150 Cobalamine is both vitamin and active coenzyme. The others require an additional component for activity: thiamine and
pyridoxine have a pyrophosphate and a phosphate added, respectively, as coenzyme; riboflavin is combined with AMP and niacin is combined with ribosyl-ADP before active as coenzymes.
protein).
152 All listed vitamins are water soluble, also vitamin C, folate, pantothenate, biotin. Fat-soluble vitamins are A, D, E, and K. 153 Thiamine (B1): major signs and symptoms is peripheral neuropathy, loss of appetite, constipation and nausea occur
early, then depression and confusion. Impaired nerve cell function leads to beriberi characterized primarily by advanced neuro-muscular symptoms. Riboflavin: inflammation of the corners of the mouth (angular stomatitis), tongue
(inflammation of glossitis) and scaly dermatitis. Niacin: dermatitis, diarrhea and dementia in severe deficiency in pellagra (rarely seen in the modern world). Pyridoxine: peripheral neuropathy, convulsions, coma in later severe form, early onset shows irritability, nervousness and depression in mild form. Cobalamin: megaloblastic anemia also involves folate (methylmalonic aciduria and homocystinuria) and pernicious anemia (due to lack of intrinsic factor protein (IF) in the stomach and hence absorption of B12 is prevented).
154 Pyridoxine is the major form in the diet, and pyridoxal phosphate (via pyridoxal kinase) is the active form of the vitamin.
All nonphosphorylated and phosphorylated forms are water soluble.
155 Pyridoxal-P forms a Schiff base intermediate with the α-NH
2 of amino acids. One of three bonds can then break yielding
(a) amination/deamination, (b) loss of carboxyl group, or (c) dehydration (serine) of the amino acid.
156 Amination converts their α-keto forms to amino acids (e.g., pyruvate to ala, OAA to asp, a-KG to glu) for amino acid
synthesis via aminases/transaminases. Deamination to the α-keto forms is the first step in catabolism. Either is used to interconvert α-keto and α-amino acid forms.
157 Iron in RBC hemoglobin, muscle myoglobin, cytochromes and iron-sulfur complexes in mitochondria are important for
oxygen transport and energy metabolism. Copper in several oxygenases, these include cytochrome c oxidase, and several cuproenzymes involved in heme synthesis, superoxide dismutase, and ceruloplasmin synthesis and function.
158 Iron anemia.
159 Ceruloplasmin in plasma scavenges superoxide and other oxygen free radicals. Ceruloplasmin is defective in Wilson’s
disease. [Ceruloplasmin produces a sky blue color in solution similar to cerulean blue pigment. Recall solutions of copper ions are blue such as copper sulfate.]
160 Ferrous iron is stored by ferritin protein in most body cells, ferric iron is transported by transferrin protein.
Ceruloplasmin is the major transport protein of copper to the body and is also essential for the regulation of the oxidation- reduction reactions, transport, and utilization of iron (see Baynes Fig 3.6). That is, ferritin-Fe2+ + ceruloplasmin-Cu2+ <–> transferritin-Fe3+ + ceruloplasmin-Cu+. Ferritin scavenges free iron (generally toxic, especially in the central nervous system). Oxygen or oxidized thiol groups reduce Cu+ to Cu2+ in ceruloplasmin. Hence, if copper is deficient, iron
metabolism is effected.
Functional hemoglobin and myoglobin contain Fe2+. Occasionally it is oxidized to Fe3+ when bound O
2 is reduced to
superoxide nonenzymatically to yield ferrihemoglobin or methemoglobin (rust brown color), which binds oxygen poorly.
Methemoglobin reductase restores the Hb(Fe2+). [in patients with metHb reductase deficiency (methhemoglobinemia)
have a dark cyanotic appearance; vitamin C treatment reduces the iron back to ferrous Fe2+..]
Copper deficiency also is associated with degeneration of vascular tissue with bleeding due to defects in elastin and collagen production. Since copper deficiency influences iron metabolism, small copper deficiency manifests as microcytic (small erythrocytes) microchromic (pale erythrocytes) anemia that is resistant to iron therapy.
161 All 20 amino acids are need to synthesize about 200 g total protein per day in the healthy 70 kg human. The body can
synthesize 12 amino acids by metabolic pathways, but eight amino acids cannot, those eight are: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine, lysine (memory acronym: Pvt. Tim Hall). Their carbon skeletons cannot be synthesized by the body thus the diet becomes essential in supplying them in an adequate amount based upon their composite average mole fraction of daily protein synthesis.
