Transcript Chemical Reactions

Chemical Reactions



All processes of life depend on the ordered
flow of energy
Metabolism – totality of an organism’s
chemical processes
Metabolic reactions are organized into
pathways that are orderly series of
ezymatically controlled reactions - two types:
1.
2.
Catabolic pathways – rxns that release energy
by breaking down complex molecules (ex.
Cellular respiration breaks down glucose into
CO2 and water)
Anabolic pathways – rxns that consume energy
to build complex molecules (ex. Photosynthesis
makes glucose, making macromolecules from
monomers)




Exergonic reactions – release energy (catabolic or
downhill)
Endergonic reactions – require input of energy (anabolic
or uphill)
Exergonic and endergonic rxns are often coupled – an
exergonic rxn provides the energy to drive an endergonic
rxn
Energy carrier molecules such as ATP couple the rxns


Chemical rxns require an initial input of energy
(activation energy) to get started – usually
supplied by kinetic energy (molecules collide
and react)
Most rxns occur more easily at higher temps
Enzymes



Spontaneous rxns proceed too slowly to sustain life at
body temp
Enzymes act as biological catalysts to speed up
chemical rxns to solve this problem
Catalyst – a molecule that speeds up a rxn by lowering
activation energy (catalyst is not used up in the rxn and
remains unchanged)
Enzymes are substrate specific


Enzymes are very specific and catalyze only a specific
type of rxn (remember, proteins have a very specific
shape allowing them to function)
Lock and Key Model – specific shape of the enzyme
allows them to bind to specific molecules and catalyze
specific rxns
Induced Fit Model
Induced Fit Model – more current model
Each enzyme has a dimple or groove called the
active site into which reactant molecules called
substrates fit
When substrate enters active
site, both substrate and active
site change shape – induced
fit (amino acids temporarily
bond with substrate or
electrical charges distort the
chemical bonds in substrate –
this promotes the reaction to
occur)
New product is expelled and
enzyme can go on and accept
a new set of substrates
Enzyme activity is affected by a cell’s physical and
chemical environment
Temperature – as temperature increases, enzyme
activity increases to a point – extreme temps disrupt
hydrogen bonds and ionic bonds holding enzyme’s
shape and cause it to denature – most human
enzymes have an optimal temp of 35o to 40o C
pH – optimal pH range for most enzymes is pH 6 – 8
1.
2.
Extremes in pH will denature enzymes
Cofactors – many enzymes require nonprotein
helpers – some bind to the active site
permanently, others bind reversibly along with
substrate (inorganic examples: zinc, iron,
copper)
3.
•
Organic cofactors are called coenzymes – most
vitamins are coenzymes
4.
Enzyme Inhibitors –
certain chemicals inhibit
the action of enzymes
a.
b.
Competitive inhibitors –
inhibitors that resemble the
shape of the normal
substrate and compete for
the active site and block it
Noncompetitive inhibitors –
inhibitors that do not
directly compete with
substrate at active site –
bind to another part of
enzyme causing it to
change shape so active site
cannot bind substrate
Metabolism can be controlled by regulating the
activity of enzymes

Many enzymes are controlled through allosteric regulation – a
regulatory molecule binds to an allosteric site on an enzyme
(specific receptor site different from active site)
1.
Allosteric enzymes have an active form (shape) and an
inactive form
2.
Allosteric activators/inhibitors bind to allosteric site and change
the shape of the enzyme to activate or inhibit the enzyme

Feedback inhibition – switching off of a
metabolic pathway by it’s end-product
which acts as an inhibitor of an enzyme
within the pathway (often by allosteric
inhibition)
Industrial Uses of Enzymes





Enzymes have long been used for industrial purposes
Leather tanning – proteases are used to soften hids and
remove hair
Brewing – enzymes in barley grains at germination are
used to convert stored starch to sugars that can be
fermented by yeast
Biotechnology – enzymes are frequently used in genetic
engineering
Lactose-free milk – some people are “lactose-intolerant”
(do not produce lactase in pancreatic juices) – cannot
digest milk and milk products


Lactase is obtained from bacteria
Milk and milk products are treated with lactase before
consumption to remove lactose