Download Lecture 11: Take your Vitamins! Enzyme Cofactors Reference

Lecture 11: Take your Vitamins! Enzyme Cofactors
Reference: Lieberman and Marks Chapter 8 and lecture notes
NOTE: This lecture contains a lot of organic chemistry and reaction mechanisms. It is
not necessary to know this detail. The goal of the lecture is to introduce cofactors and
their role in enzymology.
1. Define the term “cofactor” and explain why cofactors are important for
some enzyme reactions
A. Cofactor
-Small organic molecules or metal ions that work in concert with an
enzyme to catalyze biochemical reactions
B. Importance to Enzyme Reactions
-Catalysis by enzymes is limited to specific side chains. But these
side chains can’t do all the chemistry that is required.
-Cofactors provide special chemical reactivity or structural
properties that can drive these special reactions
2. List the various classifications of cofactors and give a one sentence
definition of each
-Loosely bound: molecule or ion will bind to the enzyme, carry out
reaction, then dissociate from the enzyme
Ex: 1. ATP will bind, then dissociate
2. Mg2+ ions bind and dissociate
-Tightly bound: may be covalently attached to an enzyme residue in the
active site, or bind non-covalently with very high affinity
Ex: 1. Heme often covalently bound, as is biotin and lipoic acid
2. Flavins bind non-covalently but very tightly, Zn2+ ions
bind by electrostatic bonds to active site residues
3. For each coenzyme listed in Table 7-1 (slide 8), list the vitamin source of the
cofactor (if there is one) and provide the metabolic role of each cofactor. This will
be very important for the nutrition class also.
4. For each cofactor example given in the slides, name the functional part of the
molecule (the “business end”) and the type of reaction it participates in (also
named in Table 7-1).
A. Metabolite Cofactors: Molecules that are produced by metabolic pathways that are
used by other enzymes to carry out key reactions
a. ATP : The functional part of the molecule is the terminal (gamma) phosphate
-It is produced by several pathways (oxidative phosphorylation,
glycolysis, fatty acid oxidation) and used as an energy donor by dozens of enzymes
b. S-adenosylmethonine (SAM): The functional part of the molecule is the
methyl group
-donates a methyl group in nearly all biochemical reactions requiring a
methyl group transfer
B. Vitamin-derived Coenzymes
a. Vitamin C: Functional group is the double bond between the two carbons with
the OH groups
-donates electrons to reduce targets
b. Coenzyme A: Functional group is the SH (thiol)
-carries acyl groups in metabolism made from Vitamin B5 and metabolic
cofactor ADP
-One of the most common cofactor used by metabolic pathways.
-Used as an acyl group carrier. Acetate most common, but can be fatty
acid or ketone group
-The sulfur atom works with the carbonyl group of the acyl group to
activate the acyl group for reactivity
C. Nicotinamide Coenzymes
a. NADP+/NADPH: Functional group is the nicotinamide portion (carbamylated
pyridine ring)
-Used in reactions Involving Hydride (H: -) ion… Carries reducing
equivalents
-Donates or accepts one proton and two electrons as a hydride ion (H: -)
-Commonly used by dehydrogenase enzymes
-A true cosubstrate: binds like a substrate and exits like a product.
-NADH biggest role? Carry electrons to mitochondria to drive ATP
production.
-NADPH used in fatty acid and cholesterol syntheses
D. Flavins
a. FMN = flavin mononucleotide and FAD = flavin adenine dinucleotide:
Functional group is two nitrogen atoms in the ring
FMN
FAD
-Electrons and protons are accepted or donated in two steps
-Biggest role? Electron transport in mitochondria to drive ATP production
-Others functions include xenobiotic drug metabolism via cytochorme P450,
lipid metabolism, antioxidant
E. Thiamine Pyrophosphate: Functional group is the carbon in the ring which acts as a
very strong nucleophile
-Decarboxylation and transketolation reactions: transfer of aldehyde groups
-Energy metabolism, essential for neuronal and neurocognitive function (used in
synthesis of acetylcholine, nucleic acids and NADPH)
F. Pyridoxyl Phosphate (Vitamin B6 derivative): Functional group is the aldehyde group
-Covalently attached to host enzyme
-Carries out reactions with amino acids, like isomerizations, decarboxylation, side chain
removal.
-PLP function can depend on NADH, cobalamin, folic acid
Ex: homocysteine levels
G. Cobalamin (Vitamin B12): Functional Group is the Cobalt ion
-Adenosylcobalamin: Catalyzes exchange reactions: Isomerizations
-Methylcobalamin: transfers methyl groups
-Homocysteine is an intermediate in the synthesis of methionine.
-THF donates the methyl group.
-Methylcobalamin drives the transfer reaction
H. Lipid-Soluble Vitamins
a. Vitamin A: Functional group is the -OH
-A (from β-carotene) used in vision
b. Vitamin K: Functional group is the naphthoquinone ring
- used in synthesis of some blood clotting proteins and carboxylation of some
glutamate residues
c. Vitamin E: Functional group is the phenol
-Vitamin E has anti-oxidant properties
d. Vitamin D: Functional group is the double bond all the way on the right of
the molecule
-
I. Heme
Vitamin D helps with Ca2+ uptake
-Example of a prosthetic group: tightly or covalently bound to host protein
-Central iron atom involved in electron transfer
-Key component of hemoglobin and of enzymes in electron transfer chain
J. Tightly-Bound Metals
a. Iron-Sulfur clusters: electron transfer reactions, especially in electron
transport chain and in TCA cycle
b. Zn2+ ions: often form electrostatic interactions with substrates to control
position of substrate or to activate a bond or reactive group