- WELCOME TO PART 5, PROTEINS AND ENZYMES.
AND IN THIS TUTORIAL,
WE'RE GOING TO LOOK AT THE SECONDARY STRUCTURE
OF PROTEINS WHICH WE KNOW IS RELATED TO THE H-BONDING
BETWEEN NEIGHBORING AMINO ACID GROUPS
WITHIN THE POLYPEPTIDE CHAIN.
LET'S GET STARTED.
SO THE SECONDARY STRUCTURE, RIGHT, WILL HAVE REGULAR FOLDING
AND IT'S IN LOCALIZED REGIONS OF THE BACKBONE, RIGHT?
SO THE WHOLE THING ABOUT CLOSE PROXIMITY
AND REALLY IT'S ABOUT THE CARBONYLS
AND THE AMINO GROUPS IN OUR BACKBONE
AND H-BONDING IS THE DEAL HERE.
SECONDARY STRUCTURES,
ALL ABOUT H-BONDING WITHIN THE POLYPEPTIDE BACKBONE.
NOW IT COMES IN TWO FLAVORS.
WE HAVE THE ALPHA HELIX WHERE THE POLYPEPTIDE BACKBONE TWISTS
AND THEN WE HAVE THE BETA SHEET WHERE IT'S MORE
OF A FOLDING AND WE GET THESE PARALLEL SHEETS.
AND SO LET'S LOOK AT THESE TWO MORE CLOSELY.
SO, ON THIS LEFT SIDE, RIGHT,
THIS IS OUR ALPHA HELIX.
SO WE LOOK AT THE PRIMARY STRUCTURE,
THE AMINO ACID SEQUENCE,
AND THEN WE NOTICE THAT AS IT SPIRALS,
WE CAN GET H-BONDING,
RIGHT, RIGHT HERE BETWEEN THE AMINE GROUP OF THE AMIDE
AND THE CARBONYL OF THE CARBOXYLIC ACID.
SO THIS H-BONDING BETWEEN NEIGHBORING AMINO ACIDS
IN THE PEPTIDE CHAIN CAN CREATE THE SPIRALING.
AND THEN HERE IS JUST
ANOTHER DIAGRAM SHOWING THE SAME THING, RIGHT?
SO HERE WE SEE OUR ALPHA HELIX AND THEN HERE IS THE H-BONDING.
I THINK IT'S A LITTLE CLEARER TO SEE IN THIS SECOND DIAGRAM.
SO ANYTIME YOU SEE WITHIN A PROTEIN STRUCTURAL,
THE SPIRALING RIBBON THEN THE AUTHOR IS COMMUNICATING
TO YOU SECONDARY STRUCTURE OF ALPHA HELIX.
THE OTHER ALTERNATIVE IS THE BETA SHEET.
NOW THE BETA SHEET, IT LOOKS--
IT HAS THIS RIBBON QUALITY RIGHT HERE, SO WE'LL SEE IT HERE.
THIS IS OUR BETA SHEET.
SO IT FOLDS BACK ON ITSELF.
AND THEN NOTICE THIS LIKE THIS ANGLED QUALITY, RIGHT?
SO IF WE LOOK AT THE BETA SHEET,
WE SEE THAT THAT THE FOLDING
LEADS TO THIS LIKE VERTICAL KIND OF LINEAR QUALITY
AS OPPOSED TO THE CIRCULAR SPIRALING OF THE ALPHA HELIX.
AND SO THE BETA SHEETS WILL SEE THAT SILK AND SPIDER WEBS ARE
FORMED WITH BETA SHEETS.
AND SO THAT'S WHY THEY'RE SO IRIDESCENT,
AS THE LIGHT COMES IN AND HITS THE SILK OR THE SPIDER WE,
IT GETS REFLECTED BACK
AND THAT REFLECTION OF LIGHT IS A CONSEQUENCE
OF THE SECONDARY STRUCTURE OF THE PROTEINS
THAT CREATE SKILL AND SPIDER WEBS,
AND THAT'S WHAT MAKES THEM SO SPARKLY IS THAT THE BETA SHEETS
ARE REFLECTING THE LIGHT.
AND SO THEN ONCE AGAIN IN HERE
WE CAN SEE THAT WE HAVE THE H-BONDING
BETWEEN THE NEIGHBORING R-GROUPS-- OR NO,
EXCUSE ME, THE NEIGHBORING AMINO ACIDS.
OKAY. ALL RIGHTY.
