Biotin

1. Biotin • Structure consists of two rings and a valeric acid side chain • Sources • liver, soybeans, egg yolk, cereals, legumes, nuts • often found bound to protein • biocytin o or biotinyllysine • avidin binds biotin and inhibits absorption • glycoprotein found in egg whites • heat labile so cooking denatures

2. Biotin • Digestion and Absorption • Protein bound biotin requires digestion by proteolytic enzymes • yields free biotin or biocytin • Biotinidase hydrolyzes biocytin to free biotin and lysine • Undigested biocytin may be absorbed and digested in plasma by biotinidase • Unmetabolized biocytin is excreted in urine

3. Biotin • Biotinidase • Deficiency due to inborn error has been documented in infants and children • Clinical features include seizures, ataxia, skin rash, alopecia, acidosis • Bioavailability • varies from 100% in corn to 0% in wheat • Absorption • most takes place in proximal SI by active transport.

4. Biotin • Transport • Free or protein bound • Uptake • related to needs of cells • Storage • most found in muscle, liver and brain • Bacterial Synthesis of biotin • may be absorbed into body or excreted in feces

5. Biotin Functions • Biotin must be activated to biotinyl 5’-adenylate • biotin reacts with a ATP (Mg required) • carboxylase joins the biotinyl moiety to form holoenzyme carboxylase with release of AMP • Biotin dependent enzymes • acetyl CoA carboxylase • pyruvate carboxylase • propionyl CoA carboxylase • Beta-methylcrotonyl CoA carboxylase

6. Biotin • Carboxylases • biotin is attached by an amide linkage • carboxy terminus of biotin is linked to epsilon amino group of a specified lysine residue of apoenzyme • chain connecting biotin and apoenzyme is long and flexible • allows biotin to move from one active site to another • see figure 9.23 and 9.24a

7. Pyruvate Carboxylase • adds a carboxyl group to pyruvate forming oxaloacetate • requires presence of acetyl CoA as well as ATP and Mg • acetyl CoA serves as allosteric activator • Fate of OAA • if surplus of ATP, gluconeogenic pathway • if deficiency of ATP, TCA cycle • see Figure 9.24b

8. Acetyl CoA Carboxylase • Initiation of fatty acid synthesis • Transfers a carboxyl group to acetyl CoA forming malonyl CoA • Activated by citrate and isocitrate • Inhibited by palmitoyl CoA

9. Propionyl CoA Carboxylase • Important for catabolism of isoleucine, threonine and methionine and odd chain fatty acids • Catalyzes the conversion of propionyl CoA to methylmalonyl CoA • Methylmalonyl CoA converted to succinyl CoA via vitamin B12 dependent enzyme

10. Beta-methylcrotonyl CoA Carboxylase • During leucine catabolism • beta-methylcrotonyl CoA is formed • carboxyl group added to beta-methylcrotonyl CoA to form beta-methylglutaconyl CoA • defect in BMCC results in accumulation of methyl crotonylglycine and hydroxyisovaleric acid • further catabolized to acetoacetate and acetyl CoA

11. Genetic Defects of Carboxylases BCAA Threonine Methionine Odd Chain FA Glucose Pyruvate Oxaloacetate Leucine 3-methylcrotonyl CoA 3-methylglutaconyl CoA Acetyl CoA Citrate CH3-crotonyl glycine OHisovaleric acid BMCC Propionyl CoA PCC PC Malonyl CoA ACC Methylmalonyl CoA Succinyl CoA Fatty Acids

12. Metabolism and Excretion • Catabolism of holocarboxylases by proteases yields biocytin • Biocytin degraded by biotinidase to yield free biotin • Some of free biotin is reused; some is further degraded to bisnorbiotin • See free biotin, bisnorbiotin and biocytin in urine primarily

13. Biotin • AI • 30 g/d (adult males and females) • Deficiency • depression, hallucinations, muscle pain, nausea, scaly dermatitis • excessive ingestion of raw egg white • GI disorders • IBD • achlorydia • Excessive alcohol ingestion • Certain medications

14. Biotin • Toxicity • none observed • Assessment • Evaluation of biotin in • blood, plasma or serum • urine