- WELCOME TO PART 8 ON PROTEINS AND ENZYMES.
HERE WE'RE GOING TO LOOK AT AN INTRODUCTION TO ENZYMES.
SO ENZYMES ARE VERY IMPORTANT BECAUSE THEY INCREASE THE RATE
OF CHEMICAL REACTIONS WITHOUT BEING CHANGED THEMSELVES
AND MOST BIOLOGICAL CATALYSTS ARE PROTEIN ENZYMES.
AND ENZYMES ARE IMPORTANT BECAUSE WITHIN OUR BODY
WE DON'T HAVE THE LUXURY OF RAISING THE TEMPERATURE
REALLY HIGH OR USING STRONG ACIDS OR BASES
BECAUSE IT WOULD BE BAD FOR OUR HEALTH.
SO WE NEED ENZYMES TO CATALYZE THEIR REACTIONS OCCURRING
IN OUR BODY.
REMEMBER THAT CATALYSTS, RIGHT, SO ENZYMES ARE CATALYSTS,
HOW DO THEY WORK?
THEY WORK BY LOWERING THE ACTIVATION ENERGY
AND THEY DO THAT BY BINDING ONE ORE MORE SUBSTRATES
INTO AN ACTIVE SITE.
SO SUBSTRATES...
SO THE REACTANTS OF AN ENZYME-CATALYZED REACTION
ARE CALLED THE SUBSTRATE.
SO BASICALLY THE REACTANTS ARE PULLED AND STRETCHED
IN THE ACTIVE SITE OF AN ENZYME USING THE INTERACTIONS
WE'VE LEARNED ABOUT IN SECONDARY AND TERTIARY STRUCTURE.
HYDROPHOBIC AND HYDROPHILIC,
AND WHAT THIS DOES IS IT LOWERS THE ACTIVATION ENERGY
TO INCREASE THEIR REACTION RATE.
SO IF WE LOOK AT THIS SYMBOLICALLY,
HERE IS OUR SUBSTRATE AND THEN WE COULD LOOK AT--
THIS WOULD BE THE ACTIVE SITE OF THE ENZYME HERE.
AND SO HERE THEY ARE SEPARATE, RIGHT?
SO WE HAVE OUR ENZYME AND WE HAVE OUR SUBSTRATE.
AND SO THEN WHEN THEY COME TOGETHER,
THEY FORM THE ENZYME SUBSTRATE COMPLEX,
SO NOW AT THIS POINT IT'S HELD TOGETHER
BY THOSE NON-COVALENT INTERACTIONS, RIGHT?
HELD TOGETHER VIA NON-COVALENT INTERACTIONS.
AND THEN WHEN WE'RE ALL DONE, THE ENZYME IS UNCHANGED,
IT LOOKS LIKE IT DID BEFORE THE JOURNEY STARTED
BUT THE SUBSTRATE LOOKS VERY DIFFERENT NOW,
IT HAS FORMED ONE OR TWO MORE PRODUCTS, NEW SUBSTANCES.
SO WHEN WE THINK OF THE WORD SUBSTRATE,
JUST THINK OF OUR REACTANT AND THEN AS OUR SUBSTRATE
FORMS THE ENZYMES SUBSTRATE COMPLEX,
AND THEN WE END UP WITH THE ENZYME BACK LIKE BEFORE
BUT NOW WE HAVE PRODUCTS.
SO, IT'S ALL ABOUT WHAT WE'VE LEARNED
AND LEARNING ABOUT THE STRUCTURE OF PROTEINS,
THESE NON-COVALENT INTERACTIONS,
SO WE REALLY WANT TO HAVE AWARENESS FOR THE R GROUPS.
SO THE SIDE CHAINS SETS ANOTHER WAY TO SAY THE R GROUP.
SO WHEN WE'RE LOOKING AT AMINO ACIDS NOW,
WE EITHER WANT TO PHYSICALLY OR MENTALLY CIRCLE R GROUPS
BECAUSE THAT'S WHAT CREATES THE TERTIARY STRUCTURE.
AND SO TAKE YOUR KNOWLEDGE OF TERTIARY STRUCTURE
AND APPLY IT TO THE ENZYME-SUBSTRATE COMPLEX,
THE SAME THINKING WORKS FOR BOTH.
SO THEN WHEN WE--WHEN WE'RE LOOKING AT THESE R GROUPS,
WE JUST WANT TO SPONTANEOUSLY BE CLASSIFYING THEM,
SO ANYTIME WE SEE, ALL CARBONS AND HYDROGENS,
WE KNOW IT'S HYDROPHOBIC.
SO FOR THIS FIRST TWO SIDE CHAINS,
WE SEE THEM AS HYDROPHOBIC.
HERE WE SEE 'OH'
SO THAT'S POLAR AND NEUTRAL.
AND THEN IF WE LOOK AT THIS ONE HERE,
WE SEE IT WOULD BE BASIC.
