Chapter 7 - ProteinObjectives:• Review the basic structure of proteins and their component amino

acids

• Learn which are the essential amino acids and why

• Understand that there are various amino acid “pools” within the body including dietary and body tissue pools

• Learn how amino acids can be used for

The synthesis of new proteins for growth or replacement of cellular proteins

Production of important non-protein nitrogen containing molecules

Oxidation as an energy source

Synthesis of glucose, ketones or fatty acids

• Understand how the liver and muscle handle amino acid metabolism, and how brain function is closely related to some amino acid metabolism

• Learn how to determine the “quality” of the protein you eat, what the recommended intakes are and why

Fig. 7-1, p. 175

Functional roles of proteins:

Structural organization

Table 7-1a1, p. 178

Table 7-1a2, p. 178

Table 7-1b, p. 179

Key Ideas: the tertiary structure of a protein is determined by its amino acid composition and amino acid side chain chemistry determines what our bodies can do with them metabolically

Sources of Protein

Exogenous Sources• Animal products such as meat, poultry, fish, and

dairy products (not including butter, sour cream, or cream cheese)

• Plant products such as grains, grain products, legumes, and vegetables

Endogenous Sources• Desquamated mucosal cells (50g protein/day)• Digestive enzymes and glycoproteins (17g/day)

(total daily requirement for protein for males is 56g/day, for females it is 46 g/day)

Table 7-2, p. 182

Digestion of Proteins – pepsin only works on exogenous proteins, all other enzymes work on both exogenous and endogenous proteins

Step 1

Amino acid transporter

Cytosol

Intracellular amino acid metabolism

Na+ /K +

ATPase

Step 2

Cell membrane

Step 3

Step 4

Step 5

Aminoacid Amino

acid

Aminoacid

Aminoacid

Aminoacid

Fig. 7-6, p. 183

Na+

Na+

Na+

Na+

Na+

Na+

Table 7-3, p. 184

Amino Acid Supplements

• Amino acids that use the same carrier system may create, depending on the amount ingested, a competition between the amino acids for absorption

• Supplements may result in impaired or imbalanced amino acid absorption

• Some amino acid supplements can cause serious illness

•L-tryptophan

•L-phenylalanine

• Some amino acids are useful

•L-lysine

PeptideCarrier

Lumen(small intestine)

Brush borderMembrane

K1 K1

Na1

Na1

Basolateral Membrane

Enterocyte

Na1

Na1

H1H1

H1H1

PeptidePeptide

Fig. 7-7, p. 184

Na1

Na1

Na1

Na1

Na1

Na1

K1 K1

Required forsome aminoacid transport

AminoAcid

Enterocyte

Lumen

AminoAcids

BasolateralMembrane

Carrier

Brush bordermembrane

Aminoacids

ADP 1

ATP

Pi

Peptides

Fig. 7-8, p. 185

Table 7-4, p. 185

Fig. 7-9, p. 186

Many dietary amino acids are used by enterocytes and never make it into the circulation. This is an example of the fates of glutamine in the enterocyte.

Fig. 7-10, p. 186

Reduced Glutathione can also be made from glutamine in enterocytes

Metabolic Fates of Amino Acids

• Synthesis – Plasma proteins (albumin, retinol-binding protein, acute phase

proteins, heat shock proteins) – Nitrogen containing non-protein compounds (glutathione,

carnitine, creatine, carnosine, choline)– Purines and pyrimidines

• Catabolism

•Transamination and deamination to keto-acids

•Energy

•Glucose

•Ketone bodies

•Cholesterol

•Fatty acids

•Ammonia disposal – urea cycle

Synthesis of plasma proteins

Synthesis of nitrogen containing, non-protein compounds

• Glutathione

• Carnitine

• Creatine

• Carnosine

• Choline

• Purine and pyrimidine bases

Table 1. Effect of Oral Creatine Supplementation on Specific Activities

Activity Summary of Studies CommentsStationary cycling sprints

Several studies support an ergogenic effect in repeated sprints but are not convincing for single sprints

Field studies with bicycles on a track are needed to simulate actual competition

Running sprints (repeated or single)

Conflicting results regarding performance enhancement

Speculation is that weight gain offsets any potential benefit

Swimming sprints (repeated or single)

Conflicting results regarding performance enhancement

Speculation is that weight gain offsets any potential benefit

Weight lifting

Some evidence of an ergogenic effect

Whether creatine supplementation truly increases muscle synthesis is being investigated. Creatine-induced weight gain may make true double-blindness difficult to achieve

