1stAid Biochem
137
S E C T I O N I I
Biochemistry
“Biochemistry is the study of carbon compounds that crawl.”
––Mike Adams
High-Yield Clinical Vignettes
High-Yield Topics DNA and RNA Genetic Errors Metabolism Protein/Cell Vitamins
This high-yield material includes molecular biology, genetics, cell bi-ology, and principles of metabolism (especially vitamins, cofactors,minerals, and single-enzyme-de?ciency diseases). When studying metabolic pathways, emphasize important regulatory steps and en-zyme de?ciencies that result in disease. For example, understanding the defect in Lesch–Nyhan syndrome and its clinical consequences is higher yield than memorizing every intermediate in the purine sal-vage pathway. Do not spend time on hard-core organic chemistry,mechanisms, and physical chemistry. Detailed chemical structures are infrequently tested. Familiarity with the latest biochemical tech-niques that have medical relevance—such as enzyme-linked im-munosorbent assay (ELISA), immunoelectrophoresis, Southern blot-ting, and PCR—is useful. Beware if you placed out of your medical school’s biochemistry class, for the emphasis of the test differs from the emphasis of many undergraduate courses. Review the related biochemistry when studying pharmacology or genetic diseases as a way to reinforce and integrate the material.
B I O
C H E M I S T RY—H I G H-Y I E L
D C L I N I C A L V I G N
E T T E S
s Full-term neonate of uneventful delivery becomes mentally retarded and hyperactive and has a musty odor →what is the diagnosis? →PKU.
s A stressed executive comes home from work, consumes 7 or 8 martinis in rapid succession be-fore dinner, and becomes hypoglycemic →what is the mechanisms? →NADH increase pre-vents gluconeogenesis by shunting pyruvate and oxaloacetate to lactate and malate.
B I O
C H E M I S T RY—H I G H-Y I E L
D T O P I C S
DNA/RNA/Protein
1.Molecular biology: tools and techniques (e.g., cloning, cDNA libraries, PCR, restriction frag-
ment length polymorphism, restriction enzymes, sequencing).
2.Transcriptional regulation: the operon model (lac, trp operons) of transcription, eukaryotic
transcription (e.g., TATA box, enhancers, effects of steroid hormones, transcription factors).
3.Protein synthesis: steps, regulation, energy (Which step requires ATP? GTP?), differences be-
tween prokaryotes and eukaryotes (N-formyl methionine), post-translational modi?cation
(targeting to organelles, secretion).
4.Acid–base titration curve of amino acids, proteins.
Genetic Errors
1.Inherited hyperlipidemias: types, clinical manifestations, speci?c changes in serum lipids.
2.Glycogen and lysosomal storage diseases (e.g., type III glycogen storage disease), I cell disease.
3.Porphyrias: defects, clinical presentation, effect of barbiturates.
4.Inherited defects in amino acid metabolism.
Metabolism
1.Glycogen synthesis: regulation, inherited defects.
2.Oxygen consumption, carbon dioxide production, and ATP production for fats, proteins, and
carbohydrates.
3.Amino acid degradation pathways (urea cycle, tricarboxylic acid cycle).
4.Effect of enzyme phosphorylation on metabolic pathways.
5.Rate-limiting enzymes in different metabolic pathways (e.g., pyruvate decarboxylase).
6.Sites of different metabolic pathways (What organ? Where in the cell?).
7.Fed state versus fasting state: forms of energy used, direction of metabolic pathways.
8.Tyrosine kinases and their effects on metabolic pathways (insulin receptor, growth factor re-
ceptors).
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B I O
C H E M I S T RY—H I G H-Y I E L
D T O P I C S(c o n t i n u e d)
9.Anti-insulin (gluconeogenic) hormones (e.g., glucagon, GH, cortisol).
10.Synthesis and metabolism of neurotransmitters (e.g., acetylcholine, epinephrine, norepineph-
rine, dopamine).
11.Purine/pyrimidine degradation.
12.Carnitine shuttle: function, inherited defects.
