Week 2 Vitamins 2 Lecture (1).pdf

B-vitamins B6, B7, B9, B12 Dr. Heisel BMS200 Overall Outcome Reminder At the end of the vitamin lectures, you will be able to describe how our bodies absorb vitamins, convert them into a useful form, and use them at a biochemical level to promote optimal health. You will also be able to ap...

B-vitamins B6, B7, B9, B12 Dr. Heisel BMS200 Overall Outcome Reminder At the end of the vitamin lectures, you will be able to describe how our bodies absorb vitamins, convert them into a useful form, and use them at a biochemical level to promote optimal health. You will also be able to apply their biochemical mechanisms to treatment of select conditions. Specific objectives to support this outcome can be found in the syllabus and at the start of each vitamin section in the ppts Pyridox- (al, ine, amine): Vitamin B6 Objectives Relate B6 absorption, metabolism, excretion and testing to the general properties of B-vitamins Relate the biochemical function of B6 to synthesis of heme, NAD, neurotransmitters, lipids (unsaturated fatty acids, ceramide) Relate the biochemical function of B6 to carbohydrate metabolism (energy production) and amino acid metabolism Develop hypothesis for the mechanism by which vitamin B can assist in improving depression, inflammation Describe the biochemical relationship between vitamin B6 deficiency and subsequent glutamate decarboxylase deficiency resulting in infantile seizures. Provide a biochemical rationale for fatigue and microcytic anemia as symptoms of B6 def. What do you already know? Name the biochemical reaction catalyzed by B6 that can be used to move amino groups. ▪ Review: What three pairs of molecules can interconvert via this reaction? Alpha Amino acid Amino acid Alpha ketoacid ketoacid Review Amino acids Alanine Aspartate Glutamate Pyruvate Oxaloacetate α-Ketoglutarate Alpha ketoacids What do you already know? In addition to transamination, B6 facilitates other types of reactions, including: ▪ Trans- and de- sulfhydration ▪ Decarboxylation Structure and function Six forms = interchangeable “vitamers” ▪ All the same except for the functional group FYI: = Aldehyde (pyridoxal, PL), alcohol (pyridoxine, PN), amine (FYI: pyridoxamine, PM), either with or without an additional phosphate To Know: Coenzyme is phosphorylated aldehyde form = pyridoxal phosphate = PLP ▪ Other B-vitamins can help with vitamer interconversions Ex: B2 helps convert PNP to the PLP ▪ Important, as only PLP acts as a coenzyme  FYI visual Structure and function Blue = functional groups, rest of molecule is the same Red = phosphate added, rest of molecule is the same Yellow = coenzyme form Absorption and metabolism What needs to happen for the phosphorylated vitamers to be absorbed? What type of enzyme is used? What type of enzyme helps convert pyridoxal (PL) to the pyridoxal phosphate coenzyme form (PLP)? Specific functions B6 and energy: ▪ Link to B3 B6 helps make the NAD + coenzyme from the amino acid ___________. ▪ Review: NAD+ is converted to NADH by which of the energy producing pathways? A – glycolysis B – beta oxidation C – citric acid cycle NADH can then go to ETC → energy ▪ Glycogenolysis B6 helps glycogen phosphorylase release glucose ▪ Glucose can then be used for glycolysis → energy Specific functions B6 and energy ▪ Heme B6 helps condense succinyl CoA and glycine to the start heme synthesis pathway Review: ▪ What vitamins can help make succinyl CoA? ▪ How does heme connect to energy? Specific functions B6 and synthesis ▪ Creation of neurotransmitters Which ones? What amino acids are they derived from? What type of B6-assisted reaction is common to all? What is a general function of each of these NTs? Amino acid: ? Amino acid: ? Amino acid: ? * *=? * GABA Inhibitory NT in brain. Review: Serotonin Dopamine Mechanism? Specific functions B6 and synthesis ▪ Gluconeogenesis Cys to pyruvate (FYI transamination and desulfhydration) Asp to oxaloacetate (transamination) Ala to pyruvate (transamination) ▪ Part of glucose-alanine cycle, green slide = review Alanine Aspartate GNG Cysteine