Download Lecture 44

Dental Biochemistry 2013
Lecture 39
Mukund Modak, Ph. D.
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Learning Goals for AIDS (HIV):
 HIV as a causative agent for AIDS ( infection target and role of CD4)
 HIV anatomy ( important structural proteins)
 HIV Life Cycle ( vRNA---RNA:DNA---ds proviral DNA(integration into
host DNA)
 HIV Genome and major gene products ( LTR , pol gene etc)
 Structural and NON-structural proteins
 Chemotherapy intervention sites and treatment strategies
 Commonly used inhibitor-drugs ; mode of their action
 Conclusions
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HIV (AIDS)
HTLV Family, Retrovirus class (lentivirus subclass)
Target: T4 lymphocytes (CD4 receptor)*
*CD4 (Ig like structure) binds MHC2 complement on the surface of
antigen presenting cell
T4 is a helper T-cell (helps both T and B cell functions); HIV
infection destroys
T4 cell, resulting in the state of immunodeficiency and ultimate
demise due to bacterial or viral infections.
Viral replication occurs via retroviral lifecycle
Viral RNA  proviral DNA  mRNA  Protein
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LIFE CYCLE OF AIDS VIRUS (HIV)
1.
HIV attaches to T4 cell
gp120 (of virus) to CD4 and CCR5 (chemokine receptor) present on
the cell surface allows entry of the virus.
2.
Viral RNA + viral RT + nucleotides = viral DNA
RT
3.
V RNA
+tRNA primer
4.
RT
RNA.DNA
ss
ss
Integrase
ds DNA
its RNase H
into host DNA
Proviral DNA
Proviral DNA Cell DNA  transcription +translation viral proteins
+RNA
New Virus
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Diploid genome (2 RNA copies/ virus particle)
RNA physically linked as a dimer by hydrogen bonds; Harbors
tRNAlys for initiating reverse transcription
RNA is single-stranded, positive sense, composed of 9749
nucleotides and has 5' cap and 3' poly-(A) tail
HIV genome encodes nine open reading frames, 15 proteins
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5’
3’
MA (matrix)
CA (capsid)
NC
(nucleocapsid)
p6
Gp160
PR (protease)
RT (reverse transcriptase)
IN (integrase
SU (Gp120)
TM (Gp41)
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LTR Region contains sites for transcriptions factors
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HIV GENE PRODUCTS AND THEIR FUNCTIONS
Structural genes
LTR
gag
pol
env
:
:
:
:
Non-structural genes1
tat
:
rev
:
vif
:
vpr
:
vpu
:
nef
:
Binding site for host transcription factors
Nucleocapsid, core protein (matrix), P-6
RT, RNase H, integrase, protease
Coat proteins (CD4 specific). Gp120 & Gp41
Transcription activator ( 100 X) binds to vRNA (TAR)
Regulation of mRNA export
Promotes infectivity
Nuclear import of viral DNA; G-2 cell cycle arrest.
enhances release of virus; also down regulates CD4
expression
suppresses v-gene expression; also regulates CD4
and MHC class I expression
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

CD4 antigen: primary receptors, binds HIV gp120
Chemokine receptors: essential co-receptors, seventransmembrane G protein-coupled receptors,
tropism
-CCR5: employed by macrophage-tropic HIV
strains involved in critical early stages of infection,
CCR5 receptor gene was mapped to human
chromosome 3p21 only 18 kb away from CCR2B
receptor gene
-CXCR4: ligand is a B cell stimulatory factor
called fusin, promotes infection /fusion of CD4+ T
cells
-CCR2: recently identified co-receptor
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-Nature of co-receptor may explain why people who
are exposed repeatedly to HIV remain uninfected
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
CCR2: heterozygous mutation in CCR2 present in all
races in the U.S. at a frequency of about 20-25%,
accounts for long term survivors
CCR5: binds to the chemokines- RANTES, MIP-1,
and MIP-1 and suppress HIV’s ability to infect cells;
32 base pair deletion in CCR5 prevents its
expression, two copies of defective CCR5 gene
confers immunity from HIV infection and a single
defective gene results in delayed AIDS progression
1 in 100 Caucasians have this double mutation and
17% have a single defective gene in contrast to only
2% African Americans with single mutation
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Chemotherapeutic Intervention Sites
in the HIV Life Cycle
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Reverse Transcriptase Inhibitors


Nucleoside Class: competitive inhibitors, bind to the
enzyme's active site, DNA chain terminators. These
drugs are phosphorylated to the triphosphate form
by host cell enzymes before being incorporated into
the growing DNA chain and inhibiting further
elongation. Eg. AZT(azidothymidine),
DDI(didanosine), 3TC(Lamivudine) etc.
Non-Nucleoside Class: these drugs do not need to
be phosphorylated to be active, bind elsewhere than
the enzyme’s active site and function in a noncompetitive fashion. These drugs work
synergistically with nucleoside analogs, exhibit high
therapeutic index and good bio-availablity. Eg.
Nevirapine, delavirdine, efavirenz etc.
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Treatment strategies
Pol (replication) and env (receptors, viral membrane) gene products
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Pol gene assembly
17 kD
65 kD
36 kD
NH2
Protease
pol
RNase H
Integrase
Protease : protease inhibitors
Pol (Rt) : AZT/dideoxynucleosides/non-nucleoside inhibitors
e.g. Nevirapine
Integrase: Issentress (Merck)
Entry Site (CCR-5): Maraviroc
Fusion Inhibition: Enfuvirtide (EFV)
Vaccines: ????
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 Peptide
mimicking compounds that are
transition state analogs. They bind the
enzyme much more tightly than the natural
substrate and function as competitive
enzyme inhibitors. Drugs in this category
include saquinavir, indinavir etc.
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 Combination
therapy, or the simultaneous
use of multiple anti-HIV drugs, is the most
effective means of controlling HIV-1
infection. Typically, one combines one
protease inhibitor with two reverse
transcriptase inhibitors
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
No effective HIV vaccine available

Attempts to develop 3 classes of anti-HIV
vaccines
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Therapeutic vaccines: designed to boost the
immune system of an already-infected person
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Protective/Prophylactic vaccines: to prevent HIV
infection in uninfected population

Perinatal vaccines: prevent mother to child
transmission
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