162 Nitrogen balance occurs when the dietary intake of utilizable nitrogen equals the nitrogen excreted (sweat, urine, feces).
Body composition of nitrogen is mostly accounted for by protein mass (about 16% N), the N in nucleic acids and some other metabolites is ignored.
163 For infants, human milk is the best source because the ratio or mole fractions of the amino acids matches the amino acids
comprising the total body proteins of the infant. Moreover, human milk protein is completely digestible and absorbable for full utilization. Bovine milk is ideal for calves for the same reason and is the next best source for humans, hence its wide commercial availability. Eggs are the next best source of protein, the high cholesterol content notwithstanding. [Beside the lactose intolerance, a portion of the population has allergic responses to cows milk.].
164 As an essential amino acid, the body can’t synthesize it to maintain its representation in the daily requirement for protein
synthesis needs. All proteins initiate with methionine, so a 50% decrease in availability could decrease protein synthesis by the same amount. Additionally, all other proteins will be affected in proportion to the number of internal methionines.
165 While essential amino acid deficiency is ongoing, the turnover of proteins will release some of the deficient amino
acid(s) back into the amino acid pool (cellular, tissue, organ, whole body) and that helps de novo protein synthesis, but the nonessential amino acids can’t be incorporated because overall protein synthesis is about 50% (met example). The nonessential amino acids are processed catabolically through oxidation which begins with deamination. The deaminated N is incorporated into urea to prevent ammonia toxicity. The net result is the loss of body N now exceeds diet intake, hence, negative nitrogen balance ensues and continues until the deficiency of essential amino acids ceases.
166 See Baynes Fig 28.7. (a) Ribose reduction, all NDP + thioredoxin-(SH)2 (NADPH) via ribonucleotide reductase -->
dNDP + thioredoxin-S2 (NADP+). (b) Uracil to thymine, dUMP + N5N10-methylene-FH4 via thymidylate synthase -->
dTMP + dihydrofolate (FH2).
167 dUMP to dTMP (see fn 167). ancillary Rx 1: FH
2 + NADPH via FH2 reductase --> FH4 + NADP+. Rx 2: FH4 + serine
via serine hydroxymethyl transferase --> N5N10-methylene-FH4 + glycine + H
2O.
168 The ribonucleotide reductase uses NADPH which has niacin (B3, nicotinic acid). The dihydrofolate reductase FH 4 uses
folate [also multiple peptide composed of glutamate].
169 A hydride is a proton with two electrons or H:–, the hydride equivalent is donated by the two cysteine –SH groups that
form a bridged thioredoxin–S-S-intrapeptide. These are reduced in turn by NADPH. Nicotinic acid is the vitamin and its amidation yields the active nicotinamide unit of NADPH. [note: During deoxyribose formation, the two –SH groups in thioredoxin (–trp-Cys-gly-pro-Cys-) donate a hydride equivalent (H–) and are oxidized to cross linked cystine disulfide.
Reduction back to a pair of cysteines (–trp-Cys-gly-pro-Cys-) is via flavoprotein thioredoxin reductase. NADPH (from pentose shunt) as electron source reduces the –S-S- back to active thioredoxin–(SH)2 . In the second reaction (reduction
of FH2 to FH4), NADPH donates the hydride + H+ (i.e., H2) hydrogenation equivalent.]
170 In addition to tetrahydrofolate transferring carbon atoms (in the forms of formaldehyde, formyl, hydroxymethyl, methyl,
or forminimo), S-adenosyl methionine (SAM) for methylations, and CO2-biotin for carboxylations. 171There are over 300 known zinc-containing enzymes. Aldolase, alcohol dehydrogenase.
172 Metallothioneines are small proteins containing about 62 amino acids with about 30 mole % as cysteine. These provide
pockets of S-donor atoms that can bind heavy metals, especially Cu2+ and Zn2+. Cadmium induces the cellular resistance
to heavy metal cytotoxicity by stimulating thionein synthesis, which is mediated in most species by the binding of metal ions either to a cysteine-rich polypeptide in the metallothioneine family or to short cysteine-containing gamma-glutamyl peptides. The thioneines are important in heavy metal detoxification and excretion.