Pyruvate Oxaloacetate  alpha KG glutamate B6 Glucose-Alanine Cycle - To Know: - Alanine from muscle carries N to liver for urea cycle - Liver makes glucose (gluconeogenesis) to send to muscle for energy - B6 helps with alanine- pyruvate transaminations Remaining details = FYI review only (not testable this term) Oxaloacetate NH3 alpha KG glutamate B6 Specific functions: B6 and synthesis ▪ Desaturated fatty acid: Gamma-linolenic acid production of anti- linoleic acid gamma-linolenic acid inflammatory delta-6-desaturase, B6 prostaglandins ▪ Cysteine (review) Made from homocysteine (HC) + and serine Serine ▪ HC may be linked to increased risk of cardiovascular disease FYI: Transulfhydration Would B6 potentially increase or decrease + this risk? FYI: Deficiencies Provide a rationale for the following: ▪ A spike in infant seizures in the 1950’s with the advent of formula Due to an error in the sterilization process that destroyed B6 - what the connection to seizures? ▪ Depression (hint: what NT’s might be important?) ▪ Microcytic anemia ▪ Inflammation All the info you need is in the slides!!  To know from figure: Yellow and blue boxes Testing Xanthurenic acid Testing ▪ Can do a tryptophan load test Test urine before and after Requires PLP tryptophan load for presence of xanthurenic acid ▪ Would levels go up or down in a B6-deficiency? Why? Biotin: Vitamin B7 Objectives Relate B7 absorption, metabolism, excretion and testing to the general properties of B-vitamins Relate the biochemical function of B7 to carbohydrate metabolism (GNG), fatty acid synthesis, energy production, heme synthesis, ketone synthesis, cholesterol synthesis Describe the mechanism by which B7 (biotin) assists in blood sugar regulation (such as in diabetes) What do you already know? What type of reaction does biotin catalyze? A – Redox B – Carboxylation C – Decarboxylation What synthesis pathway below uses B7? a) Fatty acid synthesis b) Gluconeogenesis  Structure and Function Coenzyme form of biotin has a CO2 attached and is linked to its enzyme by a lysine residue (FYI: biotin attached to a lysine =“biocytin”) CO2 comes from bicarbonate, HC03- Absorption To absorb, need to remove the attached carboxylase enzyme via proteolysis ▪ Sometimes lysine is still attached ▪ Removed by biotinidases Back to Gaston… ▪ Eggs have high levels of biotin Why would Gaston’s diet of 4-5 dozen raw eggs have caused him to be biotin deficient? ▪ Why would he have been fine if he had cooked his eggs? Hint: Eggs white contain avidin, a glycoprotein that binds biotin Specific functions B7 and synthesis ▪ Gluconeogenesis Conversion of pyruvate to oxaloacetate Carboxylation GNG Specific functions B7 and synthesis ▪ Fatty acid synthesis Review: What are the two substrates for fatty acid synthesis? What is the role of biotin? CO2 (transferred over from biotin) Enzyme name? Specific functions B7 and energy ▪ B7 helps make succinyl CoA (from propionyl CoA) Succinyl CoA → CAC and heme synthesis CH3 CO2 from biotin Methylmalonyl CoA Propionyl CoA Succinyl CoA energy What is an energy Heme CAC producing pathway that makes propionyl CoA? Energy  Odd-numbered fatty acyl CoA Beta ox Acetyl CoA Propionyl CoA CH3 FYI: Also produced by catabolism of various aa’s Heme synthesis Specific functions B7 and synthesis ▪ HMG CoA (from leucine) Note the type of enzyme that uses biotin ▪ Rest of diagram is FYI only What two main synthesis pathways does HMG CoA (and therefore biotin) feed into? ? HMG CoA ? Specific functions - other Linked to increased glucokinase activity ▪ Review: What reaction does glucokinase catalyze? What happens to the product? What is the connection to blood sugar levels? Deficiencies FYI: No hallmark condition, but tend to see two types of symptoms: ▪ Neurological Ex: Lethargy, depression ▪ Dermatological Ex: Dermatitis (dry, red, scaly skin) and brittle nails Folate: Vitamin B9 Objectives Relate B9 absorption, metabolism, excretion and testing to the general properties of B-vitamins Relate the biochemical function of B9 to purine and pyrimidine synthesis, then extrapolate to