13.Cellular/organ effects of insulin secretion.
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B I O
C H E M I S T RY —
D N A A N D R N A
Chromatin Condensed by (?) charged DNA looped twice around Think of beads on a string.
structure
(+) charged H2A, H2B, H3, and H4 histones
(nucleosome bead).H1 ties nucleosomes together in a string (30-nm ?ber). In mitosis, DNA condenses to form mitotic chromosomes.Heterochromatin Condensed, transcriptionally inactive.Euchromatin
Less condensed, transcriptionally active.
Eu =true, “truly transcribed.”Nucleotides
Purines (A, G ) have two rings. Pyrimidines (C, T, U)PUR e A s G old:PUR ines.have one ring. Guanine has a ketone. Thymine has CUT the PY (pie): a methyl.
PY rimidines.
Uracil found in RNA; thymine in DNA.
THY mine has a me THY l.
G-C bond (3 H-bonds) stronger than A-T bond (2 H-bonds).
Start and stop AUG (or rarely GUG) is the mRNA initiation codon. AUG in AUG urates codons
AUG codes for methionine, which may be removed protein synthesis.
before translation is completed. In prokaryotes the initial AUG codes for a formyl-methionine (f-met).Stop codons: UGA, UAA, UAG.
UGA = U G o A way UAA = U A re A way UAG = U A re G one
Genetic code: Unambiguous =each codon speci?es only one amino acid.
features
Degenerate =more than one codon may code for same amino https://www.wendangku.net/doc/a15721893.html,maless, nonoverlapping (except some viruses).
Universal (exceptions include mitochondria, archaebacteria, Mycoplasma,and some yeasts).Mutations in DNA
Silent =same aa, often base change in third position of Severity of damage: nonsense codon.
>missense >silent.
Missense =changed aa (conservative =new aa is similar in chemical structure).
Nonsense =change resulting in early stop codon.Frameshift = change resulting in misreading of all nucleotides downstream, usually resulting in a truncated protein.
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N 10–Formyl-Carbamoyl
Transition versus Transition =substituting purine for purine or pyrimidine Transversion =transversion
for pyrimidine.Trans con version (one type Transversion =substituting purine for pyrimidine or vice to another).versa.DNA replication
Origin of replication: continuous DNA synthesis on Eukaryotic genome has multiple leading strand and discontinuous (Okazaki fragments) origins of replication. on lagging strand. Primase makes an RNA primer Bacteria, viruses, and on which DNA polymerase can initiate replication.plasmids have only one origin DNA polymerase reaches primer of preceding of replication.fragment; 5′→3′ exonuclease activity of DNA polymerase I degrades RNA primer; DNA
ligase seals; 3′→5′ exonuclease activity of DNA polymerase “proofreads” each added nucleotide.DNA topoisomerases create a nick in the helix to 141
5'
Leading strand
DNA polymerase
Okazaki fragment
DNA ligase
RNA primer
DNA polymerase
B I O
C H E M I S T RY—
D N A A N D R N A(c o n t i n u e d)
DNA repair: single Single-strand, excision-repair–speci?c glycosylase If both strands are damaged, strand recognizes and removes damaged base. Endonuclease repair may proceed via
makes a break several bases to the 5′side. Exonuclease recombination with
removes short stretch of nucleotides. DNA polymerase undamaged homologous
?lls gap. DNA ligase seals.chromosome.
DNA/RNA/protein DNA and RNA are both synthesized 5′→3′.Imagine the incoming synthesis direction Remember that the 5′of the incoming nucleotide nucleotide bringing a gift
bears the triphosphate (energy source for bond). (triphosphate) to the 3′host.
The 3′hydroxyl of the nascent chain is the target.“BYOP(phosphate) from 5
Protein synthesis also proceeds in the 5′to 3′ direction.to 3.”
Amino acids are linked N
to C.
Types of RNA m RNA is the largest type of RNA.M assive, R ampant, T iny.
r RNA is the most abundant type of RNA.
t RNA is the smallest type of RNA.
