- WELCOME TO PROTEINS AND ENZYMES PART 7,
QUATERNARY STRUCTURE.
SO QUATERNARY, WE'LL DO THE FOUR DEGREE.
THIS ONE--THERE'S NOT AS MUCH AS MANY QUESTIONS
THAT YOU CAN REALLY GET ASKED ABOUT
BUT YOU WANT TO UNDERSTAND THE FUNDAMENTAL CONCEPTS,
THAT WE HAVE TWO OR MORE POLYPEPTIDE CHAINS
HELD TOGETHER BY NON-COVALENT INTERACTIONS
TO FORM A SINGLE 3D PROTEIN.
REMEMBERING THAT, THAT MEANS BIOLOGICAL FUNCTION, RIGHT?
SO POLYPEPTIDES BY THEMSELVES ARE NOT A PROTEIN
UNTIL THEY CAN DO A JOB INSIDE OUR BODIES.
SO THE VERY COMMON EXAMPLE IS TO LOOK AT HEMOGLOBIN
BECAUSE THERE ARE--THERE'S FOUR POLYPEPTIDE SUBUNITS
AS THE ALPHA AND BETA CHAINS. AND THEN WE HAVE THE HEME GROUP
THAT ACTUALLY BINDS TO THE OXYGEN AND THOSE ARE THERE.
AND THE HEME GROUP, IF WE WANTED TO LOOK AT--
YOU KNOW, IT OFTEN GETS SYMBOLIZED HERE.
THERE'S THE HEME WITH THE IRON.
BUT IF WE WANTED TO GET INTO THE NITTY-GRITTY DETAIL
OF WHAT IT LOOKS LIKE CHEMICALLY.
HERE IS THE STRUCTURE.
SO ONE OF THE MAIN POINTS IN THIS TUTORIAL
IS TO REMEMBER THE DIFFERENCE BETWEEN A POLYPEPTIDE
AND A PROTEIN, RIGHT?
SO A POLYPEPTIDE IS A POLYMER OF AMINO ACIDS.
AND A PROTEIN IS ALSO A POLYMER OF AMINO ACIDS.
SO POLYPEPTIDES AND PROTEINS ARE BOTH POLYMERS OF AMINO ACIDS,
BUT WHAT MAKES IT A PROTEIN IS IT HAS THAT BIOLOGICAL FUNCTION.
SO QUATERNARY STRUCTURE IS A GREAT PLACE TO TALK ABOUT
THAT BECAUSE IF WE HAD ANYONE OF THESE FOUR POLYPEPTIDES
BY ITSELF, IT'S NOT GOING TO TRANSPORT OXYGEN MOLECULES.
SO WE NEED THE COMBINATION. ANOTHER TOPIC THAT COMES UP
AS WE LOOK AT QUATERNARY STRUCTURE IS WE CAN EXPLORE
THE CONCEPT OF DENATURATION.
SO DENATURATION HAS TO DO WITH THE FACT THAT AN ACTIVE,
BIOLOGICALLY ACTIVE PROTEIN MUST BE IN ITS NATIVE CONFIRMATION,
CONFORMATIONS WOULD REALLY BE SECONDARY, TERTIARY,
AND QUATERNARY, RIGHT?
THAT WOULD BE THE CONFORMATION.
THE PRIMARY STRUCTURE, THAT'S COVALENT BONDS.
SO THOSE ARE NOT EASILY BROKEN.
HOWEVER, THE SECONDARY, TERTIARY,
AND QUATERNARY STRUCTURE,
THAT'S ALL ABOUT NON-COVALENT INTERACTIONS.
SO THESE CAN BE DISTURBED BY HEAT AND AGITATION,
CHANGES IN PH, DETERGENTS, SALTS, SOLVENTS,
THERE'S ALL KINDS OF THINGS.
AND WHAT THIS DO IS THEY CAUSE THE PROTEIN TO DENATURE.
SO WHAT DOES THAT MEAN?
THAT MEANS THAT THE PROTEIN BECOMES UNFOLDED.
