BIOCHEMISTRY Enzymes and Coenzymes PDF/PPT

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Enzymes and Coenzymes



Session 15


Session Plan
• General Characteristics of Enzymes
• Enzyme Structure
• Enzyme Nomenclature
• Enzyme Function
• Enzyme Specificity
• Factors Affecting Enzyme Activity
• Enzyme Inhibition
• Regulation of Enzyme Activity
• Medical Uses of Enzymes

NOTE: Vitamins are discussed in detail in the Nutrition Modules in your further studies.


General Characteristics of Enzymes

– Usually a protein, acting as catalyst in specific
biochemical reaction

• Each cell in the human body contains
1,000s of different enzymes

– Every reaction in the cell requires its own
specific enzyme

• Most enzymes are globular proteins
– A few enzymes are made of RNA

• Catalyze biochemical reactions involving Animation of enzyme at work
nucleic acids
• Enzymes undergo all the reactions of udent_view0/chapter2/animation__how_enzymes_wor

proteins k.html
– Enzymes denaturation due to pH or temperature

• A person suffering high fever runs the risk of ed%20Tutorials/at0302/at_0302_enzyme_catalysis.html

denaturing certain enzymes


Enzyme Structure

Composed only of protein

Composed of:

– Apoenzyme
• Conjugate enzyme without

its cofactor

• Protein part of a • The apoenzyme can’t catalyze its reaction
conjugated enzyme without its cofactor.

– The combination of the apoenzyme with the cofactor
makes the conjugated enzyme functional.

– Coenzyme (Cofactor)
• Holoenzyme = apoenzyme + cofactor

• Non-protein part of a
– The biochemically active conjugated enzyme.

conjugated enzyme


Coenzymes and cofactors

• Coenzymes provide additional chemically reactive functional groups
besides those present in the amino acids of the apoenzymes
– Are either small organic molecules or inorganic ions

• Metal ions often act as additional cofactors (Zn2+, Mg2+, Mn2+ & Fe2+)
– A metal ion cofactor can be bound directly to the enzyme or to a coenzyme

– A small organic molecule, acting as a cofactor in a conjugated enzyme

• Coenzymes are derived from vitamins or vitamin derivatives
– Many vitamins act as coenzymes, esp. B-vitamins


Enzyme definitions
Term Definition

Enzyme Protein only enzyme that facilitates a chemical reaction

Coenzyme Compound derived from a vitamin (e.g. NAD+) that assists an
enzyme in facilitating a chemical reaction

Cofactor Metal ion (e.g. Mg2+) that that assists an enzyme in facilitating a
chemical reaction

Apoenzyme Protein only part of an enzyme (e.g. isocitrate dehydrogenase)
that requires an additional coenzyme to facilitate a chemical
reaction (not functional alone)

Holoenzyme Combination of the apoenzyme and coenzyme which together
facilitating a chemical reaction (functional)


Enzyme Nomenclature • Suffix of an enzyme –ase
– Lactase, amylase, lipase or protease

• Denotes an enzyme

• Enzymes are named according
to the • Some digestive enzymes have the suffix –in

– Pepsin, trypsin & chymotrypsin
type of reaction they • These enzymes were the first ones to be studied

catalyze and/or their substrate
• Prefix denotes the type of reaction the

enzyme catalyzes

• Substrate = the reactant upon – Oxidase: redox reaction
– Hydrolase: Addition of water to break one

which the specific enzyme acts component into two parts

– Enzyme physically binds to the
substrate • Substrate identity is often used together

with the reaction type
– Pyruvate carboxylase, lactate dehydrogenase

Enzyme Substrate Enzyme/substrate complex


6En zMymae Cjloasrs ClaRsesacetiosn Coatafl yzEedn zymExaemsple s in Metabolism

Oxidoreductase Redox reaction (reduction & Examples are dehydrogenases
oxidation) catalyse reactions in which a

substrate is oxidised or reduced

Transferase Transfer of a functional group Transaminases which catalyze
from 1 molecule to another the transfer of amino group or

kinases which catalyze the 6 Major Classes
transfer of phosphate groups.

Hydrolase Hydrolysis reaction Lipases catalyze the hydrolysis of Enzymes
of lipids, and proteases catalyze Based on the type of

the hydrolysis of proteins
reaction they catalyze

Lyase Addition / removal of atoms to / Decarboxylases catalyze the
from double bond removal of carboxyl groups

Isomerase Isomerization reaction Isomerases may catalyze the
conversion of an aldose to a
ketose, and mutases transfer

functional group from one atom
to another within a substrate. The table explains

the functions of
Ligase Synthesis reaction Synthetases link two smaller

(Joining of 2 molecules into one, molecules are form a larger one. enzymes and how
forming a new chemical bond, they are classified
coupled with ATP hydrolysis) and named.