Polymerases: RNA Eukaryotes:I, II, and III are numbered as
RNA polymerase I makes r RNA.their products are used in
RNA polymerase II makes m RNA.protein synthesis. OR 1, 2,
RNA polymerase III makes t RNA.3= RMT (rhyme).
No proofreading function, but can initiate chains. RNA
polymerase II opens DNA at promoter site (A-T-rich
upstream sequence—TATA and CAAT). α-amanitin
inhibits RNA polymerase II.
Prokaryotes:
RNA polymerase makes all three kinds of RNA.
Regulation of gene
expression
Promoter Site where RNA polymerase and multiple other Promoter mutation commonly
transcription factors bind to DNA upstream from results in dramatic decrease
gene locus.in amount of gene tran-
scribed.
Enhancer Stretch of DNA that alters gene expression by binding
transcription factors. May be located close to, far
from, or even within (in an intron) the gene whose
expression it regulates.
Introns versus Exons contain the actual genetic information coding IN trons stay IN the nucleus, exons for protein.whereas EX ons EX it and are Introns are intervening noncoding segments of DNA.EX pressed.
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Splicing of mRNA Introns are precisely spliced out of primary mRNA transcripts. A lariat-shaped intermediate
is formed. Small nuclear ribonucleoprotein particles (snRNP) facilitate splicing by
binding to primary mRNA transcripts and forming spliceosomes.
RNA processing Occurs in nucleus. After transcription:Only processed RNA is (eukaryotes) 1.Capping on 5′end (7-methyl-G)transported out of the
2.Polyadenylation on 3′end (≈200 A’s)nucleus.
3.Splicing out of introns
Initial transcript is called heterogeneous nuclear RNA
(hnRNA).
Capped and tailed transcript is called mRNA.
tRNA structure75–90 nucleotides, cloverleaf form, anticodon end is opposite 3′aminoacyl end. All
tRNAs, both eukaryotic and prokaryotic, have CCA at 3′end along with a high
percentage of chemically modi?ed bases. The amino acid is covalently bound to the 3′
end of the tRNA.
tRNA charging Aminoacyl-tRNA synthetase (one per aa, uses ATP) Aminoacyl-tRNA synthetase
scrutinizes aa before and after it binds to tRNA. If and binding of charged
incorrect, bond is hydrolyzed by synthetase. The tRNA to the codon are
aa-tRNA bond has energy for formation of peptide responsible for accuracy of
bond. A mischarged tRNA reads usual codon but amino acid selection.
inserts wrong amino acid.
tRNA wobble Accurate base pairing is required only in the ?rst 2 nucleotide positions of an mRNA
codon, so codons differing in the 3rd “wobble” position may code for the same
tRNA/amino acid.
Protein synthesis: P site =peptidyl, A site =aminoacyl. ATP is used A TP = tRNA A ctivation. ATP versus GTP in tRNA charging, whereas GTP is used in binding G TP = tRNA G ripping and
of tRNA to ribosome and for translocation.G oing places.
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B I O
C H E M I S T RY —
D N A A N D R N A (c o n t i n u e d )
Polymerase chain Molecular biology laboratory procedure that is used to synthesize many copies of a desired reaction (PCR)
fragment of DNA.Steps:
1.DNA is denatured by heating to generate 2 separate strands
2.During cooling, excess of premade primers anneal to a speci?c sequence on each strand to be ampli?ed
3.Heat-stable DNA polymerase replicates the DNA sequence following each primer These steps are repeated multiple times for DNA sequence ampli?cation.
Molecular biology techniques
Southern blot
A DNA sample is electrophoresed on a gel and then DNA–DNA hybridization transferred to a ?lter. The ?lter is then soaked in a S outhern = S ame
denaturant and subsequently exposed to a labeled DNA probe that recognizes and anneals to its complementary strand. The resulting double- stranded labeled piece of DNA is visualized when the ?lter is exposed to ?lm.
Northern blot Similar technique, except that Northern blotting DNA–RNA hybridization
involves radioactive DNA probe binding to sample RNA.
