Chapter 6: Proteins, Amino Acids and Protein Turnover

Chapter 6: Proteins, Amino Acids, and Protein Turnover

Proteins: Components of All Body Tissues

  • Amino acid structure

    • Central Carbon (C)

    • Amine group (NH2)

    • Acid group (COOH)

    • Hydrogen atom (H)

    • Side chain (R group)

    • Varies for each amino acid

What Are Proteins?

  • Proteins: Large, complex molecules found in cells of all living things.

  • Dictated by genetic material (DNA).

  • Composed of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) atoms.

  • Made from 20 different amino acids.

  • Pearson Education, Inc. (2014): Educational reference for protein information.

Amino Acids

Structure of Amino Acids

  • Cartoon diagrams depicting structures:

    • General structure:

    • Amino group (H₂N) - Central Carbon (C) - Carboxyl (COOH)

    • R group varies (example diagrams include Glycine and Leucine).

Types of Amino Acids

  • Essential Amino Acids:

    • Nine amino acids that cannot be produced in sufficient quantities by the body; must be obtained from food.

  • Nonessential Amino Acids:

    • Can be synthesized by the body in adequate amounts.

Transamination

  • Process where the amine group is transferred from one acid to a different acid group and side chain.

  • Example:

    • Conversion from Valine to Glycine as shown in diagrams.

Peptide Bonds

  • Bonds formed between two amino acids through a dehydration reaction where a molecule of water (H₂O) is removed, creating a dipeptide.

  • Diagram shows formation of a dipeptide bond between two amino acids.

Cell Components

  • Cell membrane, Ribosomes, Endoplasmic reticulum, Cytoplasm (cell interior), Mitochondrion, and nucleus as key components involved in protein synthesis.

Transcription and Translation

Process Overview

  • Nucleus:

    • Transcription of genetic information from DNA to mRNA.

  • Cytoplasm:

    • Ribosomes translate mRNA code to amino acids to build proteins.

    • tRNA delivers specific amino acids required for protein synthesis.

Steps of Protein Synthesis:

  1. DNA unwinds, and a section is transcribed into mRNA.

  2. mRNA leaves the nucleus and travels to cytoplasm.

  3. mRNA binds to ribosome.

  4. tRNA binds with specific amino acids, delivering them to ribosome.

  5. Amino acids are added to growing chain.

  6. Final protein is released for further modifications or functional use.

Protein Turnover

  • Definition: Process of degrading existing proteins to provide building blocks for new proteins.

  • Amino acid pool: Includes amino acids from food and cellular breakdown, used for future protein synthesis.

    • Synthesis processes include enzymes, antibodies, other cell components, synthesis of fat, and glucose for energy.

    • Nitrogen from amino acids synthesized into urea in the liver, excreted in urine.

  • Implication: Body cannot store excess proteins or amino acids; they must be regularly consumed and processed.

Urea Excretion

  • The liver and kidneys are responsible for excreting excess nitrogen from high protein intake especially relevant in high protein diets.

Protein Structure and Denaturation

Protein Function Dependence on Structure

  • Four Levels of Protein Structure: Primary, Secondary, Tertiary, Quaternary.

  • Denaturation: Process where proteins unravel, losing their function due to heat, acid, or other damaging substances. High fever and excessive acidic or alkaline conditions can cause denaturation.

Incomplete and Complete Proteins

Definitions

  • Incomplete proteins: Do not provide all essential amino acids.

  • Complete proteins: Contain all essential amino acids.

Complementary Protein Combinations

  • Legumes and Grains

    • Legumes: limited in methionine and cysteine.

    • Grains: limited in lysine.

    • Examples: Red beans and rice, Hummus with bread, Minestrone soup.

Digestion of Dietary Proteins

  • Occurs primarily in the stomach and small intestine.

Digestive Process in key organs:

  • Mouth: Chewing reduces food size.

  • Stomach: Hydrochloric acid denatures proteins; pepsin activates to break proteins into smaller peptides.

  • Small Intestine: Proteases further digest polypeptides; absorption of amino acids into bloodstream through enterocytes.

  • Pancreas: Produces proteases released into the small intestine.

  • Liver: Processes amino acids, converting them to glucose or fat, or building new proteins.

Protein Digestibility

  • Methods for estimating protein quality: Chemical score and PDCAAS.

  • Absorption rates:

    • Animal protein & soy: ~90% digestibility.

    • Legumes: ~70-80%.

    • Grains/vegetables: ~60-90%.

Why Do We Need Protein?

Functions of Protein

  • Cell Growth, Repair, and Maintenance

  • Enzymes and Hormones Regulation

  • Fluid & Electrolyte Balance

    • Proteins control osmotic pressure in capillaries.

  • Acid-Base Balance: Proteins act as buffers.

  • Strong Immune System: Provide antibodies.

  • Energy Source: In dire situations, proteins can be converted for energy.

Dietary Protein Requirements

Recommended Daily Allowance (RDA)

  • Sedentary adults: 0.8 g/kg/day.

  • Active individuals: 1.2-2 g/kg/day.

  • Protein needs increase for children, adolescents, pregnant, and lactating women.

How to Calculate Personal Protein Needs

  • Step 1: Convert weight in lbs to kg (lbs/2.2).

  • Step 2: Multiply weight in kg by RDA (0.8 g/kg).

Protein-Energy Malnutrition Disorders

Types

  • Marasmus: Caused by severe energy and protein deficiency.

  • Kwashiorkor: Primarily protein deficiency with adequate energy intake.

Genetic Disorders linked to Protein Metabolism

  • Phenylketonuria (PKU): Inability to break down phenylalanine due to missing enzyme.

  • Sickle Cell Anemia: A genetic defect that alters mRNA instructions for hemoglobin production leading to health complications.

High Protein Diets

Effects on Health

  • Heart Disease: Associated with high intake of animal proteins and saturated fats raises LDL cholesterol.

  • Bone Health: Higher protein intake can protect against bone loss in some populations.

  • Kidney Disease: High protein diets may strain kidneys, particularly in susceptible populations.

Highlights of Nutrient-Rich Food Sources

  • Legumes: Complete protein sources, iron, fiber, low cholesterol.

  • Nuts: Reduce cardiovascular disease risk, packed with nutrients.

  • New High Protein foods: Quinoa and amaranth are examples of complete proteins being promoted in contemporary diets.