roles of folate in spina bifida, anemia, and cancer Relate the biochemical function of B9 to amino acid metabolism Discuss the implications of the methyl-folate trap Relate the biochemical function of B9 to neurotransmitter synthesis, and link to a possible role in treating depression Relate the genetic polymorphisms in the MTHFR gene to the optimal type of folate supplementation What do you already know? Which best describes the overall biochemical function of folate: A – methylation B – 1-C transfer  Structure and function Coenzyme forms have: ▪ 4 H’s added: “THF” = tetrahydrofolate ▪ Multiple glu’s attached (one shown for simplicity) ▪ A 1-C group added (FYI to N10 and/or N5) Absorption and metabolism Absorption ▪ Look at the previous slide: what do you need to remove prior to absorption? These are removed by hydrolases (aka conjugases) in the small intestine Metabolism ▪ B3 adds the H’s to create the THF form What do you think the enzyme name is? ▪ Polyglu’s and a specific 1-C group are added Various B-vitamins interconvert folate coenzyme forms  To Know: Points 1-4 2) 1-C groups are then added to the THF form. 1) NADPH helps reductase enzymes add H’s to make DHF, then THF. 4) Some THF forms help make purines or pyrimidines 3) Note all the B- vits involved in folate conversions: B2, B3, B6, B12 Absorption and metabolism Folate undergoes enterohepatic circulation, and small amounts are stored in liver on folate binding proteins (FBP) ▪ Absorption: Small intestine to gut to liver Once in liver, could be: ▪ Converted to coenzyme forms and used by liver ▪ Stored ▪ Sent to other tissues ▪ Cycled back to small intestine in the bile, another chance for absorption back to liver (enterohepatic circulation)  Liver PolyGlu -Trapped. converted Blood to coenzymes - Small amounts stored on FBP Extrahepatic MonoGlu Excreted Tissues into bile MonoGlu Bile PolyGlu duct Blood -Trapped, converted to PolyG MonoG Small intestine coenzymes Conjugases Specific functions B9 and synthesis ▪ Purines and pyrimidines Required to support DNA replication ▪ Cells requiring a high rate of cell division are adversely impacted in a folate deficiency Deficiency can lead to: Spina bifida, cancer Megaloblastic anemia Folate Deficiency Decreased purine and pyrimidine synthesis Megaloblastic anemia FYI: End up with Spina bifida Decreased DNA replication immature RBC’s Failure of vertebral & that have laminae to fuse Increased replication errors decreased O2 during fetal carrying capacity development, exposing the spinal Cancer cord - How can folate help prevent Important to cervical dysplasia from supplement with developing into cancer? folate during - Why would you not treat pregnancy to cancer with folate decrease risk supplementation? - Note that the chemotherapeutic drug “methotrexate” works by inhibiting production of THF  Specific functions B9 and synthesis ▪ Various amino acids, including methionine from homocysteine (HC) The 1-C group (methyl) on methionine comes from methyl folate. It is passed onto HC via methyl B12. FYI: HC = cys with an extra CH2 One reason to make methionine is to make the methyl donor “SAM” (S-adenosyl-methionine)  ATP S-adenosylmethionine SAM = methyl donor B12 Methyl is donated, products include: - Choline - Epinephrine - Methylated DNA Homocysteine  What vitamin deficiency would cause a “methyl folate trap”? Methylation What are the reactions implications? More on methyl folate… It is made from methyleneTHF (MTHF) via the MTHFR enzyme (FYI: methylenetetrahydrofolate reductase) ▪ MTHFR gene is highly polymorphic This can influence MTHFR activity ▪ Reduced activity = “poor converter” What type of folate supplementation would be best? ▪ Can also have increased activity= “over converter” Specific functions: other Folate levels correlate with THB levels ▪ THB (FYI tetrahydrobiopterin) is a redox coenzyme that helps with the synthesis of dopamine and serotonin Indicates a possible role for folate deficiency in depression To Know: high levels of THF correlate with high levels of THB, THB used in serotonin and dopamine synthesis