BASICALLY DENATURING IS A LOSS OF SECONDARY, TERTIARY,
AND QUATERNARY STRUCTURE.
AND THAT COULD BE 'OR'.
AND/OR QUATERNARY STRUCTURE.
SO ANYTIME THERE'S BEEN A CHANGE IN THE ENVIRONMENT
OF A PROTEIN THAT CAUSES A LOSS OF THE QUATERNARY, TERTIARY,
OR SECONDARY STRUCTURE, WE SAY THE PROTEIN HAS BEEN DENATURED.
SO IT DEPENDS ON WHAT THE CIRCUMSTANCES WERE.
SOME PROTEINS CAN SPONTANEOUSLY REFOLD.
BUT IN MANY CASES, IT'S IRREVERSIBLE
AND THEN AT THAT POINT IF THE PROTEIN IS DEACTIVATED
AND THE BODY NEEDS TO JUST CHOP IT UP TO USE
FOR SOMETHING FRESH.
SO WE THINK ABOUT IT HERE. SO HERE IS THE ACTIVE PROTEIN.
NOTICE THAT IT HAS ITS SECONDARY, TERTIARY,
AND POSSIBLY QUATERNARY STRUCTURE,
AND THEN HERE WE SEE IT AS DENATURED.
LOST-WE'VE LOST THOSE INTERACTIONS.
SO NOW LET'S HAVE A FEW WRAP UP QUESTIONS TO SEE HOW WELL
YOU'RE UNDERSTANDING PROTEIN STRUCTURE
AND THE IDEA OF DENATURATION,
OKAY. SO ABOUT HALF OF THE 223 AMINO ACIDS THAT WE FIND
IN THE DIGESTIVE ENZYME TRYPSIN ARE HYDROPHOBIC, RIGHT?
WHERE WOULD WE FIND THOSE?
SO IF THEY'RE HYDROPHOBIC, WE'RE GOING TO FIND THEM
ON THE INSIDE OF THE GLOBULE, RIGHT?
IN THE CENTER OF THE GLOBULAR PROTEIN,
REMEMBERING THAT GLOBULAR PROTEINS ARE OFTEN ENZYMES.
NOW IF WE LOOK AT THE POLAR PARTS,
THE POLAR AMINO ACID RESIDUES OF TRYPSIN,
WHERE WOULD THEY BE, RIGHT?
THOSE ARE GOING TO BE AROUND THE OUTSIDE,
THE OUTSIDE SURFACE OF TRYPSIN.
AND THEN SUPPOSE THAT A POLYPEPTIDE
CONTAINING A HUNDRED AND FIFTY AMINO ACID RESIDUES
IS SYNTHESIZED IN THE LABORATORY,
WHY IS IT NOT CORRECT TO CALL IT A PROTEIN?
BECAUSE IT DOES NOT HAVE BIOLOGICAL FUNCTION, OKAY?
AND THEN LET'S KEEP THIS DISTINGUISHED.
WHAT PARTS OF THE PROTEIN STRUCTURE IS OR ARE AFFECTED
BY HYDROLYSIS?
SO IN HYDROLYSIS,THAT'S BREAKING COVALENT BONDS WITH WATER.
SO WHEN WE'RE TALKING ABOUT HYDROLYSIS OF PROTEINS,
WE'RE TALKING ABOUT THE PRIMARY AMINO ACID STRUCTURE.
BEING BROKEN.
SO TYPICALLY THE HYDROLYSIS CONDITIONS
WILL ALSO CAUSE PROBLEMS WITH THE SECONDARY, TERTIARY,
AND QUATERNARY STRUCTURE BUT THE MAIN THING
ABOUT HYDROLYSIS IS THAT IT HAS THE POWER TO GET ALL THE WAY
TO THE PRIMARY STRUCTURE WHICH IS THE AMINO ACID SEQUENCE.
SO THIS CONCLUDES OUR TUTORIAL ON THE QUATERNARY STRUCTURE
OF PROTEINS
AND PLEASE TAKE A FEW MINUTES NOW
TO WORK ON SOME HOMEWORK PROBLEMS
TO REINFORCE YOUR UNDERSTANDING.