Enzyme Active Site
• Active site

– The specific portion of an enzyme (location)
where the substrate binds while it undergoes
a chemical reaction

• The active site is a 3-D ‘crevice-like’ cavity
formed by secondary & tertiary structures
of the protein part of the enzyme

– Crevice formed from the folding of the protein
• Aka binding cleft

– An enzyme can have more than only one
active site

– The amino acids R-groups (side chain) in the
active site are important for determining the
specificity of the substrate

Stoker 2014, Figure 21-2 p750


Enzyme – Substrate Complex

• When the substrate binds to the enzyme active site an
Enzyme-Substrate Complex is formed temporarily
– Allows the substrate to undergo its chemical reaction much faster

Timberlake 2014, Figure 4, p.738 Timberlake 2014, Figure 3, p.737


Lock & Key Model of Enzyme Action
• The active site is fixed, with a rigid shape (LOCK)

• The substrate (KEY) must fit exactly into the rigid enzyme (LOCK)

• Complementary shape & geometry between enzyme and substrate
– Key (substrate) fits into the lock (enzyme)

• Upon completion of the chemical reaction, the products are released from the active site, so the next
substrate molecule can bind

Stoker 2014, Figure 21-3 p750


Stoker 2014, Figure 21-4 p751

Induced Fit Model of Enzyme Action

• Many enzymes are flexible & constantly change their shape
– The shape of the active site changes to accept & accommodate the

• Conformation change in the enzyme’s active site to allow the substrate to


• Analogy: a glove (enzyme) changes shape when a hand (substrate) is
inserted into it


Enzyme Specificity
• Absolute Specificity

– An enzyme will catalyze a particular reaction for only one substrate
– Most restrictive of all specificities

• Not common

– Catalase has absolute specificity for hydrogen peroxide (H2O2)

– Urease catalyzes only the hydrolysis of urea

• Group Specificity
– The enzyme will act only on similar substrates that have a specific functional


• Carboxypeptidase cleaves amino acids one at a time from the carboxyl end of the
peptide chain

• Hexokinase adds a phosphate group to hexoses


Enzyme Specificity
• Linkage Specificity

– The enzyme will act on a particular type of chemical bond, irrespective of the
rest of the molecular structure

– The most general of the enzyme specificities

• Phosphatases hydrolyze phosphate–ester bonds in all types of phosphate esters

• Chymotrypsin catalyzes the hydrolysis of peptide bonds

• Stereochemical Specificity
– The enzyme can distinguish between stereoisomers
– Chirality is inherent in an active site (as amino acids are chiral compounds)

• L-Amino-acid oxidase catalyzes reactions of L-amino acids but not of D-amino


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Factors Affecting Enzyme Activity

Enzyme activity
• Measure of the rate at which an enzyme converts substrate to products

in a biochemical reaction

4 factors affect enzyme activity:
• Temperature

• pH

• Substrate concentration: [substrate]

• Enzyme concentration: [enzyme]


Stoker 2014, Figure 21-6 p753
Temperature (t)

• With increased t the EKIN increases
– More collisions
– Increased reaction rate

• Optimum temperature (tOPT) is the t,
at which the enzyme exhibits
maximum activity
– The tOPT for human enzymes = 370C

• When the t increases beyond tOPT
– Changes in the enzyme’s tertiary

structure occur, inactivating & denaturing
it (e.g. fever)

• Little activity is observed at low t


Stoker 2014, Figure 21-7 p753

• Optimum pH (pHOPT) is the pH, at which the
enzyme exhibits maximum activity

• Most enzymes are active over a very narrow
pH range

– Protein & amino acids are properly maintained

– Small changes in pH (low or high) can result in
enzyme denaturation & loss of function

• Each enzyme has its characteristic pHOPT,
which usually falls within physiological pH
range 7.0 – 7.5

• Digestive enzymes are exceptions:
– Pepsin (in stomach) – pHOPT = 2.0

– Trypsin (in SI) – pHOPT = 8.0


Stoker 2014, Figure 21-8 p754
Substrate Concentration

• If [enzyme] is kept constant & the [substrate] is

– The reaction rate increases until
a saturation point is met
• At saturation the reaction rate stays the same even if

the [substrate] is increased

– At saturation point substrate molecules are bound to
all available active sites of the enzyme molecules

• Reaction takes place at the active site
– If they are all active sites are occupied the reaction is

going at its maximum rate
• Each enzyme molecule is working at its maximum


– The incoming substrate molecules must “wait their turn”


Stoker 2014, Figure 21-9 p755

Enzyme Concentration
• If the [substrate] is kept constant & the

[enzyme] is increased
– The reaction rate increases

– The greater the [enzyme], the greater the
reaction rate

– The rate of an enzyme-catalyzed reaction is

always directly proportional to the amount
of the enzyme present

• In a living cell:
– The [substrate] is much higher than the

• Enzymes are not consumed in the reaction

• Enzymes can be reused many times


Stoker 2014, p756


Key concept: function of an enzyme

What is the function of an enzyme in a chemical reaction?