Western blot Sample protein is separated via gel electrophoresis and Antibody–protein hybridization
transferred to a ?lter. Labeled antibody is used to bind to relevant protein.
Southwestern blot Protein sample is run on a gel, transferred to a ?lter, DNA–protein interaction
and exposed to labeled DNA. Used to detect DNA–protein interactions as with transcription factors (e.g., p53, jun ).
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COMMON GENETIC DISEASES DETECTABLE BY PCR
Bio.53, 58, 70, 87, 94, 95, 96
B I O
C H E M I S T RY —G E N E T I C E R R O R S
Modes of inheritance
Autosomal dominant
Often due to defects in structural genes. Many Often pleiotropic and, in many generations, both male and female, affected.
cases, present clinically after puberty. Family history crucial to diagnosis.
Autosomal recessive
25% of offspring from 2 carrier parents are affected. Commonly more severe than Often due to enzyme de?ciencies. Usually seen in dominant disorders; patients only one generation.
often present in childhood.X-linked recessive
Sons of heterozygous mothers have a 50% chance of Commonly more severe in being affected. No male-to male transmission.males. Heterozygous females may be affected.Mitochondrial Transmitted only through mother. All offspring of Leber’s hereditary optic
inheritance
affected females may show signs of disease.
neuropathy, mitochondrial myopathies.
Genetic terms
Variable expression Nature and severity of the phenotype varies from one individual to another.Incomplete penetrance Not all individuals with a mutant genotype show the mutant phenotype.Pleiotropy One gene has more than one effect on an individual’s phenotype.
Imprinting Differences in phenotype depend on whether the mutation is of maternal or paternal origin (e.g., Angelman’s syndrome [maternal], Prader-Willi syndrome [paternal]).Anticipation Severity of disease worsens or age of onset of disease is earlier in succeeding generations (e.g., Huntington’s disease).
Loss of heterozygosity
If a patient inherits or develops a mutation in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops. This is not true of oncogenes.Hardy–Weinberg If a population is in Hardy–Weinberg equilibrium, then:
population p 2+ 2pq + q 2= 1genetics
p + q = 1
p and q are separate alleles; 2pq = heterozygote prevalence.
DNA repair defects
Xeroderma pigmentosum (skin sensitivity to UV light), ataxia-telangiectasia (x-rays), Bloom’s syndrome (radiation), and Fanconi’s anemia (cross-linking agents).Xeroderma Defective excision repair such as uvr ABC exonuclease. Results in inability to repair pigmentosum
thymidine dimers, which form in DNA when exposed to UV light.Associated with dry skin and with melanoma and other cancers.
Fructose Hereditary de?ciency of aldolase B. Fructose-1-phosphate accumulates, causing a decrease intolerance
in available phosphate, which results in inhibition of glycogenolysis and gluconeogenesis.
Symptoms: hypoglycemia, jaundice, cirrhosis.
T reatment: must decrease intake of both fructose and sucrose (glucose +fructose).
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Bio.60, 84
Bio.84
Bio.64
B I O
C H E M I S T RY —G E N E T I C E R R O R S (c o n t i n u e d )
Galactosemia
Absence of galactose-1-phosphate uridyltransferase. Autosomal recessive. Damage is caused by accumulation of toxic substances (including galactitol) rather than absence of an essential compound.
Symptoms: cataracts, hepatosplenomegaly, mental retardation.
Treatment: exclude galactose and lactose (galactose +glucose) from https://www.wendangku.net/doc/a15721893.html,ctase de?ciency
Age-dependent and/or hereditary lactose intolerance (blacks, Asians).Symptoms: bloating, cramps, osmotic diarrhea.Treatment: avoid milk or add lactase pills to diet.Pyruvate
Causes backup of substrate (pyruvate and alanine), L ysine and L eucine—the only dehydrogenase resulting in lactic acidosis.purely ketogenic amino de?ciency Findings: neurologic defects.
acids.
