Hormones transported in blood by carrier proteins: Answers A-B A The statement refers to a hormone that binds to an intracellular (either nuclear or cytoplasmic) receptor. B The statement refers to a hormone that binds to receptors in the cell membrane A deficiency of this enzyme can lead to a collagen with higher than normal solubility: Answers AE A End terminal peptidase B Signal peptidase C Lysyl Hydroxylase D Prolyl Hydroxylase E Lysyl Oxidase You are treating a patient with high urine levels of cystathionine, which results from low activity of cystathionine gamma-lyase. Cystathionine requires a co-factor (pyridoxal phosphate, B6). Your patient has normal levels of vitamin B6 and a normal amount of cystathionine gamma-lyase protein, but a lab assay shows low activity levels. The low activity is resolved by addition of excess vitamin B6 and so you order B6 supplementation for your patient. The urine cystathionine levels subsequently normalize. A genetics analysis shows your patient has mutation causing a single amino acid change in the enzyme. Which if the following best explains the efficacy of vitamin supplementation? Answers A - E A The mutation is loss of a critical lysine that pyridoxal phosphate requires to bind to the enzyme. B Vitamin B6 is a cofactor hydroxylation of collagen IV in kidney, which explains the higher levels of excretion of cystathionine. C A mutation at the cofactor binding site raises the Km for pyridoxal phosphate. D The mutation reduces stability of cystathionine gamma-lyase causing low levels of the protein. E Vitamin B6 is conjugated to cystathionine and slows its excretion in urine