What happens to the enzymes when the body
temperature rises from 37ᵒC to 42ᵒC?

If an enzyme has broken down and is non-functional,
what would happen to the chemical reaction
normally facilitated by the enzyme? Explain.



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Enzyme Inhibition

– A substance that slows down or stops the normal catalytic function of

an enzyme by binding to the enzyme

• Three types of inhibition:
– Reversible competitive inhibition

– Reversible non-competitive inhibition

– Irreversible inhibition


Reversible Competitive Inhibition
• A competitive inhibitor resembles the

– Inhibitor competes with the substrate for binding to

the active site of the enzyme

– If an inhibitor is bound to the active site:
• Prevents the substrate molecules to access

the active site
– Decreasing / stopping enzyme activity

• The binding of the competitive inhibitor to the
active site is a reversible process

– Add much more substrate to outcompete the
competitive inhibitor

• Many drugs are competitive inhibitors:
– Anti-histamines inhibit histidine decarboxylase,

which converts histidine to histamine
Stoker 2014, Figure 21-11 p758


Stoker 2004, Figure 21.11, p.634

Reversible Noncompetitive Inhibition
• A non-competitive inhibitor decreases

enzyme activity by binding to a site on the
enzyme other than the active site

– The non-competitive inhibitor alters the tertiary
structure of the enzyme & the active site

• Decreasing enzyme activity

• Substrate cannot fit into active site

– Process can be reversed only by lowering the
[non-competitive inhibitor]

• Example:
– Heavy metals Pb2+ & Hg2+ bind to –SH of

Cysteine, away from active site
• Disrupt the secondary & tertiary structure

Stoker 2004, Figure 21.12, p.634


Stoker 2014, p759

Irreversible Inhibition
• An irreversible inhibitor inactivates an enzyme

by binding to its active site by a strong covalent bond

– Permanently deactivates the enzyme

– Irreversible inhibitors do not resemble substrates

• Addition of excess substrate
doesn’t reverse this process
– Cannot be reversed

• Chemical warfare (nerve gases)

• Organophosphate insecticides


Stoker 2014, p760


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Allosteric Enzymes
• Allosteric enzymes have a quaternary

– Are composed of 2 or more protein chains
– Possess 2 or more binding sites • Binding of a regulator molecule

to its regulatory site causes

• 2 types of binding sites: changes in 3-D structure of the
enzyme & the active site

– One binding site for the substrate
– Binding of a Positive regulator

• Active site up-regulates enzyme activity
• Enhances active site, more able

– Second binding site for a regulator molecule to accept substrate

• Regulatory site
– Binding of a Negative regulator

(non-competitive inhibitor)
• Active & regulatory binding sites are down-regulates enzyme activity

distinct from each other in shape & location • Compromises active site, less
able to accept substrate


The different effects

Positive & Negative

on an
Allosteric enzyme

Stoker 2014, Figure 21-13 p762


Feedback Control Example:
The degradation of glucose

• A process in which activation or inhibition of one of the earlier reaction through a metabolic pathway
steps in a reaction sequence is controlled by a product of this reaction can be regulated in several
sequence. ways

The enzyme PFK is
allosterically inhibited by

– One of the mechanisms in which allosteric enzymes are regulated
the product ATP

– Most biochemical processes proceed in several steps & each step is Glycolysis (makes ATP) is
catalyzed by a different enzyme slowed when cellular ATP

• The product of each step is the substrate for the next step / enzyme. is in excess
Observe animation of feedback control

Reaction 1: converts reagent A Reaction 2: converts reagent B Reaction 3: converts reagent C
into product B into product C into product D


Proteolytic Enzymes • Most digestive & blood-clotting enzymes
are proteolytic

– Blood clotting enzymes break down
& Zymogens proteins within the blood so that they can

form the clot

• 2nd mechanism of allosteric enzyme regulation • Platelets interspersed with tangled
protein (collagen and thrombin)

– Production of an enzyme in an inactive form

• Activation of a zymogen requires the
– Activated when required (right time & place)

removal of a peptide fragment from the
• Activated aka “turned on”

zymogen structure
– Changing the 3-D shape & affecting the

• Proteolytic enzymes catalyze breaking of active site
peptide bond in proteins • E.g. Pepsiongen (zymogen)

– To prevent these enzymes from destroying the >>> Pepsin (active proteolytic enzyme)
tissues, that produced them, they are released in
an inactive form = ZYMOGENS


