Organic cofactors are called coenzymes

o Some cofactors are inorganic ions (Ex. Fe⁺⁺)

• Active site Enzyme location where substrates bind and react and where products are formed and released

o The active site is like a lock and the substrate a key that fits the lock (Fig. 8.5)

Main classes of metabolic reactions

1. Condensation (Ligation --joining reactions) Fig. (8.8a)

Synthesis of complex molecules by formation of bonds between smaller molecules

H₂O is usually removed during bond formation

2. Hydrolysis (Fig. 8.8b)

Breakdown of complex organic molecules

H₂O is usually added to break the bond

3. Addition or removal of a functional group

Exs.
1.) Glucose + ATP ----------> Glucose-P + ADP (Fig. 8.16)

2.) Pyruvic acid ------------> acetaldehyde + CO₂

4. Oxidation-reduction reactions

Transfer of electrons or hydrogen atoms from one compound to another

AH₂ + B -------------> A + BH₂

Enzyme names.

• Add "-ase" suffix to substrate or type of reaction
  

Ex.

o Lipase: catalyzes reaction that converts lipids into glycerol and fatty acids

                  lipase
lipid ------------------> glycerol + fatty acids

o Dehydrogenase: H is removed from the substrate

                        dehydrogenase
SH₂ --------------------------> P + 2H

Factors that affect rate (speed) of an enzyme-catalyzed reaction

• Temperature

o Increasing temperature increases rate --to a point

o Too high temp. denatures proteins (disrupts structure), preventing substrate binding and ability to catalyze the reaction

• pH

o Neutral pH is optimal for activity and stability of many enzymes

o Strongly acidic or basic solutions denature proteins

• Concentration of substrate and products

o Higher substrate concentration increases activity

o High product concentration inhibits activity

• Inhibitors

o Chemicals that decrease or eliminate activity

o May chemically modify active site or compete with substrates for binding to active site (Fig. 8.10)

Oxidation-Reduction Reactions

Oxidation = removal of electrons

Reduction = addition of electrons

Electrons are removed from a compound and transferred to another compound

Electrons may be transferred: singly, with H atoms or with hydride anions

An oxidation-reduction reaction involves 2 reactions (called half reactions) that occur together

AH₂ -----> A + 2H                     Oxidation half reaction

B + 2H -----> BH₂                     Reduction half reaction
___________________

AH₂ + B -----> A + BH_{2     Net reaction (2 electons, as 2 H atoms, were transferred from A to B)}

Oxidation-reduction reactions release energy

During catabolism, some of the energy is captured and stored in high energy bonds between phosphate groups of adenosine triphosphate (ATP)

ATP makes the energy available to the cell when needed

Glucose C₆H₁₂O₆

Contains energy stored in the electrons that form the covalent bonds between the atoms

Nutrient used by many organisms as:

o Source of energy for synthesis of ATP

o Carbon skeletons for synthesis of cell constituents

o Electrons for oxidation-reduction reactions involved in some anabolic reactions

Presence or absence of O₂ determines:

o Amount of energy available to the cell

o Type of metabolic end products

Overview

1. Glycolysis

Glucose is oxidized to pyruvic acid, NAD⁺ is reduced to NADH and 2 ATPs are produced

2. Fermentation

Occurs if O₂ is not present

Regenerates NAD⁺ and disposes of electrons in organic waste products

No ATP is produced

3. Respiration

Occurs if O₂ is present

(some microorganisms can use other compounds in place of O₂, Ex. nitrate, sulfate)

Completes oxidation of pyruvic acid, regenerates NAD⁺ and produces a lot more ATP

Major steps

1.) 2 ATPs used to phosphorylate glucose, a 6-carbon sugar

2.) The product is split into two molecules of a 3-carbon compound (glyceraldehyde 3-P)

3.) Glyceraldehyde 3-P oxidized to pyruvic acid

a.) 2 NAD⁺ reduced to 2 NADH

b.) 4 ATPs produced

• Net result:

Glucose + 2 NAD⁺ + 2 ADP + 2 P ------> 2 pyruvic acid + 2 NADH + 2 ATP

• For glycolysis to continue, NAD⁺ must be regenerated

Two ways depending on whether O₂ is absent or present

a. Fermentation (anaerobic environments) or

b. Respiration (aerobic environments)

• NAD⁺ (nicotinamide adenine dinucleotide) is a coenzyme used by many enzymes that catalyze oxidation-reduction reactions involving transfer of 2 electrons, often as a hydride ion, H-

When a substrate is oxidized, NAD⁺ is reduced to NADH (Fig. 8.14)

• Purpose is to regenerate NAD⁺ so that glycolysis can continue when O₂ is not present