Treatment: increased intake of ketogenic nutrients.Glucose-6-G6PD is rate-limiting enzyme in HMP shunt (which G6PD de?ciency more phosphate
yields NADPH). NADPH is necessary to keep prevalent among blacks.dehydrogenase glutathione reduced, which in turn detoxi?es free H einz bodies: altered
de?ciency
radicals and peroxides. ↓NADPH in RBCs H emoglobin precipitates leads to hemolytic anemia due to poor RBC within RBCs.
defense against oxidizing agents (fava beans, sulfonamides, primaquine) and antituberculosis drugs. X-linked recessive disorder.Glycolytic enzyme Hexokinase, glucose-phosphate isomerase, aldolase, RBCs metabolize glucose
de?ciency
triose-phosphate isomerase, phosphate-glycerate anaerobically (no mitochon-kinase, enolase, and pyruvate kinase de?ciencies dria) and thus solely depend are associated with hemolytic anemia.on glycolysis.
Glycogen storage 12 types, all resulting in abnormal glycogen metabolism diseases
and an accumulation of glycogen within cells.Type I
Von Gierke’s disease =glucose-6-phosphatase de?ciency. Findings: severe fasting hypoglycemia, ↑↑glycogen in liver.Bio.82
Type II
Pompe’s disease =lysosomal α-1,4-glucosidase P ompe’s trashes the P ump de?ciency.
(heart, liver, and muscle).
Findings: cardiomegaly and systemic ?ndings, leading to early death.Bio.79
Type III Cori’s = de?ciency of debranching enzyme α-1,6-glucosidase.Type V
McArdle’s disease =skeletal muscle glycogen M cArdle’s:M uscle.phosphorylase de?ciency.
Findings: ↑glycogen in muscle but cannot break it V ery P oor C arbohydrate down, leading to painful cramps, myoglobinuria with M etabolism.
strenuous exercise.
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Bio.86
Bio.62
Homocystinuria
Defect in cystathionine synthase. T wo forms:
Results in excess homocystine 1.De?ciency (treatment: ↓Met and ↑Cys in diet)in the urine. Cysteine 2.Decreased af?nity of synthase for pyridoxal becomes essential.
phosphate (treatment: ↑↑vitamin B 6in diet)Cystinuria
Common (1/7000) inherited defect of tubular amino COLA
acid transporter for C ystine, O rnithine,L ysine, and A rginine in kidneys. Excess cystine in urine can lead to the precipitation of cystine kidney stones.Maple syrup urine Blocked degradation of branched amino acids (Ile, Val,Urine smells like maple syrup. disease
Leu) due to ↓α-ketoacid dehydrogenase.
Think of cutting (blocking) Causes severe CNS defects, mental retardation, and branches of a maple tree.
death.
Amino acid derivatives
Phenylketonuria
Normally, phenylalanine is converted into tyrosine Screened for at birth.(nonessential aa). In PKU, there is ↓phenylalanine Phenylketones =phenylacetate, hydroxylase or ↓tetrahydrobiopterin cofactor. T yrosine phenyllactate, and becomes essential and phenylalanine builds up, leading phenylpyruvate in urine.to excess phenylketones.
Findings: mental retardation, fair skin, eczema, musty body odor.
Treatment: ↓phenylalanine (contained in Nutrasweet)and ↑tyrosine in diet.
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Bio.65
Bio.71
Tryptophan
NAD +/NADP +
serotonin
Phenylalanine
NE
melanin
tyrosine
dopamine
dopa
Histidine histamine Glycine
porphyrin heme
epi
Arginine
creatine Bio.54
Bio.77
B I O
C H E M I S T RY —G E N E T I C E R R O R S (c o n t i n u e d )
Alkaptonuria
Congenital de?ciency of homogentisic acid oxidase in the degradative pathway of tyrosine. Resulting alkapton bodies cause dark urine.Also, the connective tissue is dark. Benign disease. May have arthralgias.Albinism
Congenital de?ciency of tyrosinase. Results in an Lack of melanin results in an inability to synthesize melanin from tyrosine. Can increased risk of skin cancer.
result from a lack of migration of neural crest cells.Adenosine ADA de?ciency can cause SCID. Excess ATP and SCID =severe combined
deaminase dATP imbalances nucleotide pool via feedback (T and B) immunode?ciency de?ciency
inhibition of ribonucleotide reductase. This
disease. SCID happens to prevents DNA synthesis and thus lowers lymphocyte kids (remember “bubble count. First disease to be treated by experimental boy”).
human gene therapy.
