Understanding Enzyme Catalyzed Bi-Substrate Reactions
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Introduction
Enzymes play a critical role in facilitating biochemical reactions in living organisms. They are proteins that act as catalysts, speeding up reactions by providing an alternative pathway with a lower activation energy. Within our body, numerous enzyme-catalyzed reactions take place, with a notable type referred to as bi-substrate reactions, where an enzyme processes two different substrate molecules and converts them into products. This article delves into two main categories of bi-substrate reactions: sequential reactions and double displacement reactions, explaining their mechanisms and differences.
What are Bi-Substrate Reactions?
Bi-substrate reactions involve an enzyme that catalyzes the conversion of two substrate molecules – let’s call them A and B – into two products, C and D. The general representation of such a reaction can be written as:
A + B ⇌ C + D
In this context, understanding how the substrates bind and how products are released is vital since these processes directly influence the reaction's pathway and efficiency.
Types of Bi-Substrate Reactions
Bi-substrate reactions can be broadly categorized into two types:
- Sequential Reactions
- Double Displacement Reactions
Each type of reaction has unique characteristics and mechanisms that dictate how substrates interact with the enzyme and how products are formed and released.
Sequential Reactions
Sequential reactions are defined by the requirement that both substrates must bind to the enzyme’s active site simultaneously, leading to the formation of a ternary complex – an intermediate complex made up of the enzyme and both substrates (E: A + B).
Ordered Sequential Reactions
In ordered sequential reactions, the binding order of the substrates is crucial. The substrates must attach to the active site in a specific sequence, and the release of products also follows this order.
- Example: The reaction catalyzed by lactate dehydrogenase during glycolysis is a classic example:
- NADH binds first to the enzyme.
- Pyruvate then binds, forming the ternary structure.
- The products, lactate and NAD+, are released in a defined order: lactate is released first, followed by NAD+.
Random Sequential Reactions
In contrast, random sequential reactions allow for substrates to bind in any order, meaning the sequence of binding does not impact the overall reaction outcome. Once both substrates bind, they still form a ternary complex.
- Example: A reaction catalyzed by creatine kinase:
- Either ATP or creatine can bind first to the enzyme.
- Upon forming the ternary complex, the reaction proceeds to create phosphocreatine and ADP.
- The products can be released in any sequence.
Key Distinctions in Sequential Reactions
- In ordered sequential reactions, the arrangements of substrate binding and product release are crucial to the reaction pathway.
- Random sequential reactions have more flexible substrate interactions, offering different possible sequences without affecting the reaction.
Double Displacement Reactions
Double displacement reactions, also referred to as ping-pong reactions, differ fundamentally in how substrates interact with the enzyme. In this case, only one substrate binds at a time, leading to sequential reactions of substrates and products.
Mechanism of Double Displacement Reactions
- Substrate A binds to the enzyme and undergoes a transformation, resulting in product C and a modified enzyme.
- Product C is released, and now the enzyme is primed to accept substrate B.
- Substrate B binds to the modified enzyme, transferring the group that was attached from substrate A, resulting in product D.
- The enzyme is released in its original form, despite being modified during the reaction process.
Example of a Double Displacement Reaction
A well-known example includes the reaction where aspartate and 2-oxoglutarate react, catalyzed by the enzyme aspartate transaminase.
- The reaction involves a transfer of the amino group and transformation of substrates one at a time, emphasizing the ping-pong nature of substrate interaction.
Critical Differences between Reaction Types
Understanding the differences in reaction types can aid in the study of biochemical pathways and enzymatic functions:
- Binding Requirement: Sequential reactions require both substrates to bind simultaneously, whereas in double displacement reactions, one substrate binds at a time.
- Mechanism: The process of forming products differs; sequential reactions usually form an intermediary complex before proceeding, while double displacement treats substrates individually, leading to changes in the enzyme's state.