Fig. 7-16, p. 192

Pyrimidine synthesis

Fig. 7-17a, p. 193

Purine synthesis from ribose phosphate

Fig. 7-17b, p. 193

Gout

Fig. 7-20, p. 195

Amino Acid Metabolism

• Deamination or transamination

• Nitrogen recycling/disposal via urea cycle

• Use of remaining carbon skeletons

Fig. 7-21, p. 196

Deamination

Urea

Fig. 7-22, p. 196

Transamination

Fig. 7-23, p. 197

A typical blood chemistry panel might include the following tests:

General Metabolism

Kidney Function

Electrolytes Liver Function Thyroid Pancreas

GLU  (Glucose)LDH  (Lactate dehydrogenase)CPK  (Creatine phosphokinase)

BUN (Blood Urea Nitrogen)CREAT Creatinine

Na  (Sodium)K (Potassium)Cl (Chloride)CA (Calcium)PHOS Phosphorus

ALP  (Alkaline phosphatase)ALT (Alanyl amino transferase)ALB  (Albumin)GGT  (Gamma-glutamyl transpeptidase)SGPT   (Serum glutamate pyruvate transaminase)TP  (Total Protein)CHOL  (Cholesterol)GLOB  (Globulin)TBILI (Total Bilirubin)

T3 Triiodothyronine)T4  Thyroxine

AMY (Amylase)LIP (Lipase)

Fig. 7-24, p. 199

Fate of carbon skeletons from amino acids

If acetylcoA is abundant, but oxaloacetate is not

glucogenic

ketogenic

Metabolic fates of selected amino acids

• Phenylalanine – tyrosine, melanin, thyroid hormones, L-dopa, dopamine, norepinephrine, epinephrine, acetyl coA

• Tryptophan – serotonin, melatonin, picolinate, NAD+, nicotinamide, NADP+, acetylcoA, pyruvate, xanthurenic acid

• Methionine – homocysteine, spermidine, tetrahydrofolate, taurine, sulfate, succinylcoA

• Lysine – carnitine, acetylcoA• Threonine – succinyl coA, pyruvate, acetaldehyde

Fig. 7-31, p. 207

Fates of amino acids not used for the synthesis of body proteins

How do organs other than the liver get rid of the ammonia biproduct of amino acid metabolism?• Ammonia to glutamate to glutamine

(glutamine synthetase; ATP; Mg2+ or Mn2+)

• Alanine to pyruvate to glucose (transamination)

• Branched chain amino acids to glutamate to glutamine to alanine (transamination)

Fig. 7-34, p. 209

Alanine-glucose cycle

Fig. 7-38b, p. 213

Fig. 7-38c, p. 213

Kidneys prefer:

Glycine, alanine, glutamine, glutamate, phe, and aspartate

From these it produces:

Arginine, histidine, serine and maybe tyrosine

It is the only organ other than the kidney that has gluconeogenic enzymes and is critically important during fasting.

Table 7-5, p. 214

You can also excrete urea via the skin, and can lose nitrogen via hair and skin cell loss. These losses are referred to as insensible nitrogen losses.

Fig. 7-38a, p. 213

How to Determine Protein Quality?

• Digestibility• Composition with respect to indispensable

amino acids– High quality or complete– Low quality or incomplete

• Quantitative evaluation methods– Amino acid score– Digestiblity corrected amino acid score– Protein efficiency ratio– Nitrogen balance– Biological value– Net protein utilization

Table 7-6, p. 220

Table 7-7, p. 222

FoodServing

Weight in grams

Protein grams % Daily Value

Hamburger, extra lean

6 ounces 170 48.6 97

Chicken, roasted

6 ounces 170 42.5 85

Fish 6 ounces 170 41.2 82

Tuna, water packed

6 ounces 170 40.1 80

Beefsteak, broiled

6 ounces 170 38.6 77

Cottage cheese

1 cup 225 28.1 56

Cheese pizza 2 slices 128 15.4 31

Yogurt, low fat 8 ounces 227 11.9 24

Tofu 1/2 cup 126 10.1 20

Lentils, cooked 1/2 cup 99 9 18

Skim milk 1 cup 245 8.4 17

Split peas, cooked

1/2 cup 98 8.1 16

Whole milk 1 cup 244 8 16

Lentil soup 1 cup 242 7.8 16

Kidney beans, cooked

1/2 cup 87 7.6 15

Cheddar cheese

1 ounce 28 7.1 14

Macaroni, cooked

1 cup 140 6.8 14

Soymilk 1 cup 245 6.7 13

Egg 1 large 50 6.3 13

p. 227a

p. 227b

p. 228