Activation of a Zymogen

Stoker 2014, Figure 21-14 p763


Covalent Modification of Enzymes

• Covalent modifications are the 3rd mechanism of enzyme activity regulation
– A process of altering enzyme activity by covalently modifying the structure of the enzyme

• Adding / removing a group to / from the enzyme

• Most common covalent modification = addition & removal of phosphate group:
– Phosphate group is often derived from an ATP molecule

• Addition of phosphate = phosphorylation is catalyzed by a Kinase enzyme

• Removal of phosphate = dephosphorylation is catalyzed by a Phosphatase enzyme

– Glycogen synthase: involved in synthesis of glycogen
• Deactivated by phosphorylation

– Glycogen phosphorylase: involved in breakdown of glycogen
• Activated by phosphorylation.


Vitamins as Coenzymes Stoker 2014, Figure 21-20 p779

• Many enzymes require B vitamins as coenzymes
– Allow the enzyme to function

• Coenzymes serve as temporary carriers of atoms or functional groups
– Coenzymes provide chemical reactivity that the apoenzyme lacks

– Important in metabolism reactions to release energy from foods
• E.g. redox reactions where they facilitate oxidation or reduction

• B vitamins don’t remain permanently bonded to the apoenzyme
– After the catalytic action the vitamin is released & can be repeatedly used by various enzymes

– This recycling reduces the need for large amounts of B vitamins


Key concept: sites with enzymes, coenzymes

Why is an enzymes active site important to the
function of the enzyme?

Why is the enzymes regulatory binding site important
for controlling the activity of the enzyme?

Why are coenzymes (derived from vitamins)
important to the function of some enzymes?



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Drugs Inhibiting Enzyme Activity
• Many prescription drugs inhibit enzymes

• ACE Inhibitors
– Inhibit Angiotensin-Converting Enzyme

• Lowers blood pressure

• Sulfa drugs
– Antibiotics acting as competitive inhibitors of bacterial enzymes

• Involved in conversion of PABA to Folic acid
– Deficiency of folic acid retards bacterial growth, eventually killing them

• Penicillin’s
– β-lactam antibiotics inhibit transpeptidase

• Transpeptidase enzyme strengthens the cell wall
– Forms peptide cross links between polysaccharides strands in bacterial cell walls

– Without transpeptidase enzyme (inhibited by Penicillin) >>> weakened cell wall, bacteria dies


Medical Uses of Enzymes
• Enzymes can be used in diagnosis & treatment of certain diseases

• Lactate dehydrogenase (LDH) is normally not found in high levels in
blood, as it is produced in cells

– Increased levels of LDH in the blood indicate
myocardial infarction (MI) (Heart attack)

– Tissue plasminogen activator (TPA) activates the enzyme plasminogen that
dissolves blood clots

• Used in the treatment of MI

• There is no direct test to measure urea in the blood
– Urease converts urea into ammonia, which is easily measured

& is used as urea indicator
• Blood Urea Nitrogen (BUN) is used to measure kidney function

– High urea levels in the blood indicate kidney malfunction


• Isoenzyme catalyze the same reaction

in different tissues in the body
– e.g. lactate dehydrogenase (LDH) consists of 5 isoenzymes

• Each isoenzyme of LDH has the same function
– Converts lactate to pyruvate

• LDH1 isoenzyme is more prevalent in heart muscle

• LDH5 form is found in skeletal muscle & liver

• Isoenzymes can be used to identify the damaged or diseased organ or tissue
• It is a marker for a particular location

• If LDH1 isoenzyme was found in the blood >>> indicates heat muscle damage


Stoker 2014, Table 21-3 p768


Stoker 2014, Table 21-7 p780


Readings & Resources
• Stoker, HS 2014, General, Organic and Biological Chemistry, 7th edn,

Brooks/Cole, Cengage Learning, Belmont, CA.
• Stoker, HS 2004, General, Organic and Biological Chemistry, 3rd edn,

Houghton Mifflin, Boston, MA.
• Timberlake, KC 2014, General, organic, and biological chemistry:

structures of life, 4th edn, Pearson, Boston, MA.
• Alberts, B, Johnson, A, Lewis, J, Raff, M, Roberts, K & Walter P 2008,

Molecular biology of the cell, 5th edn, Garland Science, New York.
• Berg, JM, Tymoczko, JL & Stryer, L 2012, Biochemistry, 7th edn, W.H.

Freeman, New York.
• Dominiczak, MH 2007, Flesh and bones of metabolism, Elsevier Mosby,

• Tortora, GJ & Derrickson, B 2014, Principles of Anatomy and Physiology,

14th edn, John Wiley & Sons, Hoboken, NJ.
• Tortora, GJ & Grabowski, SR 2003, Principles of Anatomy and Physiology,

10th edn, John Wiley & Sons, New York, NY.


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