Proteins

1
Proteins

• Amino Acids

2
Protein Structure

• Nearly 50 of dry weight of an animal cell is
  protein
• Structural components of cell
• antibodies
• hormones
• enzymes

3
Protein is an Amino Acid Polymer

• Great variety of proteins due to sequence of
  amino acids
• 20 different amino acids found in proteins
• proteins vary in size
• Hemoglobin 64,500 molecular wt 650 aas
• Myosin muscle protein 468,000 molecular wt 4500
  aas

4
Structure of Amino Acids

• Backbone contains a nitrogen in an amino group
  and two carbons.
• N -- C-- C

5
All Amino acids contain.

• C-C-N backbone
• A side group
• Phosphorus
• 1 and 2
• 2 and 3

6
Four Levels of Structure

• Primary sequence of amino acids the main wire
  of a slinky
• Secondary H bonding between O and H of primary
  sequence to form helix or beta pleated sheet the
  slinky helix
• Tertiary folding back of secondary structure
  like a slinky getting all messed up

7
Which of the following levels of structure are
primarily stabilized by Hydrogen bonding?

• Primary
• Secondary
• Tertiary
• Quaternary

8
Levels of Structure

• Quaternary subunits associated with each other
• Hemoglobin has four subunits

9
Formation of Insulin

• Beta cells of pancreas detect that blood
  glucose is elevated
• Genes turned on to produce preproinsulin
• mRNA formed and moved into Endoplasmic Reticulum
  where ribosomes read the message and form the
  primary sequence of amino acids

10
Insulin production

• Preproinsulin is modified to proinsulin in the ER
• Proinsulin is further modified to insulin and
  packaged in Golgi Apparati to be sent out into
  the blood to help lower Blood Glucose

11
Three General Types of Proteins

• Globular Proteins insulin, plasma albumins,
  globulins, hemoglobin
• hydrophobic inside and hydrophilic outside
• compactly folded and coiled
• Fibrous Proteins keratin (hair, feathers,
  claws, wool, skin)
• silk
• repeating gly-ser-gly-ala-gly-ala

12
Third Type of Protein

• Collagen
• skin, cartilage, bone, intercellular glue
• Triple helix of 25 glycine, 25 proline and
  hydroxyproline
• Proline is an inflexible amino acid because it is
  an imino acid with side chain attached to
  backbone in two locations
• Therefore it is a long extended helix

13
Amino Acids and Optical Activity

• Central Carbon is an asymmetric carbon in almost
  all amino acids (except glycine)
• Optical activity means that amino acids can be D
  or L
• L amino acids found in food
• D amino acids found in bacteria cell walls

14
Amino acids as pH buffers

• In solution, Amino acids have two weak acid
  groups
• -COOH and -NH3
• both are able to lose a H to contribute to the
  H concentration of the solution
• At physiologic pH, generally amino acids exist as
  zwitterions, with a -COO- and a NH3 with out
  a net charge
• They can accept or give off an H as needed

15
Amino Acids Classified by R Groups

• Non polar or hydrophobic
• aliphatic alanine , valine, leucine, isoleucine,
  methionine
• aromatic phenylalanine, tryptophan,
• Polar but uncharged
• OH serine, threonine, tyrosine
• SH cysteine
• amide asparagine, glutamine

16
Classes of Amino Acids

• Polar and Charged
• cationic lysine, arginine, histidine
• anionic glutamic acid, aspartic acid
• imino acid
• Proline side group attached at central carbon
  and amino nitrogen

17
Essential Amino Acids

• 8 (or 9 or 10)
• Isoleucine, leucine, lysine, methionine,
  phenylalanine, threonine, tryptophan, valine
  (histidine, arginine)
• Cho, et al Long term Effects of low histidine
  intake on men.
• Histidine is indispensable with a low N diet