Lesch–Nyhan Purine salvage problem owing to absence of HGPRTase, LNS =L acks N ucleotide syndrome
which converts hypoxanthine to inosine monophos-S alvage (purine).
phate (IMP)and guanine to guanosine monophos-phate (GMP). X-linked recessive. Results in excess uric acid production.
Findings: retardation, self-mutilation, aggression, hyperuricemia, gout, and choreoathetosis.Ehlers–Danlos Faulty collagen synthesis causing:Sounds like “feller’s damn syndrome
1. Hyperextensible skin loose” (loose joints).
2. Tendency to bleed
3. Hypermobile joints
10 types. Inheritance varies from autosomal dominant (type IV) to autosomal recessive (type VI) to X-linked recessive (type IX).
Osteogenesis Clinically characterized by multiple fractures occurring May be confused with child imperfecta
with minimal trauma (brittle bone disease), which may abuse.occur during the birth process, as well as by blue sclerae due to the translucency of the connective tissue over the choroid. Caused by a variety of gene defects resulting in abnormal collagen synthesis.The most common form is autosomal-dominant with abnormal collagen type I synthesis.
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Bio.51
Bio.50
Bio.70
Bio.56
Bio.75
Sphingolipid Components of nerve tissue.
components
Lysosomal storage Each is caused by a de?ciency in one of the many diseases
lysosomal enzymes.
Fabry’s disease
Caused by de?ciency of α-galactosidase A, resulting in X-linked recessive.
accumulation of ceramide trihexoside. Finding: renal failure.Bio.57
Gaucher’s disease
Caused by de?ciency of β-glucocerebrosidase, leading Autosomal recessive.
to glucocerebroside accumulation in brain, liver, spleen, and bone marrow (Gaucher’s cells with characteristic “crinkled paper” enlarged cytoplasm).Type I, the more common form, is compatible with a normal life span.Bio.63
Niemann–Pick disease
De?ciency of sphingomyelinase causes buildup of
Autosomal recessive. No man sphingomyelin and cholesterol in reticuloendothelial picks (Niemann–Pick) his and parenchymal cells and tissues. Patients die by nose with his sphinger.age 3.Bio.73
Tay–Sachs disease
Absence of hexosaminidase A results in
Autosomal recessive.GM 2ganglioside accumulation. Death occurs by Tay-saX sounds like age 3. Cherry-red spot visible on macula. Carrier he X osaminidase.
rate is 1 in 30 in Jews of European descent (1 in 300 for others).Bio.81
Metachromatic De?ciency of arylsulfatase A results in the accumulation Autosomal recessive.
leukodystrophy of sulfatide in the brain, kidney, liver, and peripheral nerves.Bio.72
Krabbe’s disease
Absence of galactosylceramide β-galactosidase Autosomal recessive.
leads to the accumulation of galactocerebroside in the brain. Optic atrophy, spasticity, early death.
Bio.69
Hurler’s syndrome De?ciency of α-L -iduronidase; results in corneal Autosomal recessive.clouding and mental retardation.Bio.67
Hunter’s syndrome
De?ciency of iduronate sulfatase. Mild form of Hurler’s X-linked recessive.
with no corneal clouding and mild mental retardation.Bio.66
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CERAMIDE
+ fatty acid
Serine + palmitate
CEREBROSIDE
B I O
C H E M I S T RY —M E TA B O L I S M
ATP
Base (adenine), ribose, 3 phosphoryls. 2 phosphoanhydride bonds, 7 kcal/mol each.