1. NADH is oxidized back into NAD⁺

2. Pyruvic acid is reduced to fermentation end products

Organic acids

Alcohols

Gases: CO₂ and H₂

• The fermentation end products define the type of fermentation

Types of fermentation (Fig. 8.26)

1. Homolactic acid fermentation (lactic acid only end product)

Electrons are transferred from NADH to pyruvic acid

Lactic acid is a catabolic waste product (fermentation end product) used to dispose of electrons and regenerate NAD⁺

Lactic acid production lowers pH

Lactic acid bacteria -- Gram positive rods and cocci

Normal flora of vaginal and intestinal tract that inhibit growth of pathogens

Used to manufacture food: Ex. yogurt, sour cream, cheese, saukraut

Important for production of silage for feeding to livestock

2. Alcoholic fermentation (ethanol and CO₂ end products)

Carried out by yeasts --used to make bread, wine, beer and ethanol for fuel

Some bacteria also do this

3. Other types of fermentation produce different end products (Fig. 8.26)

Analysis of fermentation end products helps identify anaerobic microorganisms

Ex. Enterobacter sp. produce acetoin that is detected with the Voges-Proskauer test

• Requires O₂
• Glucose is completely oxidized to CO₂
• Much more ATP is produced

Involves four processes:

1. Oxidation of pyruvic acid to acetyl-Coenzyme A

2. Oxidation of acetyl-CoA to CO₂ via Krebs cycle (TCA cycle)

3. Regeneration of NAD⁺ and transport of electrons to O₂

4. Formation of ATP via oxidative phosphorylation

1. Oxidation of pyruvic acid (Fig 8.21)

Pyruvic acid + NAD⁺ Coenzyme A ----------> Acetyl-CoA + CO₂ + NADH

2. Krebs cycle

• Each acetyl group is oxidized, producing 2 molecules of CO₂ (steps 3 and 4, below)

o Completes the oxidation of all 6 carbons of glucose

• Electrons (4 pairs per acetyl group) from oxidation are transferred to NAD⁺ and FAD

o Three NADH and one FADH₂ are produced (steps 3, 4, 6 and 8)

• Small amount of usable energy is produced -as ATP by bacteria and plants or GTP by animals (step 5)

• NAD⁺ and FAD must now be regenerated and ATP produced

3. Electron transport chain

• Located in a membrane (Fig. 8.23)

o Cell membrane of prokaryotic cells (Fig. 8.24c)

o Mitochondrial membrane of eukaryotic cells

• NADH and FADH₂ are oxidized

o Regenerates NAD⁺ and FAD for reuse in glycolysis and Krebs cycle

• Electrons are transported via a chain of carriers to O₂

o O₂ is reduced to water inside cell

4 electrons + O₂ + 4 H+ ---------> 2 H₂O

o H⁺ are pumped across membrane to exterior of prokaryotic cells

o Results in higher [H⁺ ] outside cell than inside

o This concentration gradient is a form of potential energy

4. Oxidative phosphorylation(Fig. 8.23 and 8.24)

• H⁺ moves back across the membrane and into the cell via chemiosmosis
• ADP is phosphorylated to ATP

The reaction is catalyzed by the enzyme ATP-ase

ATPs Produced

Metabolic process	Fermentation	Respiration
Glycolysis	2	8
Pyruvate to acetyl-CoA	-	6
Krebs cycle	-	24
Total ATPs	2	38

• Glycolysis and the Krebs cycle are called central metabolic pathways because the different organic compounds used for carbon and energy by the cell (carbohydrates, proteins and lipids and others) are broken down into common intermediates that enter these pathways where they are oxidized to CO₂ to produce ATP.

Ex.

Polysaccharides like starch are hydrolyzed to glucose which enters central metabolism via glycolysis

Fatty acids from lipids are oxidized to acetyl-CoA which enters via the Krebs cycle

Proteins are hydrolyzed to amino acids which are broken down to intermediates that enter the Krebs cycle as pyruvic acid, acety-CoA or carboxylic acids.

Some organic pollutants are hydrolyzed and oxidized to acety-CoA that enters the Krebs cycle

• Catabolism is coupled to anabolism when carbon intermediates of glycolysis and the Krebs cycle are removed from the pathways (rather than being completely oxidized to CO₂) and used with ATP in biosynthetic pathways to make cell constituents.

Uses for ATP generated during catabolism

1. Biosynthesis (anabolism) --formation of covalent bonds to produce:

o Lipids, amino acids, enzymes, membrane proteins, nucleotides, DNA, RNA, peptidoglycan, etc.

2. Active transport of substances across cell membrane

3. Movement (motility)

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Last updated: Feb. 22, 2007/jh