CONTROL OF
METABOLIC
PATHWAYS
Explain what the
“one gene – one polypeptide”
hypothesis means in terms of
producing functional proteins.
Enzymes
Describe the
properties of enzymes and their mode of action:Active site – one or many particular regions of specific
shape &/or charge that make up the cleft, into which
substrate molecules are drawn in (*induced fit model)
Specificity – the protein is determined by the order of
the bases in the DNA, in turn the triplets coding for the
amino acid sequence of the polypeptide chain (1o
structure), and the folding (α-helices and β-pleated
sheets – held by H-bonds b/w CO and NH grps , 2o
structure, and cysteine aa forming disulfide bridges that
give the precise shape, 3o structure) and aggregation of
chains to form globular proteins (4o structure). This
makes enzymes not only specific in their shape (*lock
and key design) and function but also where they are
produced (controlled by the rate the reactants and
products enter and exit the organelles).
Enzymes are proteins whose synthesis is controlled by DNA.
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Activation energy – enzymes are biological
catalysts (speed up reactions without being used
up in them). This is achieved by lowering the
‘energy of activation’, due to there lock and key
design and induce fit capability, thus influencing
reactant bond stability.
Control of metabolic pathways – regulating
the amount of enzyme activity [Speed forward
stimulation (keeping a substrate at a low level
by continuing to be produced) and Negative
feedback (deactivating the enzyme when there
is an abundance of end-product = End product
inhibition)] or restriction/compartmentation
within an organ/organelle.
Anabolic enzymes :A + B  AB
Catabolic enzymes :
AB A + B
Metabolic disorders –
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eg. Phenylalanine (raft of disorders
depending on which enzyme is faulty)
– we are now able to screen for these
disorders eg. PKU.
Regulation of gene action
– consist of the structural
gene (codes for an enzyme),
promoter (to which the RNA
polymerase binds to begin
mRNA synthesis) and operator
sites (to which the repressor can
bind to block synthesis).
 Operon
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Gene induction – eg. Lac operon in E.coli
– consists of an operon with a regulator
gene (which produces the repressor),
located just upstream. When lactose is
available some is converted into inducer
allolactose, this binds to the repressor
thus transcription occurs.
Gene repression – eg. By end product – in
the Lac operon absence of lactose, results
in the repressor being produced and
binding to the operator, thus preventing
transcription.
Transcriptional control in Eukaryotes –
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Eukaryotes have ‘control elements’, in
non-coding region (introns), that
regulate transcription by binding
‘transcription factors’
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Eukaryotic genes have a promoter region
immediately upstream and an enhancer
sequence further upstream.
Transcription is activated when the hairpin
loop in the DNA brings the transcription
factors/ activators attached to the
enhancer in contact with the RNA
polymerase bound to the promoter.
Transcription is deactivated when a
terminator sequence is reached. [NB.
Terminator/stop codons stop translation].
Control elements selectively activate genes
at appropriate stages of cell division.