Table of Contents
Fundamentals of Biochemistry
Biochem 380 - Fall 2006
Lecture 020
Outline
- Announcements
- Questions from previous lecture
- Section 7.5: FAD and FMN
- Section 7.6: Coenzyme A
- Section 7.7: Thiamine Pyrophosphate (TPP)
- Section 7.9: Biotin
- Section 7.10: Tetrahydrofolate
- Section 7.11: Cobalamin
Announcements
- Lecture 19 notes are online
- We will skip section 7.8
- Chris (TA) will not have office hours tomorrow
- Quiz on Friday on Chapter 6
Questions
- Any questions on the material from the previous lecture?
Section 7.5: FAD and FMN
- Last time, we discussed the coenzyme NAD, which is a 2-electron carrier in oxidation-reduction reactions.
FAD and FMN are two more coenzymes that act as electron carriers
- In contrast to NAD, these coenzymes are prosthetic groups that are tightly bound to their enzymes,
usually noncovalently
- Like NAD, FAD and FMN can be reduced by receiving 2 electrons and a hydrogen (combined
as a hydride ion).
However, FAD and FMN receive an additional proton and are reduced to FADH2 and FMNH2
- FAD and FMN also be partially reduced to an intermediate state by accepting just a single electron
Flavin Adenine Dinucleotide (FAD)
- FAD accepts 2 protons and 2 electrons at two nitrogens located on the isoalloxazine ring
- Like NAD, the cofactor consists of an adenine nucleotide connected via a diphosphate linkage, but
the second part is only a 'pseudo-nucleotide' with a linear 5-carbon chain
FAD is Partly Derived from Riboflavin
- FAD is partly derived from the vitamin riboflavin (vitamin B2):
- This vitamin is characterized by a yellow color which is seen in the oxidized form of the isoalloxazine ring
- The term 'flavin' comes from the latin word flavus, meaning 'yellow'
- Deficiencies in riboflavin are associated with diseases such dermatitus
The FAD Reaction
- The reduction reaction below shows the addition of 2 hydrogens and two electrons to the isoalloxazine ring:
Multiple Oxidation States of FAD
Section 7.6: Coenzyme A
- Another important coenzyme is the cosubstrate Coenzyme A (CoA)
- Unlike the electron carriers above, CoA is an activated carrier of carbon groups
- The reactive group on CoA is the terminal sulfhydryl, which can form bonds with acyl or acetyl carbon
compounds
- The resulting thioester C-S bond is more easily broken than the corresponding ester C-O bond, making
the activated carrier acetyl-CoA a good donor of acetyl groups in oxidation and biosynthesis reactions
Coenzyme A is Partly Derived from Pantothenic Acid
- Coenzyme A is partly derived from the vitamin pantothenic acid (vitamin B5):
- This vitamin is a very common compound that is present in most foods, which is why its name
is derived from the latin word pantos, meaning 'everywhere'
- Consequently, there are not any common dietary diseases associated with pantothenic acid
Section 7.7: Thiamine Pyrophosphate
- Thiamine Pyrophosphate (TPP) is a coenzyme derived from the vitamin thiamine
(Vitamin B1). TPP is synthesized by the transfer of a pyrophosphoryl group from ATP
- TPP is used as a prosthetic group in many decarboxylase enzymes. The reactive
portion of the coenzyme is the C2 carbon, which has a lowered pKa when bound to enzymes
- Deprotonation of the C2 carbon produces a reactive carbanion, a powerful nucleophile that
can attack carbonyl carbons adjacent to carboxylate groups
- The carbanion is stabilized by the adjacent nitrogen and sulfur atoms in the thiazole ring,
as can be seen in the mechanism for the decarboxylation of pyruvate
TPP and Pyruvate Decarboxylase
Section 7.9: Biotin
- In contrast to the decarboxylase reaction of TPP, biotin is a carrier of carbon dioxide
that is used to add carboxyl groups in biosynthetic reactions
- Biotin is a prosthetic group that is covalently attached to an enzyme by an amide linkage
to the end of a lysine side chain on the enzyme to form biocytin
- Like TPP, it has a ring structure that incorporates nitrogen and sulfur.