173 For metalloenzymes, deficiency of the metal cofactor inhibits enzyme activity because the chemistry of the catalysis
involves the metal ion. In Wilson’s disease, the liver’s capacity to synthesize ceruloplasmin is impaired leading to chronic accumulation of copper in body tissues associated with hemolysis and damage to both liver (cirrhosis) and brain cells. The activities of copper, zinc, iron, and other metalloenzymes will be affected by the excessive copper accumulation, which can interfere with absorption and utilization of other metal ions.
174 Vit D: bones, curved legs. Vit C: subcutaneous and other hemorrhages, muscle weakness, soft, swollen bleeding gums,
osteoporosis, and poor wound healing and anemia. Niacin: dermatitis, diarrhea, and dementia. Thiamine: beri-beri, early loss of appetite, constipation, and nausea, later depression, peripheral neuropathy and instability and impaired nerve function with advanced neuromuscular symptoms. Folate: megaloblastic anemia due to failure to synthesize nucleic acids (folate-dependent, 1-carbon metabolism). Polycythemia (erythrocytosis) is an increase in the hematocrit
175 Water soluble: vit C, niacin (B
3), thiamine (B1), and folate are water soluble [also flavin (B2), pantothenic acid (B5),
pyridoxine (B6), biotin, cobalamin (B12)]. Lipid or fat soluble: Vit D [also vit A, E, K].
176 Vit D (rickets): fish oils and egg yolks. Vit C (scurvy): citrus fruits. Niacin (the 3 Ds): meats, nuts, legumes. Thiamine
(beri-beri): seeds, nuts, wheat germs, legumes, lean meat. Folate (anemia): yeast, liver, leafy vegetables.
177 After dTMP synthesis, dihydrofolate reductase converts FH
2 to FH4 (tetrahydrofolate). DHF reductase is inhibited by
folate antagonists such as methotrexate (also related drug aminopterin). If FH4 cannot be regenerated, the 1-carbon-FH4
derivatives pool becomes used up, 1-carbon metabolism ceases, the cell dies. Thus, methotrexate blocks FH4,
regeneration, the next ancillary reaction cannot regenerate N5N10-methylene-FH
4 from serine and continued dUMP to
dTMP synthesis ceases.
178 Pentose shunt. First reaction via glucose-6P-dehydrogenase and 3rd dehydrogenase (an oxidative decarboxylation)
regenerate NADP+ to NADPH, which supplies the H
2 equivalents to FH2 reductase to regenerate FH2 toFH4. 179 The 3rd carbon (the hydroxymethyl side chain –CH
2OH) of serine, is transferred to FH4 onto N5N10. 180 The dUMP to dTMP reaction produces FH
ECM is a complex network of secreted macromolecules located in the extracellular space. The EMC of skin and bone provides the structural framework of the body. In all tissues the EMC has a central role in regulating basic cellular processes, including proliferation, differentiation, migration and cell-cell interactions. The EMC macromolecular network is composed of collagens, elastin, glycoproteins and proteoglycans, secreted by connective tissue cells such as fibroblasts and epithelial cells.
182 The total body mass is about 25% collagen and collagens are the primary structural components of the EMC in
connective tissues. Collagen’s three polypeptide chains [19 different collagen types, composed of 34 related, but distinct polypeptide chains] of 600 to 3000 residues are wound about each other in an extended α-helix conformation found in globular proteins. Glycine has the smallest side-chain (H), therefore the Gly-X-Y tripeptide allows the triple helical stands to fit closer together. The X and Y (most often pro and hydroxy-pro) confer rigidity to the molecule because of their bulk and limited rotation about the α-amino group. Intra- and interchain helices are stabilized by hydrogen bonds mostly between NH and C=O groups. The side chains of the X and Y groups point outwards from the helix where they can form lateral interactions with other triple helices or proteins (4º) stated in the correct answers of the test question.
183 The long cylindrical-shaped collagen molecules align in parallel quarter-staggered arrays (like bricks in a wall) such that
the ends overlap and appear as bands across the fibers (resemble fibrin fiber staggered-arrays) when viewed in the electron microscope.
184 Glucose, mannose, galactose, ribose are simple sugars. Glucosamine, mannosamine, galactosamine, found in complex
carbohydrates, and deoxyribose are derived sugars. Ribose is in RNA, deoxyribose is in DNA.
185 Glycoconjugates are formed usually with glucuronic acid to detoxify molecules that have a carboxyl or hydroxyl group.