IF WE THINK ABOUT THE FIRST AMINO ACID
AS BEING PART OF THE SUBSTRATE,
WHICH OF THE OTHER THREE AMINO ACIDS,
A, B, OR C WOULD WE EXPECT TO SEE
IN THE ACTIVE SITE OF THE ENZYME?
SO WE KNOW LIKE INTERACTS WITH LIKE,
LIKE DISSOLVES LIKE, RIGHT?
SO WE WOULD EXPECT AMINO ACID 'A'
BECAUSE IT CAN SHARE HYDROPHOBIC INTERACTIONS WITH THE SUBSTRATE.
SO NOW LET'S LOOK A LITTLE MORE CLOSELY AT THE ENZYME
SUBSTRATE COMPLEX.
SO THERE ARE TWO MODELS TO DESCRIBE THIS INTERACTION,
THE OLDER MODEL IS CALLED THE LOCK AND KEY.
AND THIS WAS THE FIRST ONE PROPOSED,
AND IT'S A VERY RIGID MODEL THAT WE THINK ABOUT
THAT THE ENZYME AND THE SUBSTRATE
HAVE TO HAVE A PERFECT FIT, JUST LIKE A LOCK WITH A KEY.
IT'S A VERY RIGID IDEA.
BUT WE HAVE LEARNED, IN OUR STUDY OF CONFIRMATIONS
AND THE MOVEMENT THAT WE KNOW THAT THESE MOLECULES
ARE FLEXIBLE.
AND SO A NEWER MODEL FOR DESCRIBING
THE ENZYME-SUBSTRATE COMPLEX IS CALLED THE INDUCED FIT,
AND IT HAS TO DO WITH THE IDEA AND NOTICE THAT PHYSICALLY
THE SHAPES AREN'T PERFECT MATCHES
BECAUSE WE KNOW THAT THE ENZYME IS A VERY LARGE MOLECULE
AND SO THE ENZYME IS FLEXIBLE.
AND CAN CHANGE ITS SHAPE TO ACCOMMODATE THE SUBSTRATE,
SO JUST TO MAKE SURE THAT EVERYBODY'S CLEAR,
THIS IS OUR SUBSTRATE REPRESENTED HERE.
AND THEN THIS IS OUR ENZYME, THE LARGE PART.
OKAY. SO NOW LET'S LOOK AT THIS.
SO THIS PART HERE IS REPRESENTING THE ACTIVE SITE
OF THE ENZYME.
SO LET'S GO AHEAD AND WE'LL JUST CIRCLE ALL THAT.
OKAY. SO HERE'S OUR ENZYME REPRESENTED IN BLUE,
SO WE'RE LOOKING AT THE ACTIVE SITE OF AN ENZYME.
AND NOW WE'RE GOING TO LOOK AT IT OVER TIME,
AND THEN THIS PART HERE, RIGHT, SO HERE IS OUR SUBSTRATE.
SO THE SUBSTRATE COMES IN,
AND WE NOTICE HERE'S A HYDROPHOBIC POCKET,
SO THIS HYDROPHOBIC INTERACTION, RIGHT HERE, HELPS TO HOLD
THE SUBSTRATE IN THE ACTIVE SITE.
SO IT HELPS FORM THE ENZYME-SUBSTRATE COMPLEX.
SO THROUGH THAT LONDON FORCES, THIS NONPOLAR GROUP HELPS
TO INTERACT WITH HYDROPHOBIC POCKET,
AND THEN WE CAN SEE THROUGH H BONDING.
SO HERE WE HAVE AN OXYGEN AND NITROGEN
AND SO THEN NOTICE WHAT HAPPENS RIGHT HERE THAT WE START
TO HAVE A REACTION OCCURRING AND HERE'S OUR CLEAVAGE SITE,
AND SO WHILE THIS PORTION OF THE SUBSTRATE
IS BEING HELD THROUGH THE NONPOLAR INTERACTIONS,
WE HAVE A BOND FORMING BETWEEN THE ENZYME AND THE SUBSTRATE
AND THEN NOTICE WE GET CLEAVAGE
AND SO NOW WE HAVE A FREE PEPTIDE
WHICH WOULD BE OUR PRODUCT.
AND SO THAT'S AN EXAMPLE OF HOW WELL ONE PART
OF THE ENZYME IS HOLDING ON TO THE SUBSTRATE,
ANOTHER PART OF THE ENZYME CAN ACTUALLY PERFORM THE REACTION.
AND SO NOW THAT WE'VE UNDERSTOOD IT AT A MOLECULAR LEVEL,
LET'S LINK THIS TO OUR UNDERSTANDING
OF REACTION PATHWAYS AND REACTION ENERGETICS.
SO LET'S COMPLETE THE VISUALIZATION
OF THE DIFFERENT ASPECTS.
