index
Announcements
Questions
Chapter 7: Coenzymes and Vitamins
Terminology of Cofactors
Section 7.1: Essential Ions
Carbonic Anhydrase is a Metalloenzyme
Mechanism of Carbonic Anhydrase, Step 1
Mechanism of Carbonic Anhydrase, Step 2
Section 7.2: Coenzymes
The Source of Coenzymes
Some Vitamins and Associated Diseases
Major Coenzymes
Vitamin C
Section 7.3: Nucleotide Cosubstrates
S-Adenosylmethionine is a Methyl Carrier
Uridine Diphosphate (UDP) is a Glucose Carrier
Section 7.4: NAD⁺ and NADP⁺
Struture of NAD⁺ and NADP⁺
Oxidation of Lactate
Questions
References
Next Lecture: Sections 7.5 - 7.11

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 019

Outline

Announcements
Questions from previous lecture
Section 7.1: Essential Ions
Section 7.2: Coenzymes
Section 7.3: Nucleotide Cosubstrates
Section 7.4: NAD⁺ and NADP⁺

Announcements

Lecture 18 notes are online

Questions

Any questions on the material from the previous lecture?

Chapter 7: Coenzymes and Vitamins

In Chapter 7, we'll take a whirlwind tour through many of the major coenzymes and their vitamin precursors
Coenzymes are enzyme cofactors, which provide chemical properties for catalysis that are not available in the limited repertoire of the 20 amino acid side chains
These accessory molecules, along with mineral cofactors, are needed for many of the core reactions of cellular metabolism. Many cofactors cannot be synthesized in humans or other animals, and must instead be obtained from vitamins and minerals in the diet

Terminology of Cofactors

Cofactors consist of two main groups: essential ions and coenzymes
Essential ions include minerals such as iron, zinc and magnesium that participate in substrate binding and catalysis. They can either be loosely bound as activator ions or tightly bound, in metalloenzymes
Coenzymes are organic compounds that bind specific groups to be transferred between reactants. Cosubstrates are loosely bound coenzymes and prosthetic groups are tightly bound

Section 7.1: Essential Ions

Over a quarter of all known enzymes require metal cations for full activity
Loosely-bound activator ions are important for binding substrates. An example is magnesium, Mg²⁺, which is found in many enzymes that bind nucleic acids
The positively-charged magnesium balances the negatively-charged phosphate groups, and its coordination geometry helps to stabilize and orient substrates such as ATP:

Carbonic Anhydrase is a Metalloenzyme

Carbonic anhydrase is a metalloenzyme that tightly binds a zinc ion at its reactive center
As we have already seen, carbonic anhydrase is a very proficient enzyme (10⁷ rate enhancement) that helps to maintain the equilibrium between CO₂ and carbonic acid, H₂CO₃, in the blood buffer system
It does this by catalyzing the following reaction:

CO₂ + H₂O → H⁺ + HCO₃^-

The mechanism of the reaction involves both acid-base catalysis and covalent catalysis, and uses the zinc ion to create a reactive hydroxide ion from water

Mechanism of Carbonic Anhydrase, Step 1

In the first step of the reaction mechanism, a water molecule is already bound on the enzyme from a previous reaction
The water has been ionized into a hydroxide ion bound to the zinc ion at the center of the active site
Three histidine residues position the zinc ion at the center, and an additional basic residue helps to abstract the proton from water
A CO₂ molecule then enters and reacts with the hydroxide to produce an intermediate that remains covalently bound to the enzyme

Mechanism of Carbonic Anhydrase, Step 2

In the second step, a second water enters and reacts with the zinc ion, displacing the bound bicarbonate ion and associated proton
The simplicity of the mechanism helps to contribute to the very high catalytic rate
Other zinc metalloenyzmes activate bound water molecules in a similar way

Section 7.2: Coenzymes

Coenzymes are small organic molecules that bind to enzymes and participate in the catalytic cycle
They are divided into two groups, depending on the tightness of their association with an enzyme. The first group, cosubstrates, act like other substrates in enzyme-catalyzed reactions
After binding and reacting, a cosubstrate dissociates with its bound reactive group from one enzyme and can then transfer the group to another. Such cosubstrates are 'recycled' over and over again, acting as shuttles or 'carriers' between pairs of enzyme-catalyzd reactions
Examples of cosubstrates include ATP and NAD⁺
The second type of coenzyme is a tightly-bound molecule called a prosthetic group. It remains bound to an enzyme throughout the reaction
An enzyme without its prosthetic group is called an apoenzyme. When the prosthetic group is added, the combination is referred to as the holoenzyme
Examples of prosthetic groups include the electron carrier FAD and the methyl carrier methylcobalamin

The Source of Coenzymes

Coenzymes are required in many different enzyme-catalyzed reactions that are found in all forms of life
Consequently, every species must obtain these coenzymes somehow. 4 of the 5 kingdoms of life, prokaryotes, protists, fungi, and plants, are able to synthesize most coenzymes from basic inorganic building blocks
In contrast, animals (including humans) need to obtain many of their coenzymes through the diet. Since coenzymes, like enzymes, can be reused for multiple reactions, they usually need to be supplied only in small amounts (micrograms or milligrams per day)
Many coenzymes are obtained from slight chemical modifications of precursors called vitamins. Vitamin comes from the term 'vital amine', coined by Casimir Funk, who discovered that rice husks contained a trace material (later shown to be thiamine) that could prevent the disease beriberi
Beriberi is one of many nutritional deficiency diseases that result from a lack of an essential mineral or vitamin in the diet. Symptoms of berberi include weight loss, heart problems and death

