Lecture 11 Cofactors Vitamins Cofactors Introduction Many enzymes

Lecture 11 Cofactors & Vitamins

Cofactors – Introduction • Many enzymes do not need any additional components to show full activity. - However, others require non-protein molecules called cofactors for activity. • Cofactors are organic or inorganic molecules that are required for the activity of a certain enzymes. - Apoenzyme = enzyme without (-) cofactor. - Holoenzyme = enzyme with (+) cofactor.

Cofactors – Introduction • Two types of cofactors: 1. Inorganic cofactors – essential ions (e. g. metal ions and ironsulfur clusters). 2. Organic cofactors – coenzymes (e. g. flavin and heme).

Cofactors - Essential Ion Cofactors • Activator ions – bind reversibly to enzyme and often participate in substrate binding. • Metal activated enzymes – require or are stimulated by addition of metal ions (i. e. Mg 2+, is required by many ATP requiring enzymes). • Metal ions of metalloenzymes – cations that are tightly bound to enzyme and participate directly in catalysis (Fe, Zn, Cu, Co). - i. e. Zinc protease (angiotensin converting enzyme).

Coenzymes • Coenzymes are small organic molecules (organic cofactors) that can be loosely or tightly bound to an enzyme. • Coenzymes include NADH, NADPH and adenosine triphosphate. These molecules transfer chemical groups between enzymes. • Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. • Two classes of coenzymes: 1. Co-substrates. - altered in reactions and regenerated to original structure in subsequent reactions. - disassociated from active site. - shuttle chemical groups among different enzyme reactions. 2. Prosthetic groups. - remains bound to enzyme.

Coenzymes • Examples: 1. Metabolite coenzymes – synthesized from common metabolites. 2. Nucleoside triphosphates – (ATP) can donate phosphates, pyrophosphates, adenosyl groups. 3. S-adenosylmethionine (SAM) – donates methyl groups. 4. Nucleotide sugars (uridine diphosphate glucose = UDP-glucose) - transfer sugars in carbohydrate metabolism.

Vitamin Derived Coenzymes • Must be obtained from diet. • Synthesized by microorganisms and plants. • Vitamins must be enzymatically transformed to function as a coenzyme. Vitamin Coenzyme Thiamin (B 1) Riboflavin (B 2) Niacin(B 3) Pyridoxal (B 6) Cobalamin (B 12) Pantothenate (B 5) Biotin Folate Vitamin A Vitamin K Thiamin-pyrophosphate FMN & FAD NAD+/NADH/NADP+/NADPH Pyridoxal phosphate adenosyl- and methylcobalamin Coenzyme A Biotin Tetrahydrafolate Retinal Vitamin K

Vitamin - Thiamin (B 1) • First vitamin discovered (Vital amine = Vitamin). • Deficiencies lead to disease called Beriberi (neurological disorders, heart problems, anorexia). • Example of thiamin reaction - Thiamin pyrophosphate (TPP). • Thiazolium ring is the chemically active part of TPP. • Serves as a cofactor in decarboxylation reactions of keto acids a prosthetic group in transketolases.

Vitamin - Riboflavin (B 2) • Water soluble vitamin. • Severe deficiencies lead to growth retardation, reproductive problems and neural degeneration. • Meat, dairy products, dark green vegetables, legumes and grains are good sources. • Form FMN and FAD.

Vitamin - Riboflavin (B 2) • FMN and FAD are important for electron transfers in many biological reactions. • FAD and FMN can transfer electrons one or two at a time. Quinone form Hydroquinone form semiquinone form

Vitamin - Niacin (B 3) • Also known as nicotinic acid. • Deficiencies lead to pellagra (dermatitis, diarrhea, dementia). • Can be synthesized from tryptophan in the liver. • Required in relatively high amounts compared to other vitamins. - Niacin is a precursor to NAD+/NADH and NADP+/NADPH, which play essential metabolic roles in living cells. - Niacin is involved in both DNA repair, and the production of steroid hormones in the adrenal gland.

Vitamin - Niacin (B 3) • NAD+ / NADP+. - Serve as cofactors in oxidation/reduction reactions. - Act as co-substrates for dehydrogenases. - Reduction of NAD+/NADP+ and oxidation of NADH/NADPH occurs 2 e- at a time. - NADH is coupled with ATP production in mitochondria. - NADPH is an important reducing agent in biosynthetic reactions.

Vitamin - Pantothenic acid (B 5) • Pantothenic acid is water soluble vitamin. • Important coenzyme to produce other biologically important molecules (co-enzyme A). • Co-enzyme A involved in acyl group transfer. • Hydrophobic acyl groups (fatty acids) are made more water soluble with co-enzyme A attached.

Vitamin - Pyridoxal (B 6) • Pyridoxal is one of the three natural forms of vitamin B 6, along with pyridoxamine and pyridoxine. • All three forms of vitamin B 6 are heterocyclic organic compounds. • All these forms are converted in the body into a single biologically active form, pyridoxal 5 phosphate (PLP). - Important in amino acid metabolism. - Bound to enzyme through reactions with lysine. - PLP functions in transamination, decarboxylation, isomerization, side-chain elimination reactions involving amino acids.

Vitamin - Biotin (Vitamin H/Vitamin B 7) • Water soluble vitamin. • Produced by gut microflora which supplies ½ Required Daily Allowance. • Deficiencies are rare. • Consuming 6 raw eggs a day can cause deficiencies due to the presence avidin. • Biotin cofactor. - Involved in ATP dependent carboxylation reactions. - Covalently bound to enzyme through amide linkage with lysine. - Important biotin enzymes are acetyl-Co. A carboxylase, pyruvate carboxylase.

Vitamin - Folate (Vitamin B 9) PABA glutamate pterin • Water soluble vitamin. • Folate important during pregnancy to prevent neural tube defects in fetus (i. e. spina bifida). • Vitamin B 9 deficiencies cause folate deficiencies. • Has a poly-glutamate tail formed by gamma-carboxy and alpha amino groups (unusual peptide bond). • Pterin ring important functional group.

Vitamin - Folate (Vitamin B 9) • Tetrahydrofolate (THF). - Folate is converted to THF by the addition of 4 hydrogens to the pterin ring. - Important in transfer of one-carbon units of other molecules such as methanol, formaldehyde or formic acid.

Vitamin - Cobalamin (B 12) • Water soluble Vitamin. • Corrin ring with Cobalt cation. • Involved in intramolecular rearrangements, methyl group transfer, reduction of ribonucleotides to deoxyribonucleotides • Forms radical species.

Vitamin - Fat soluble Vitamins • Some vitamins are fat soluble (i. e. vitamins A, D, E, K). - Vitamin A (retinol) derived from β-carotene is important for vision, regulation of gene expression during cell differentiation and teratogenic. - Vitamin D – important in calcium absorption, regulates intestinal absorption and deposition in bones. - Vitamin E – antioxidant. - Vitamin K –needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue.

Vitamin - Ubiquinone/Plastoquinone • Lipid soluble electron carriers. • Important in electron transport chains. • Can accept or donate electrons one or two at a time.

Vitamin - Cytochromes • Protein coenzyme. • Heme containing proteins. • Fe 3+ can undergo reversible one electron reduction. • Important in redox reactions.