SO, WE LOOK HERE AT UNCATALYZED AND A CATALYZED REACTION,
SO LET'S HAVE A LITTLE BIT OF REVIEW
ABOUT OUR REACTION ENERGIES,
LOOKING AT THESE TWO REACTIONS ARE THEY ENDOTHERMIC
OR EXOTHERMIC, RIGHT?
THEY ARE EXOTHERMIC.
HOW DID WE FIGURE THAT OUT?
THIS IS ALWAYS ABOUT THE ENERGY DIFFERENCE
BETWEEN THE REACTANTS AND PRODUCTS AND WE SEE THAT
THE PRODUCTS ARE LOWER IN ENERGY THAN THE REACTANTS.
SO WE WOULD SAY THAT'S NEGATIVE DELTA H
AND HEAT WILL BE RELEASED.
NOW WHICH REACTION IS CATALYZED?
SO WE HAVE THE REACTION ON THE LEFT
AND WE HAVE THE REACTION ON THE RIGHT.
TO LOOK FOR THE CATALYST, WHAT DO WE DO?
WE LOOK AT THE ACTIVATION ENERGIES.
SO WE SEE WITH THIS FIRST REACTION,
WE HAVE A VERY LARGE ACTIVATION ENERGY
AND WITH THE SECOND REACTION, IT IS MUCH SMALLER.
SO THE REACTION ON THE RIGHT IS THE ONE THAT HAS
THE CATALYST PRESENT
AND WE RECOGNIZE THAT BECAUSE WE SEE A SMALLER
ACTIVATION ENERGY WHICH CORRESPONDS
TO A FASTER REACTION RATE.
SO WE'VE ALREADY TAKEN CARE OF THIS,
WE'VE ALREADY DRAWN IN THE ACTIVATION ENERGY.
NOW LET'S SEE, WHAT ELSE DO WE NOTICE ABOUT THESE TWO GRAPHS?
DOES THE ENZYME CHANGE THE ENERGIES OF THE REACTANTS
AND PRODUCTS?
WE LOOK BACK HERE, WHAT DO WE NOTICE?
NO, RIGHT?
NO. THE ENZYME DOES NOT CHANGE THE ENERGIES
OF THE REACTANTS AND PRODUCTS, RIGHT?
SO NO, AND SO WHAT DOES THAT MEAN?
THERE'S NO CHANGE IN DELTA H, RIGHT?
THE DELTA H, THE VALUE IS THE SAME FOR BOTH REACTIONS,
REMEMBERING THAT DELTA H, THE ENTHALPY IS DETERMINED
FROM THE ENERGY DIFFERENCE BETWEEN THE REACTANTS
AND PRODUCTS.
SO DOES AN ENZYME CHANGE THE EQUILIBRIUM AMOUNTS
OF REACTANTS AND PRODUCTS?
SO WE WOULD SAY NO, RIGHT?
BECAUSE THE EQUILIBRIUM IS DETERMINED...
DETERMINED BY DELTA H,
SO EQUILIBRIUM IS ALL ABOUT THERMODYNAMICS.
SO CATALYSTS ARE ALL ABOUT...
SO WHAT DOES AN ENZYME DO?
SO AN ENZYME IS A CATALYST AND WHAT THEY DO
IS THEY LOWER THE ACTIVATION ENERGY TO INCREASE
THE REACTION RATE.
SO THE CATALYST DOESN'T CHANGE THE DESTINATION, RIGHT?
SO THERMODYNAMICS, WE CAN THINK OF AS THE DESTINATION.
THE ENZYME IS THE CATALYST, IT'S THE RATE,
SO REMEMBER THAT THE TERM WE USED THERE IS KINETICS.
SO ENZYMES TELL US HOW FAST WE GET TO OUR DESTINATION,
AND IT'S THE THERMODYNAMICS THAT TELLS US ABOUT THE DESTINATION
WHERE WE'RE GOING BECAUSE WE TYPICALLY, ENERGETICALLY
WE ALSO WANT TO GO TO THE LOWEST ENERGY.
SO THAT WRAPS UP OUR INTRODUCTION TO ENZYMES,
LOOKING AT IT AT BOTH A CHEMICAL LEVEL
AND THE INTERACTIONS BETWEEN THE ENZYME AND THE SUBSTRATE.
AND THEN TAKING THAT INTERACTION AND SHOWING HOW WE REPRESENT
IT SYMBOLICALLY ON REACTION ENERGY DIAGRAMS.
AND THEN JUST KEEPING IN MIND THE DIFFERENCE BETWEEN
THERMODYNAMICS AND KINETICS.
SO WHEN WE'RE TALKING ABOUT ENZYMES,
IT'S ALL ABOUT KINETICS AND SPEEDING UP REACTIONS.
PLEASE TAKE SOME TIME NOW TO WORK A FEW HOMEWORK PROBLEMS,
TO REINFORCE YOUR UNDERSTANDING.