Download SOLUTE TRANSPORT

Bio 119
Solute Transport
7/11/2004
SOLUTE TRANSPORT
READING:
BOM-10
Sec. 4.7
Membrane Transport Systems p. 71
DISCUSSION QUESTIONS
BOM-10: Chapter 4; #6, #8
1.
What are the 4 essential features of carrier mediated transport?
2.
What does it mean to say that "carrier mediated transport mechanisms exhibit saturation
kinetics"?
3.
What are two essential features of active transport?
4.
What argument/s can be produced to rationalize the relative prevalence of passive transport
mechanisms in higher eukaryotes compared to the prevalence of active transport
mechanisms in bacteria?
6.
In what two fundamentally different ways is energy coupled to active transport?
7.
How does group translocation violate the strict definition of active transport?
8.
How is the capacity to transport sugars maintained by bacteria even when they are energy
limited?
10.
Characterize each of the transport systems diagrammed below. [antiport vs. symport;
electroneutral vs. electrogenic]
OUT
IN
HSO4-
H+
H+
lactose
H+
Na+
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GENERAL
In E. coli, the genome contains 427 genes for transport proteins, appox. 10% of the entire genome.
Passive Diffusion vs. Protein Carrier Mediated Transport
Solutes entering cell by
passive diffusion.
Solutes requiring carriers.
Solute
H 2O
O2
N2
CO2
glycerol
urea
tryptone
glucose
ClNa+
Permeability
Coefficient
mol/sec cm2
10-2
10-6
10-6
10-7
10-7
10-10
10-12
ESSENTIAL FEATURES OF PROTEIN CARRIER MEDIATED TRANSPORT
1. High degree of solute specificity.
2. Competitive inhibition (by substrate analogs; may exert bacteriostatic effects)
3. Inducible by specific solutes (transcriptional regulation predominant)
4. Abolished by specific mutations in genes coding for transport proteins(some mutations
may eliminate energy coupling, thus converting active transport to facilitated diffusion)
5. Saturation kinetics (carrier saturation is unusual in nature)
Vmax
Vmax/2
Km
[So]-[Si]
Facilitated Diffusion vs Active Transport
For bacteria, intracellular solute concentrations are typically higher than extracellular concentrations.
This is in contrast to the cells of multicellular eukaryotes, which are typically bathed in fluids whose
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solute content is maintained at high levels by the organisms itself. Therefore, microorganisms
depend more on active transport while higher eukaryotes frequently use passive diffusion.
ESSENTIAL FEATURES OF ACTIVE TRANSPORT
1.[S] in > [S] out
2.Energy Coupling
Formally, energy used to alter Km in < Km out, thereby shifting [Si] > [So]
I.
II.
PASSIVE DIFFUSION
Equilibrium State
Energy Coupled ?
[S] in = [S] out
NO
CARRIER MEDIATED TRANSPORT
A.
FACILITATED DIFFUSION
[S] in = [S] out
NO
B.
GROUP TRANSLOCATION
[S*] in > [S] out
YES/NO
C.
ACTIVE TRANSPORT
[S] in > [S] out
YES
1.
Simple Transport
2.
ABC System (ATP-Dependent)
SIMPLE TRANSPORT
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uniport vs. symport vs. antiport
electroneutral vs. electrogenic
typically 12 trans-membrane alpha-helices
OUT
+
K
HSO
4
lactose
+
HSO
4
lactose
+
Na
PM
IN
K
+
Na
+
[H ]
electroneutral
antiport
[MASS]
electrogenic
symport
[ELECTRICAL
+ MASS]
electroneutral
symport
[MASS]
electrogenic
uniport
[ELECTRICAL]
“ABC” ATP-DEPENDENT TRANSPORT
typically involve periplasmic binding proteins
examples:
arg,his,leu
gal,malt, rib
Experimental distinction from ∆p-coupled active transport:
blocked by ionophoric uncouplers
rather than ATPase inhibitors
GROUP TRANSLOCATION
Group translocation systems are unique to bacteria.
In a group translocation process, the solute is chemically altered (by phosphorylation, for example)
as it is transported into the cell. The altered form of the solute is typically more ionic than original
solute and therefore is likely to have lower membrane solubility. The carrier protein does not
bind/transport the altered solute as efficiently as the original solute. Although the altered solute thus
accumulates at high concentration in the cell relative to the exterior concentration of unaltered
solute, this does not fit the strict definition of active transport. The altered solute is trapped in the cell
and "cheats" the concentration gradient. Moreover, the modification of the solute is usually the first
step in its biochemical utilization of the substrate.
Sugar Phosphotransferase System of Gram-Negative Bacteria is classic example:
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Note that this process is energy coupled in the sense that a high energy phosphate bond in PEP that
could otherwise phosphorylate ADP is used instead to provide for the phosphorylation of the glucose
substrate. Additionally, note that energy is conserved in the sense that the phosphorylation would be
required (at the expense of an ATP) anyway by the first step of glucose catabolism.
Other sugars (mannose, fructose, N-acetylglucosamine, -glucosides) are also transported by this
system. E II proteins are specific for each sugar and are specifically induced, while HPr and E I are
used for all substrates. E I and HPr mutants show facilitated diffusion for all substrates of the system.
E Ia,b mutants show facilitated diffusion for a specific substrate. E IIc mutants fail to transport a
specific substrate.
During nutrient starvation [ATP] declines but glycolysis is regulated so that [PEP] remains relatively
high. Thus when sugar substrates reappear, the system is poised to act. (Active transport systems
would not be efficient under these conditions.)
Other Group Translocation Systems in E. coli
OUT
IN
free fatty acids
HS - CoA
fatty acyl - S - CoA
purines/pyrimidines
phosphoribosyl - PP
nucleotide monophosphates + PP
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