SO LET'S LOOK A LITTLE MORE CLOSELY
AT THE SECONDARY STRUCTURE.
SO THE SECONDARY STRUCTURE IS ALL ABOUT ALPHA HELIX
AND BETA SHEETS.
SO LET'S LOOK AT THOSE A LITTLE MORE CLOSELY AND COMPARE.
NOW WHAT WE'LL SEE IS THAT THE SECONDARY STRUCTURE
DISTINGUISHES SEVERAL DIFFERENT TYPES OF FIBROUS PROTEINS.
FOR EXAMPLE, FIBROUS PROTEINS
WILL SEE OUR STRUCTURAL PROTEINS.
SO THEY TEND TO BE WATER INSOLUBLE
AND THERE'S OFTEN SMALL FIBRILS TWIST
TOGETHER INTO LARGER BUNDLES.
YOU CAN THINK OF IT, IT'S VERY SIMILAR TO A ROPE.
AND IF WE LOOK AT THE STRUCTURE DOWN HERE WE CAN SEE AN EXAMPLE.
WE CAN SEE THAT THE SMALLER FIBRILS ARE THEN TWISTED
INTO LARGER BUNDLES
AND WE CAN SEE WHERE THAT WOULD GIVE STRENGTH AND SUPPORT.
THE NUMBER OF DISULFIDE BONDS WILL AFFECT THE HARDNESS
AND FLEXIBILITY.
AND SO I THINK INTUITIVELY WE CAN RECOGNIZE, RIGHT,
THAT AS WE INCREASE THE DISULFIDE BRIDGES,
WE INCREASE THE STIFFNESS.
SO IF WE'RE COMPARING OUR FINGERNAILS TO OUR HAIR,
WE WOULD EXPECT TO FIND MORE DISULFIDE BRIDGES
IN OUR FINGERNAILS THAN OUR HAIR.
AND THAT'S WHEN WE DO HAIR STRAIGHTENING OR PERMS.
THAT'S BASICALLY WHAT WE'RE DOING
IS WE BREAK THE DISULFIDE BRIDGES AND THEN
WE MAKE SURE EVERYONE KNOWS WHAT THAT LOOKS LIKE, RIGHT?
AND THEN WE REFORM THEM.
AND SULFUR--
WHEN SULFUR IS BONDED TO ORGANIC MOLECULES,
IT TENDS TO HAVE KIND OF A STINKY SMELL.
AND THEN AS I MENTIONED ON THE PREVIOUS PAGE,
SILK AND SPIDER WEBS,
THOSE HAVE THE BETA SHEETS.
THE IMPORTANT THING TO KNOW ABOUT THE FIBROUS PROTEINS
AND THE SECONDARY STRUCTURE IS THAT THERE IS ONLY ALPHA HELIXES
OR BETA SHEETS IN FIBROUS PROTEINS, RIGHT?
SO WE GET ONE OR THE OTHER, NOT BOTH.
AND THEN HERE IS A SCHEMATIC OF COLLAGEN,
AN EXAMPLE OF A FIBROUS PROTEIN,
AND NOTICE HOW THERE IS A COMBINING
OF THE THREE ALPHA HELIXES, RIGHT?
SO YOU CAN TRULY JUST THINK ABOUT--
I THINK ROPES ARE REALLY GOOD EXAMPLE
FROM OUR LIFE EXPERIENCE.
AND FIBROUS PROTEINS ARE A BIG DEAL.
HERE ARE SOME EXAMPLES OF DIFFERENT FIBROUS PROTEINS
AND THEN DIFFERENT PLACES THAT THEY OCCUR IN ANIMALS
AND THEIR FUNCTION.
SO NOTICE SEEING THE KERATINS SHOW UP
FOR SKIN AND WOOL AND HOOVES AND SILK.
THE COLLAGENS ARE MORE THE CONNECTIVE TISSUE.
THE ELASTIN AS WELL, LIGAMENTS.
MYOSIN ARE FOUND IN MUSCLE AND FIBRIN HELP TO--
FOR BLOOD CLOTS.
SO THAT CONCLUDES OUR TUTORIAL ON THE SECONDARY STRUCTURE.
THE MAIN THING TO KEEP IN MIND IS THE ALPHA HELIX
VERSUS THE BETA SHEET AND THAT IT'S H-BONDING
WITHIN THE POLYPEPTIDE BACKBONE.
PLEASE TAKE SOME TIME NOW TO WORK A FEW HOMEWORK PROBLEMS
TO REINFORCE YOUR UNDERSTANDING.