Conclusion
Both sequential and double displacement reactions play essential roles in the enzymatic reactions within our cells. Understanding these differences aids in grasping how enzymes operate at a biochemical level, highlighting their specificity and the intricate balance of substrate interactions leading to product formation. By analyzing these reactions, we unveil the complexities of cellular metabolism and energy production, which are fundamental to sustaining life.
inside our body and inside our cells there are many different types of enzyme catalyzed reactions that can be
generalized by the following chemical equation so we have an enzyme that basically takes two different substrate
molecules and B and transforms them into these products C and D and these types of enzyme catalyzed reactions are known
as multiple substrate or bi-substrate reactions and we can categorize these bi-substrate reactions into two types we
have sequential reactions and we have double displacement reactions and in both of these reactions the enzymes
active side can basically accommodate both of these different types of substrate molecules a and B now let's
begin by focusing on sequential reaction so what do we mean by sequential reaction well in a sequential reaction
the defining property of a sequential reaction is basically the presence of a ternary structure and a ternary
structure is a structure in which the active site of the enzyme is filled with the two substrate molecules a and B so
the defining property of sequential reactions is that all the substrates the two substrates and B have to bind to the
active side of the enzyme to form the ternary structure that consists of the enzyme and the two substrates before the
catalysis process can actually take place and we can convert those reactants into the products now we can further
subdivide sequential reactions into two types we have ordered sequential reactions and we have random sequential
reactions and let's begin by focusing on the ordered sequential reaction so in this order sequential reaction the order
at which the two substrate molecules the two reactants bind onto the active site as well as the order at which the two
products are released by the active it actually matters and one common example that describes an order
sequential reaction is a reaction that takes place in the process of glycolysis so remember in glycolysis the ultimate
goal is to basically produce pyruvate molecules and NADH molecules now in the process of anaerobic respiration we
basically take the pyruvate molecules and we reduce the pyruvate molecules into lactate by using NADH the NADH acts
as a coenzyme and it basically is transformed into nad plus and the enzyme that catalyzes this reaction is known as
lactate dehydrogenase so how exactly is this reaction and ordered sequential reaction so to see what we mean by that
let's take a look at the following five diagrams so in diagram a we basically have the active side this Kravis is the
active side of the lactate dehydrogenase and what must happen is this coenzyme the NADH will bind into the active side
of that enzyme first before the pyruvate actually binds only then only when we buy the NADH into the active side can
the pyruvate actually move into the active side and bind into the active side and once the pyruvate moves in we
form this ternary structure in which we have the pyruvate and the NADH inside the active side of that enzyme once
again ternary simply means we have three molecules the enzyme and the two substrate molecules now once we form
this complex only now can we actually transform the pyruvate into lactate and the NADH into the nad plus and once we
form the two products so lactate in this case is product c and nad plus is product d once we form these two
products only then can they be released from the active of that ends on and because this is an
ordered sequential reaction the order at which these two products are released also matters and in this particular
reaction we're always going to find that it's the lactate that leaves first followed by that nad plus molecule so in
an ordered sequential reaction to actually catalyze the transformation of the reactants into the products we have
to form the ternary structure this complex in which the active site contains those two substrates and we
also have to basically bind these two reactants into the active site in a specific order and once we form the
products those su products must be released also at a specific order and this is in contrast to random sequential
reaction so now let's focus on random sequential reactions unlike in ordered sequential reactions
in random sequential reactions the order at which the two reactants bind into the active site and the order at which the
two products are released from the active site does not actually matter it takes place completely at random now
to demonstrate a random sequential reaction let's discuss a specific reaction that is catalyzed by an enzyme
known as a creatine kinase so creatine kinase is this enzyme that essentially transforms ATP molecules and creatine
a high-energy molecule that is used by the cell by the muscle cells of our body so to demonstrate how this reaction
takes place let's take a look at the following four diagram so this green structure is the active site of the
creatine kinase and what happens is we have these two substrate molecules the ATP is our a and the creatine is
our be and notice in this particular reaction because we're dealing with a random sequential reaction the order at
which these two reactants the two substrates actually bind onto the active side does not matter we can either have
