Chapter 7: Coenzymes and Vitamins Flashcards
molecules required by enzymes for catalysis
cofactors
what are the two families of cofactors?
- essential ions
2. coenzymes
- usually metal ions
- often the essential minerals in your diet - activator ions for enzymes
essential ions
- usually organic compounds
- act as group transfer agents - they accept and donate specific chemical groups
coenzymes
what are the two classes of essential ions?
- activator ions (loosely bound)
2. metal ions of metalloenzymes (tightly bound)
what are the two classes of coenzymes?
- cosubstrates (loosely bound)
2. prosthetic group (tightly bound)
enzymes that require a cofactor but do not have one bound
apoenzymes (apoproteins)
an enzyme together with the cofactor(s) required for activity
holoenzyme (holoprotein)
what else can the term ‘holoenzyme’ be applied to?
- enzymes that contain multiple protein subunits
- proteins that transport small molecules
what do more than a quarter of all enzymes require to achieve full catalytic activity?
metallic cations
- either have an absolute requirement for added metal ions or are stimulated by the addition of metal ions
- the metal is not undergoing any changes
metal-activated enzymes
- contain firmly bound metal ions at their active sites
- usually transition metals (often iron and zinc)
- metal often undergoes a change in redox state (involved in the mechanism)
metalloenzymes
- coenzymes that act as susbtrate in enzyme-catalyzed reactions
- it is altered in the course of the reaction and dissociates from the active site
- subsequent reaction regenerates the original structure
cosubstrates
- remain bound to the enzyme throughout the course of the reaction
- can be covalently or noncovalently bound to the apoenzyme
- must also be returned to its original form after catalysis
prosthetic groups
what are the two broad classes of vitamins?
- water soluble
2. lipid soluble
required daily in small amounts because they are easily excreted in the urine and cellular stores are unstable
water soluble vitamins
can be stored, usually in the liver, so excessive intake can be toxin (hypervitaminoses)
lipid soluble vitamins
vitamin source: N/A
metabolic roles: transfer of phosphoryl or nucleotidyl groups
mechanistic role: cosubstrate
adenosine triphosphate (ATP)
vitamin source: N/A
metabolic roles: transfer of methyl groups
mechanistic role: cosubstrate
S-Adenosylmethionine
vitamin source: N/A
metabolic roles: transfer of glycosyl groups
mechanistic role: cosubstrate
uridine diphosphate glucose
vitamin source: Niacin (B3)
metabolic roles: oxidation-reduction reactions involving two-electron transfers
mechanistic role: cosubstrate
Nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+)
vitamin source: Riboflavin (B2)
metabolic roles: oxidation-reduction reactions involving two-electron transfers
mechanistic role: prosthetic group
flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
vitamin source: Pantothenate (B5)
metabolic roles: transfer of acyl groups
mechanistic role: cosubstrate
coenzyme A (CoA)
vitamin source: Thiamine (B1)
metabolic roles: transfer of multi-carbon fragments containing a carbonyl group
mechanistic role: prosthetic group
thiamine pyrophosphate (TPP)
vitamin source: Pyridoxine (B6)
metabolic roles: transfer of groups to and from amino acids
mechanistic role: prosthetic group
Pyridoxal phosphate (PLP)
vitamin source: Biotin (B7)
metabolic roles: ATP-dependent carboxylation of substrates or carboxyl-group transfer between substrates
mechanistic role: prosthetic group
Biotin
vitamin source: Folate
metabolic roles: transfer of one-carbon substituents, especially formyl and hydroxymethyl groups; provides the methyl group for thymine in DNA
mechanistic role: cosubstrate
Tetrahydrofolate
vitamin source: Cobalamin (B12)
metabolic roles: intramolecular rearrangements, transfer of methyl groups
mechanistic role: prosthetic group
Cobalamin
vitamin source: N/A
metabolic roles: oxidation of a hydroxyalkyl group from TPP and subsequent transfer as an acyl group
mechanistic role: prosthetic group
Lipoamide
vitamin source: vitamin A
metabolic roles: vision
mechanistic role: prosthetic group
Retinal
vitamin source: vitamin K
metabolic roles: carboxylation of some glutamate residues
mechanistic role: prosthetic group
Vitamin K
vitamin source: N/A
metabolic roles: lipid-soluble electron carrier
mechanistic role: cosubstrate
Ubiquinone (Q)
vitamin source: N/A
metabolic roles: electron transfer
mechanistic role: prosthetic group
Heme group