Here, the reactive site is at the N1 nitrogen, which receives carboxyl groups to form carboxybiotin
Biotin and Pyruvate Carboxylase
- An example is seen in the carboxylation of pyruvate. Here, a carboxyl group is added to biotin
in an ATP-dependent reaction with bicarbonate
- Next, the enolate form of pyruvate attacks the carboxyl group to form the 4-carbon compound oxaloacetate
and the regenerated coenzyme
Biotin and Avidin
- Biotin is synthesized by intestinal bacteria, so dietary deficiencies are rare in normal diets. However,
a protein in raw egg whites, avidin, binds very tightly to biotin, making it unavailable for absorption
- Cooking the eggs will denature the avidin and cause it to lose its affinity for biotin
- This strong binding property has been used in laboratory techniques in which biotin is covalently
attached to target compounds, which can then be selectively retained on columns with immobilized avidin
in affinity chromatography
Section 7.10: Tetrahydrofolate (THF)
- The coenzyme tetrahydrofolate (THF) is a carrier of one-carbon units in a variety of oxidation states
- THF is derived from the vitamin folic acid, or folate, which is found in yeast, liver and green plants, and so
takes its name from folium, which is Latin for 'leaf'
- The coenzyme consists of a pteridine ring joined to p-aminobenzoate connected to a chain of one or more glutamate residues
(polyglutamate tail)
Oxidation States of THF
- Because THF can exist in so many different oxidation states, it has been difficult to characterize biochemically,
but is increasingly recognized to be important in a number of pathways for single-carbon metabolism
- The following table shows the three different oxidation states of the carbon held in THF:
- The most oxidized state, equivalent to formic acid, consists of three convertable groups: the formyl,
formimino and methenyl groups
One-Carbon Conversions of THF
- Conversions between the multiple groups and oxidation states of carbon held by THF are shown below:
Nutritional Importance of Folate
- The nutritional importance of folic acid has come to be widely recognized only fairly recently, with FDA
guidelines for folic acid enrichment of foods becoming established in 1998
- Adequate amounts of folic acid in the diet are especially important for women of child-bearing years
because it has been shown to reduce the risk of birth defects such as spina bifida, a disease that can result in
deformities and brain damage
- Folic acid may also help to reduce of high levels of homocysteine in the blood, which has been associated with
certain types of Coronary Heart Disease (CHD)
- Additional benefits may include prevention of cancers (cervical, bronchial, colon and breast) and Alzheimers
disease
- However, the cause and prevention of these diseases often involves more than just nutritional deficiencies.
Other factors such as environmental influences and genetic polymorphisms are likely to play a role as well.
Section 7.11: Cobalamin
- The final coenzymes we'll cover today are derived from the vitamin cobalamin (vitamin B12)
- This is the largest and most complex vitamin, and the last to be isolated.
- Dorothy Hodgkin received the Nobel prize in 1964 for determining the crystal structure of vitamin B12
(Section 4.2)
- Robert Woodward performed a total synthesis of B12 in 1973 in a tour de force of organic chemistry
(100 step reaction)
- Vitamin B12 is only synthesized in bacteria, not plants or animals.
Vegetarians obtain vitamin B12 from microorganisms.
- An inadequate supply of cobalamin can result in the disease of pernicious anemia,
which leads to a decrease in the production of red blood cells
- Cobalamin is used to produce two different coenzymes, methylcobalamin and adenosylcobalamin.
These are prosthetic groups used in methyl transfer and group transfer reactions
Cobalamin Structure
- The principal component of cobalamin is the corrin ring system, which is similar to the
heme group in hemoglobin
- The reactive center contains a cobalt atom, which can be bound to either a methyl group or a
doxyadenosyl group
Cobalamin and Methyl Transfer
- An example of a reaction catalyzed by methylcobalamin is the conversion of homocysteine to
methionine
- A byproduct of the reaction is the creation of tetrahydrofolate
Questions
- Questions about the material covered today?
References
- MINIREVIEW Folic Acid: Nutritional Biochemistry, Molecular Biology, and Role in Disease Processes
Mark Lucock
Molecular Genetics and Metabolism 71, 121–138 (2000)
(Biochemistry of folate metabolism)
- How Folate Can Help Prevent Birth Defects
http://www.pueblo.gsa.gov/cic_text/health/folate/796_fol.html
Paula Kurtzweil
Federal Citizen Information Center web site (FCIC)
(Discusses importance of folate intake for women of child-bearing years)
- Some good sources of folate
http://www.pueblo.gsa.gov/cic_text/health/folate/folate.pdf
Federal Citizen Information Center web site (FCIC)
(Brief list of foods that are a good source of folate)
Next Lecture: Sections 7.12 - 7.16
- Read Sections 7.12 - 7.16