pathways Review at home: Deficiencies Review the presentation ▪ What are four conditions that a folate deficiency might contribute to? Cobalamin: Vitamin B12 FYI: Only in animal products or supplements/ fortified foods Objectives Relate B12 absorption and metabolism to the general properties of B-vitamins Describe the role of the GIT in the absorption of B12 Relate the biochemical function of B12 to the energy production, the creation of SAM, and the activity of folate Relate B12 excretion and testing to the general properties of B-vitamins Structure and Function Called “cobal”amin due to attached cobalt ▪ FYI: Supplement forms are often either “cyano” (CN) or “hydroxo” (OH) Coenzyme forms have either an adenosyl group or methyl group in place of CN (or OH) Structure and Function Methylcobalamin coenzyme ▪ Methionine synthase reaction: transfers methyl group from methyl folate to HC Makes methionine and regenerates THF ▪ Remember: deficiency leads to methyl folate trap Adenosylcobalamin coenzyme ▪ Helps with formation of succinyl CoA from propionyl CoA CH3 B7 (CO2) B12 Heme Methylmalonyl CoA (No B12) CAC Propionyl CoA Succinyl CoA (Methylmalonic acid) - can be used to test for B12 deficiency Energy Absorption and metabolism B12 from food needs to be released from proteins ▪ Done by pepsin and HCl in stomach Stomach: B12 carried by R-proteins ▪ Also found in saliva, continue to act in stomach ▪ Protect B12 from hydrolysis and bacterial use Duodenum: B12 released from R-protein and picked up by intrinsic factor (IF) ▪ IF made in stomach, acts in duodenum ▪ Carries B12 to a B12-IF receptor in ileum for absorption  B12 Absorption and metabolism Enterocytes: Uptake of B12-IF-R via receptor-mediated endocytosis Lysosome ▪ Receptor is recycled B12 IF ▪ Vesicle fuses with lysosome IF is degraded B-12 is released B12 transported from enterocytes into blood ▪ Carried to tissues bound to transcobalamin II (TCII) Tissues: Uptake via receptor mediated endocytosis Once in cell, chaperone proteins facilitate conversion to adenosyl- and methyl- cobalamin in the appropriate compartment ▪ Which compartment do you think each one typically ends up in? Absorption and metabolism Like B9, also undergoes enterohepatic circulation and storage ▪ This time, significant amounts are stored for long periods of time Mostly as adenosyl cobalamin Main site = liver, secondary = muscle FYI: A deficiency may not show up for months or years once B12 intake stops, due to use of stored B12 Deficiency Can be caused by hypochorhydria (more common in elderly patients) – why? Can be secondary due an IF deficiency ▪ What routes of B12 supplementation would be needed in this case, and why? Most symptoms are similar to B9 deficiency, including megalobastic anemia ▪ FYI: megaloblastic anemia due B12 deficiency = pernicious anemia Deficiency Clinical scenario: ▪ A patient with megaloblastic anemia comes to see you. You suspect either a B12 or B9 deficiency, and you decide to try folate supplementation while waiting for lab results. Turns out you picked the wrong vitamin deficiency - the labs indicate a B12 deficiency. However, your B9 supplementation alone still ended up correcting the anemia. How? B12 deficiency = methyl folate trap: can’t regenerate THF from methyl THF Need THF to ultimately feed into purine and pyrimidine synthesis. Can provide THF as a supplement to circumvent lack of THF creation from B12. Deficiency Preview: an important deficiency symptom that B- 12 does NOT share with B9 is neurological deficits ▪ Looks similar to Alzheimer’s ▪ Can take months/years to appear – why? ▪ Back to your clinical scenario: What is the danger of correcting a B12 deficiency anemia with B9? Excretion and testing Testing ▪ Blood test for B12 ▪ Other options include blood tests for the following: ▪ Homocysteine ▪ Methylmalonic acid What is the rationale for these tests - would levels of each be elevated or decreased in a B12 deficiency? Why?  CH3 B7 (CO2) B12 Methylmalonyl CoA (No B12) Propionyl CoA Succinyl CoA Methylmalonic acid CAC Heme SAM production B12 Homocysteine