Aerobic metabolism of glucose produces 38 A TP via malate shuttle, 36 A TP via G3P shuttle.Anaerobic glycolysis produces only 2 A TP per glucose molecule.
A TP hydrolysis can be coupled to energetically unfavorable reactions.Activated carriers
Phosphoryl (ATP)
Electrons (NADH, NADPH, FADH 2)Acyl (coenzyme A, lipoamide)CO 2(biotin)
One-carbon units (tetrahydrofolates)CH 3groups (SAM)Aldehydes (TPP)
Glucose (UDP-glucose)Choline (CDP-choline)
G-protein-linked second messengers
Receptor G protein class
Major functions
α1q ↑ vascular smooth muscle contraction α2i ↓ sympathetic out?ow, ↓insulin release
β1s ↑ heart rate, ↑contractility, ↑renin release, ↑ lipolysis β2s Vasodilation, bronchodilation, ↑glucagon release M 1q CNS
M 2i ↓heart rate
M 3
q
↑exocrine gland secretions
Signal molecule ATP →cAMP via adenylate cyclase.precursors
GTP →cGMP via guanylate cyclase.
Glutamate →GABA via glutamate decarboxylase (requires vit. B 6).Choline →ACh via choline acetyltransferase (ChAT).
Arachidonate →prostaglandins, thromboxanes, leukotrienes via cyclooxygenase/lipoxygenase.
Fructose-6-P →fructose-1,6-bis-P via phosphofructokinase (PFK), the rate-limiting enzyme of glycolysis.
1,3-BPG →2,3-BPG via bisphosphoglycerate mutase.
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Receptor
Lipids
PIP 2
G G G IP 3
[Ca 2+]in
DAG
Protein kinase C
Protein kinase A
ATP
cAMP
cAMP
Phospholipase C
Receptor
Adenylcyclase
Receptor
Adenylcyclase Bio.54
NAD+/NADPH NAD+is generally used in catabolic processes to carry NADPH is a product of the
reducing equivalents away as NADH. NADPH is HMP shunt and the malate
used in anabolic processes as a supply of reducing dehydrogenase reaction.
equivalents.
S-adenosyl-ATP +methionine →SAM. SAM transfers methyl SAM the methyl donor man. methionine units to a wide variety of acceptors (e.g., in synthesis
of phosphocreatine, high-energy phosphate active
in muscle ATP production). Regeneration of meth-
.
ionine (and thus SAM) is dependent on vitamin B
12
Metabolism sites
Mit ochondria Fatty acid O xidation (β-oxidation), A cetyl-CoA Mit y OAK.
production, K rebs cycle.
Cytoplasm Glycolysis, fatty acid synthesis, HMP shunt, protein
synthesis (RER), steroid synthesis (SER).
Both Gluconeogenesis, urea cycle, heme synthesis.
Hexokinase versus Hexokinase is found throughout body.Only hexokinase is feedback glucokinase Glucokinase (lower af?nity [↑K
] but higher capacity inhibited by G6P.
m
]) is predominantly found in the liver.
[↑V
max
151
BIOCHEMISTRY—METABOLISM(continued) Regulation of metabolic pathways
152
Metabolism in major organs
153
154
Glycolysis Glucose-6-P ?s
regulation,Hexokinase/glucokinase *
irreversible Fructose-6-P Fructose-1,6-BP
ATP ?s
, AMP ⊕, citrate ?s ,enzymes
Phosphofructokinase fructose 2, 6-BP ⊕(rate-limiting step)Phosphoenolpyruvate Pyruvate
ATP ?s
, alanine ??s ,fructose-1,6-BP ⊕Pyruvate Acetyl-CoA
ATP ?s
, NADH ??s ,acetyl-CoA ??s
dehydrogenase
* Glucokinase in liver; hexokinase in all other tissues.
Gluconeogenesis, irreversible enzymes
P yruvate carboxylase In mitochondria. Pyruvate →oxaloacetate.
Requires biotin, ATP .