Some Vitamins and Associated Diseases

Major Coenzymes

Vitamin C

Vitamin C is the simplest vitamin in structure. It consists of a lactone ring, a condensation of the carboxylate group of a carbohydrate with a hydroxyl group to produce an internal ester
Like the cosubstrate NAD⁺, vitamin C is a reducing agent involved in electron tranfers, where ascorbic acid is oxidized to dehydroascorbic acid
The nutritional importance of vitamin C was recognized over 400 years ago, when sailors began to carry limes and other citrus fruits in order to avoid getting scurvy
Scurvy is associated with tissue degeneration that results from the breakdown of collagen fibers. Vitamin C is needed for regenerating the enzyme that catalyzes the synthesis of hydroxyproline, which provides additional hydrogen bonds between the collagen chains

Section 7.3: Nucleotide Cosubstrates

Nucleotides, the building blocks of nucleic acids, are used as coenzymes for many different reactions. Nucleotides are at the core of metabolism as carriers of both energy and information
ATP is the most common nucleotide. Nucleoside triphosphates are composed of three parts: an aromatic base, a ribose sugar and three phosphoryl groups linked together by high energy bonds
Phosphoryl group transfer is the most common role of ATP as a cosubstrate. Phosphorylation is used for reversible covalent modifications of enzymes. The hydrolysis of the phosphoryl groups is also as a source of energy in coupled reactions
Different types of nucleotides are associated with different areas of metabolism. ATP is involved in energy production and kinase-mediated regulation. GTP, guanosine triphosphate, is found signal transduction. UTP, uridine triphosphate, is a coenzyme in carbohydrate metabolism, and CTP is seen in lipid metabolism

S-Adenosylmethionine is a Methyl Carrier

S-adenosylmethionine is a coenzyme synthesized from methionine and ATP:
Methionine + ATP → S-adenosylmethionine + P_i + PP_i
The resulting sulfonium group is highly reactive and is the main donor of methyl groups in biosynthetic reactions

Uridine Diphosphate (UDP) is a Glucose Carrier

Section 7.4: NAD⁺ and NADP⁺

Nicotinamide adenine dinucleotide (NAD⁺) is a carrier of hydride ions. It is derived from the vitamine niacin, shown above
The closely-related coenzyme NADP⁺ is distinguished by the addition of a phosphate group on the 2' hydroxyl of its adenosine component
Niacin, also called nicotinic acid, is an essential precursor to NAD⁺, although it can also be derived from the degradation of tryptophan, an essential amino acid
Because nicotinamide is a derivative of pyridine, the nucleotide in the coenzyme is sometimes referred to as a pyridine nucleotide

Struture of NAD⁺ and NADP⁺

The electron transfer to NAD⁺ involves the addition of a hydride ion to the C4 carbon of the ring of the pyridine nucleotide
This produces a rearrangement of the electron structure in the ring so that the positively charged nitrogen receives an electron, giving it a non-bonded electron pair
Note that the positive charge of NAD⁺ refers only to the charge on the nitrogen, as the molecule has an overall negative charge from the phosphate groups

Oxidation of Lactate

A reaction involving NAD⁺ is seen in the oxidation of lactate to pyruvate
One hydrogen and two electrons are transferred to NAD⁺, reducing it to NADH, along with the production of a free proton
The reverse of this reaction is seen in anaerobic metabolism, where pyruvate is converted to lactate, to regenerate NAD⁺ in the absence of oxygen

Questions

Questions about the material covered today?

References

Function and Mechanism of Zinc Metalloenzymes,
Keith A. McCall, Chih-chin Huang and Carol A. Fierke,
Journal of Nutrition. 2000;130:1437S-1446S

Next Lecture: Sections 7.5 - 7.11

Read Sections 7.5 - 7.11

Table of Contents

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 019

Outline

Announcements

Questions

Chapter 7: Coenzymes and Vitamins

Terminology of Cofactors

Section 7.1: Essential Ions

Carbonic Anhydrase is a Metalloenzyme

Mechanism of Carbonic Anhydrase, Step 1

Mechanism of Carbonic Anhydrase, Step 2

Section 7.2: Coenzymes

The Source of Coenzymes

Some Vitamins and Associated Diseases

Major Coenzymes

Vitamin C

Section 7.3: Nucleotide Cosubstrates

S-Adenosylmethionine is a Methyl Carrier

Uridine Diphosphate (UDP) is a Glucose Carrier

Section 7.4: NAD+ and NADP+

Struture of NAD+ and NADP+

Oxidation of Lactate

Questions

References

Next Lecture: Sections 7.5 - 7.11

Section 7.4: NAD⁺ and NADP⁺

Struture of NAD⁺ and NADP⁺