Enzymes conjugate either through an ester or an ether type of linkage. Glucuronic acid conjugates with bilirubin. [Another conjugate: glycine + benzoic acid gives hippuric acid, a compound used to measure liver function. Glucuronic acid HOOC-(CHOH)4-CHO is formed from glucose by oxidation of the 1° alcohol of the last carbon, i.e., C6-OH.]
Glycolipids are classified into four groups: cerebrosides, sulfatides, globosides, and gangliosides. In all these classes, the polar head-group – comprising the sugars – is attached to ceramide by a glycosidic bond at the terminal OH of sphingosine.
186 Proteoglycans are gel forming components of the ECM. Those listed are glycosaminoglycans (GAGs) composed of
repeated disaccharides: hyaluronic acid (GlcNAc and GlcUA) and is the longest of the GAGs (250-25,000 residues); heparin (IdUA and GlcNAc); chondroitin sulfates (GlcUA and GalNAc), dermatan sulfates (IdUA and GalNAc), heparin sulfates (IdUA and GlcNAc), and keratin sulfates (Gal and GlcNAc) have sulfate on some of their amino groups.
187 There are seven crystal systems: cubic, tetragonal, orthorhombic or rhombic, monocline, triclinic, hexagonal,
rhombohedral or trigonal. Calcium phosphate is in the hexagonal system (four axes: three equal coplanar axes at 60°, and one at right angles to them) and is known as hydroxyapatite found in bone and enamel.
188 Enamel hydroxyapatite is the most calcified and hardest mineral in the body. In decreasing hardness is enamel > dentine
~ cemented > bone. Mineralogists us the Mohs Scale of Hardness: from softest is 1. talc 2. gypsum, 3. calcite, 4. fluorite, 5 apatite, 6. feldspar, 7. quartz, 8. topaz, 9. corundum or sapphire, to hardest 10. diamond.
189 Amelogenins are highly conserved 7-25 Kd proteins unique to enamel; they compose about 90% of developing enamel
ECM concerned with enamel mineralization and are high in proline and glutamate. Excreted by the ameloblasts, the amelogenins are bipolar, nonglycosylated proteins with some phosphoserine residues; the genes are on both human X and Y genes. DNA sequences for human X and Y genes, bovine X and Y genes, opossum and mouse genes are known. Amelogenins self-assemble into nanospheres (10-20 µm diameter) of about 100 monomer units, with the C-terminals on the surface. Initially they bind to the protoenamel on the a- and b-faces allowing crystal growth only on the c-face surface; as crystallization proceeds, during maturation, the amelogenins are lost allowing growth on the a- and b-faces.
190 Exchange of F– for OH– yields fluoroapatite, which is harder than hydroxyapatite.
191 Diacylphosphoglycerol (DAG) is glycerol esterified to two fatty acids; esterification of the end OH of DAG with
phosphate yields a phospholipid. Additional esterification the phosphate by choline, serine, ethanolamine, or inositol yields the corresponding phosphatidyl choline, etc. All these are found in cellular membranes.
192 Ethanolamine is decarboxylated serine. Choline is derived from ethanolamine by three N-methylations (SAM) to yield a
quaternary nitrogen with a positive charge. Betaines are amino acids with fully methylated amino groups. Thus, the simplest betaine (of glycine) is derived from choline by two oxidization steps: –OH to –CHO aldehyde then to –COOH yielding glycine betaine or betaine: (CH3)3N+–CH2–COOH. Methionine is salvaged from betaine + homocysteine by a
transmethylase which also yields dimethylglycine (two additional demethylations yield sarcosine then glycine).
taurine, and aspartate decarboxylase yields β-alanine (constituent of pantothenate) [Also important are lysine to
cadaverine; tyrosine to tyramine; histidine to histamine.]. The vitamin is B6 (pyridoxine) is active as pyridoxine phosphate
in the decarboxylases that use pyridoxine phosphate.
196 GABA is the abbreviation for γ-amino-butyric acid derived from glutamine. 197 GABA is the major inhibitory neurotransmitter in brain tissue.
198 Myoglobin is a 16.9 kd single polypeptide containing heme in muscle; hemoglobin is a 63 kd tetramer of heme-
containing globin polypeptides (A2B2) in red cells. Both bind oxygen. Myoglobin oxygen binding curve is a steep hyperbolic curve that indicates a high oxygen binding affinity even at 10-20 mm-Hg pO2 that ensures mitochondria are
supplied evenly and well.