Activated by acetyl-CoA.P EP carboxykinase In cytosol. Oxaloacetate →phosphoenolpyruvate.Requires GTP .F ructose-1,6-In cytosol. Fructose-1,6-bisphosphate →P athway P roduces bisphosphatase
fructose-6-P
F resh
G lucose.
G lucose-6-phosphatase In cytosol. Glucose-6-P →glucose
Above enzymes found only in liver, kidney, intestinal epithelium. Muscle cannot participate in gluconeogenesis.
Hypoglycemia is caused by a de?ciency of these key gluconeogenic enzymes listed above (e.g., von Gierke’s disease, which is caused by a lack of glucose-6-phosphatase in the liver).Bio.82
Pentose phosphate Produces ribose-5-P from G6P for nucleotide synthesis.
pathway
Produces NADPH from NADP +for fatty acid and steroid biosynthesis and for maintaining reduced glutathione inside RBCs.Part of HMP shunt.
All reactions of this pathway occur in the cytoplasm.
Sites: lactating mammary glands, liver, adrenal cortex—all sites of fatty acid or steroid synthesis.
Cori cycle
Transfers excess reducing equivalents from RBCs and muscle to liver, allowing muscle to function
anaerobically (net 2 ATP).
BIOCHEMISTRY—METABOLISM (continued )
155
Pyruvate
The complex contains three enzymes that require ?ve The complex is similar to the dehydrogenase cofactors: pyrophosphate (from thiamine), lipoic α-ketoglutarate dehydroge-complex
acid, CoA (from pantothenate), FAD (ribo?avin), nase complex (same cofac-NAD (niacin).
tors, similar substrate and Reaction: pyruvate +NAD ++CoA →acetyl-CoA +action).
CO 2+NADH.
Cofactors are the ?rst 4 B vitamins plus lipoic acid:B 1(thiamine; TPP)B 2(FAD)B 3(NAD)
B 5(pantothenate →CoA)Lipoic acid
Pyruvate metabolism
TCA cycle
Produces 3NADH, 1FADH 2, 2CO 2, 1GTP per acetyl CoA =12ATP/acetyl CoA (2×everything per glucose)α-Ketoglutarate dehydrogenase complex requires same cofac-tors as the pyruvate dehydro- genase complex.
C indy I s K inky S o S he F ornicates M ore O ften.
Glucose
Acetyl CoA CO 2
2 + NADH
FADH Pyruvate
dehydrogenase
ATP
Acetyl-CoA NADH CO 2 + NADH
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B I O
C H E M I S T RY —M E TA B O L I S M (c o n t i n u e d )
Electron transport chain and oxidative phosphorylation
Electron transport 1 NADH →3ATP; 1 FADH 2→2ATP
chain Oxidative
1.Electron transport inhibitors (rotenone, antimycin A, CN ?, CO) directly inhibit phosphorylation electron transport, causing ↓of proton gradient and block of ATP synthesis.
poisons
2.ATPase inhibitor (oligomycin) directly inhibits mitochondrial ATPase, causing ↑of proton gradient, but no ATP is produced because electron transport stops.
3.Uncoupling agents (2,4-DNP) increase permeability of membrane, causing ↓of proton gradient and ↑oxygen consumption. ATP synthesis stops. Electron transport continues.
Liver: fed state vs. fasting state
Fatty acid
Fatty acid synthesis =cytosol.
Fatty acid degradation occurs metabolism sites
Fatty acid degradation =mitochondria.
where its products will be Fatty acid entry into mitochondrion is via carnitine consumed—in the shuttle (inhibited by cytoplasmic malonyl-CoA).mitochondrion.
Fatty acid entry into cytosol is via citrate shuttle.
Cholesterol Rate-limiting step is catalyzed by HMG-CoA reductase, Lovastatin inhibits HMG-CoA synthesis
which converts HMG-CoA to mevalonate. T wo-thirds reductase.of plasma cholesterol is esteri?ed by lecithin-cholesterol acyltransferase (LCAT).
y c i n
A m y t a l R o t e n o n e
A n t
i m y
c i n A
N -, N 3-, C
O
Amino acids glycerol Glucose