the ATP binds followed by the creatine or we can have the creatine bind first followed by the
ATP but because this is a sequential reaction just like in the ordered sequential reaction case we have to
actually form that ternary structure that structure that contains the enzyme bound to the two substrate molecules in
this case ATP and creatine only when we form this three molecule complex can we begin the process of catalyzation and
only now can we transform the ATP into the ADP and the creatine into the phosphor creatine so basically what the
enzyme does is it catalyzes the transferring of the phosphate group from the ATP onto that creatine molecule to
create the high-energy phosphor creatine that can be used by the muscle cells as a high energy source now once we form
these su products now these two products can be released from the active side but because we're dealing with a random
sequential reaction unlike in this case in this case it doesn't matter which one of these
products is released first we can either have the ADP come out first followed by the phosphor creatine or we can have the
phosphor creatine release first followed by adp so either ATP or creatine can enter the active site first once both of
those reactants the substrates into the active site only then can we actually transform the two reactants into the two
products and once we form the two products the order at which the two products actually depart does not matter
so this is the difference between ordered sequential reactions and random sequential reactions and the
underlining fact about these two reactions is that the two substrate molecules have to bond to the active
side before that reaction actually takes place so we have to form this three molecule structure known as the ternary
structure now let's move on to the double displacement reaction also sometimes known as a ping-pong reaction
so what exactly is the major difference between the sequential reaction and the double displacement reaction well just
like the sequential reaction this is the equation that generalizes the double displacement reaction but unlike in the
sequential case in which these two molecules a and B have to by inside the active side together for that reaction
to take place in this particular case in a double displacement reaction what happens is first one of these molecules
let's say a binds onto the active side of that enzyme and a group on a is transferred into that enzymes active
side and that modifies that enzyme and it also transforms our reactant a into let's say product C and then product C
is released and once product C is released we now have that modified enzyme and only now can the second sub
should be actually bind onto the active side of that modified enzyme to form an intermediate between the enzyme and that
reacted B and now that group that was transferred into the active side of the enzyme by a is transferred on to this
reactant B and that transforms that reactant B into product D and that basically ultimately releases that
product D from the active site of the enzyme and now the enzyme the original and the original enzyme in its original
form is released by that reaction so the major difference between the sequential reaction and the
double displacement reaction is for the reaction to actually take place in a sequential reaction both have to be
bound onto the active side but in this case only one has to be bound for that reaction to actually take place and in
fact the two substrates bind and are catalyzed at different times inside the active side of the enzyme that follow
these double displacement reactions so to see what we mean let's take a look at the following general case so in these
reactions the first substrate binds and changes the enzyme to produce an enzyme substrate intermediate complex and this
can be seen in the following diagram so let's suppose we have substrate a that binds unto the active site of the enzyme
to produce this intermediate what happens next is some type of group for example a phosphoryl groups
the molecule a can be transferred onto that onto the active side of that enzyme and that modifies that enzyme and that's
why we have this asterisk the star symbol on top of this intermediate so in this intermediate once we transfer the
group onto the enzyme we transform a into product C and now the product C can be released from that complex and so now
we have this modified complex e with the star on top now next the second substrate B goes and binds into the
active side of that enzyme to form this intermediate and now that group that was transferred by a into the active site of
e is transferred onto that molecule B and that transforms the second substrate B into the product D and finally this
intermediate in this intermediate D is released from that enzyme and that we forms our original enzyme so notice in
this reaction like in any enzyme catalyzed reaction even though the enzyme might be modified intermediately
at the end that enzyme is produced and exists in its original unchanged form so the defining property of this reaction
is that the first substrate is converted to the product and then released before the second substrate binds to the active
site of the enzyme in addition we see that the enzyme is actually modified in its intermediate stages but even though
it's modified at the end it is released unmodified unchanged so these are the two types of reactions sequential
reactions and double displacement reactions that commonly take place inside our cells and which can be
generalized by using the following equation in which we have these two reactants two substrate molecules and