Oxygenation and deoxygenation of Hb takes place in stages: Hb4 <=> Hb4O2 <=>Hb4O4 <=> Hb4O6 <=> Hb4O8
Hemoglobin’s sigmoid curve indicates a positive cooperative binding among the four subunits that increases toward saturation such that each oxygen bound increases the binding affinity of the unoccupied hemes for oxygen. [The Hill coefficient (exp n) of the Hill equation (y/100 = Kxn/(1 + Kxn) is 2.5 (at pCO
2 of 40mm, plasma pH 7.44) indicating each
Hb tetramer averages about 2.5 molecules of bound oxygen and changes in pO2 account for sufficient oxygen exchange to
accommodate respiratory needs. For myoglobin n = 1, for the hyperbolic curve, no cooperative binding consistent with mass action.]
199 Porphyrins contain four pyrroles, each with differing side chains (see Baynes, Fig 27.4). Two 5-ALA (from the
mitochon-drion) condense in the cytosol to yield PBG (porphobilinogen), then 4 PBG condense to form
uroporphyrinogen III, to co-porphyrinogen III, and back into the mitochondrion as protoporphyrinogen IX, then to protoporphyrin IX and finally to heme.
200 Addition of ferrous iron via ferrochelatase to protoporphyrin IX yields metalloporphyrin called heme, in the final step of
heme synthesis within the mitochondria.
201 The steep hyperbolic oxygen binding curve of myoglobin indicates high affinity even at low pO2 (10mm, 80% sat.)
needed to store a maximum amount of oxygen inside muscle cells for release to mitochondria. Hemoglobin’s sigmoidal binding curve is ideal for facile oxygen exchange required for transport (binding, dissociation) of oxygen between lungs and peripheral tissues.
202 The g-carboxylate group provides a spatial geometry for the negative charge of both groups to bind Ca2+ cation in a
coordinate complex.
203 The function of N-terminal cluster of chelated calcium ions (about 10-12) of the coagulation zymogen helps it to bind to
exposed negative charges of phospholipids membranes ruptured in endothelial cells that line blood vessels.
204 The extremely dilute factors in circulation are concentrated together in close proximity by their electrostatic binding to
the exposed phospholipids. Concentration and proximity of substrate zymogen and activating protease greatly facilitates the individual rates and therefore the zymogen cascade series activation rates even more.
205 Osteoclastin has similar vitamin K-dependent γ-carboxyglutamate calcium chelating clusters suggesting a role for
vitamin K deficiency in osteoporosis.
206 Oxalate (HOOC-COOH). Alpha: malonate (HOOC-CH
2-COOH); Beta: succinate, citrate. Gamma: a-KG. Delta: ? 207 Glycolysis; bisphosphoglycerate mutase converts 1,3-BPG to 2,3-BPG in near molar equivalence to hemoglobin
subunits. The 2,3BPG facilitates the oxygenation of Hb in the lungs and dissociation of oxygen in the tissue capillaries. 2,3-BPG is degraded to 3-PG via bisphosphoglycerate phosphatase, which is the next substrate in glycolysis after 1,3- BPG.
208 DeoxyHHb has a lower affinity than Hb-O
2 thereby assisting oxygen gas diffusion into tissue cells after oxygen
dissociation.
209 Carbon dioxide reacts with water via carbonic anhydrase to yield carbonic acid, which dissociates into H+ and
bicarbonate. The resulting increase in acidic protons is taken up by Hb-O2 , which has a higher pKa yielding HHb-O2,
which has lower affinity for oxygen thereby yielding deoxyHHb, which circulates back to the lungs. The other molecule is 2,3-BPG, a negative allosteric effector of oxygen affinity for Hb. Putting the two negative allosteric effectors together, H+ and 2,3-BPG, facilitates both oxygen dissociation from hemoglobin in the capillaries and diffusion of oxygen into
cells.
210 The carbon dioxide reacts with each N-terminal of the β-chains to form a carbamino –NH-COO– group. Carbamino-Hb
accounts for about 13-15% and carbamino-plasma proteins is about 4% or about 17-19% of the CO2 carriage is
carbamino-protein. Dissolved CO2 is about 3.5%; HCO3– is about 80% (RBC + plasma). All together CO2 transport from