18
Non Essential Amino Acids

• Methionine ----gt cysteine
• Phe ----gt tyrosine

19
Protein Requirement

• Based on N balance studies estimation of minimum
  good quality protein for maintenance of nitrogen
  equilibrium
• Adjustment of requirement to allow for poor
  utilization of proteins from a mixed diet
  compared to good quality protein
• Adjustments for growth, pregnancy, lactation

20
Nitrogen Balance

• NB N intake - N losses
• N losses N urine N feces N skin

21
Protein RDA Vs WHO

• Protein RDA 0.8 grams/kg
• WHO 0.34 grams/kg
• Why the difference?
• Corrections from mean need to 1) 75 efficiency
  usage of protein 2) allowance for mixed
  protein 3) mean 2 SD
• Result is 0.8 g/kg

22
Factors Affecting N Balance

• Physiological state
• growth N balance in children, pregnancy
• production N balance in lactation, egg laying
• physiological stress - N balance in burns,
  trauma, fever
• Nutritional status
• switch from high N to low N diet - N balance for
  3 to 4 days

23
Nitrogen Balance

• With high N intake
• 0 N balance large intake of N offset with high
  breakdown of protein and N release
• With switch to low Protein/ N intake body
  continues to breakdown protein until adjustment
  to low protein is made
• With Low N intake
• - N balance body uses PRO as energy source

24
Energy Intake and N Balance

• Protein compartments and amino acid pool

25
Protein Status and Protein Quality

• Protein Status
• Dietary Intake compare to RDA
• Anthropometric Height, weight, mid-arm muscle
  area circumference, head circumference, BMI
• Clinical observations hair color, skin color
• kwashiorkor Phe--gtTyr--gtMelanin
• edema protein breakdown and tissue water

26
Protein Status

• Biochemical indicators
• Total serum proteins
• Serum albumin
• Serum prealbumin

27
Protein Quality and Quantity

• If all energy, vitamin, and mineral needs are
  met, Nitrogen balance is determined by
• 1. Amounts of essential Amino Acids in the diet
  and
• 2. The total Nitrogen intake

28
Protein Quality and Quantity

• Proteins rich in EAAs are needed in smaller
  quantities than proteins poor in EAAs
• If you have enough EAAs but little total Nitrogen
  in the diet, the growth rate and nitrogen balance
  can be improved by adding NEAAs or nitrogen in
  the form of urea to the animals diet
• Protein need based on both EAA and N intake

29
Methods of measuring Protein Quality

• Role of protein in diet is to supply amino acids
  to produce proteins in the body
• A complete protein is one that supplies all the
  EAAs in amounts adequate for human use
• High quality protein is one that supplies the
  EAAs in the right proportions
• limiting AA the EAA in short supply

30
Protein Quality Chemical and Biological Methods

• Chemical scoring
• Amino acid analysis of a protein
• Compare to ideal amino acid makeup
• composition of egg white or milk protein
• a test protein with a limiting amino acid of 70
  of the standard would receive a chemical score of
  70
• AA Score mg of AA in 1 gram test pro x 100
• mg of AA in 1 gram ref.
  pro

31
Biological Methods of Determining Protein Quality

• Biological Value the amount of protein N that
  is retained from a given amount of protein that
  has been digested and absorbed
• Bv N retained x100
• ----------------
• N absorbed

32
B v determination

• Bv I - fecal - urine x 100
• ----------------------
• I - fecal

33
Net Protein Utilization

• NPU N retained x100
• ----------------
• N intake

34
True Digestibility

• T d N absorbed x 100
• ------------------
• N intake

35
Protein Efficiency Ratio

• PER weight gain (grams) x 100
• --------------------------------
• Protein intake (grams)

36
Protein Slope-Ratio Assay

• Dose response curve. Proteins are fed at varying
  levels and weight gain is plotted
• Ratio of egg albumin is the comparison protein
  and given relative slope of one

37
Protein Digestion

• Protein is denatured in stomach
• Pepsin splits some peptide bonds in stomach
• In duodenum, pancreatic enzymes enter and break
  specific peptide bonds to release mostly amino
  acids and some dipeptides for absorption