Unit 04: Human Nutrition and Protein Metabolism

Unit 04: Introduction to Human Nutrition and Metabolism of Proteins and Amino Acids

• Proteins are the most abundant nitrogen-containing compounds in the diet and body
• Formed by the polymerization of L-α-amino acids through peptide bonds
• Amino acids have similar central structures but different side chains
• Side chains determine the various metabolic and physiological roles
• Amino acids cannot be synthesized fast enough by the body to meet growth and maintenance needs

Key Points

• Protein and amino acid overview
• Amino acid structure and function
• Essential (indispensable) amino acids
• Conditionally essential amino acids
• Protein and amino acid requirements
• Methods for determining protein needs
• Protein quality and digestibility
• Animal vs. plant protein

4.1 Introduction to Proteins

• Proteins are the most abundant nitrogen-containing compound in the diet and body.
• Classes of complex biomolecules include proteins, DNA, RNA, polysaccharides, and lipids.
• Proteins are formed by the polymerization of L-α-amino acids via peptide bonds.
• Protein structure and functions are determined by the specific amino acid chain and order.
• Multimeric proteins are composed of two or more polypeptide chains, each called a subunit.
• Proteins are essential for cell function and organ activity.
• Proteins function as building blocks for proteins, enzymes, hormones, and antibodies.

4.1 Protein Structure

• Proteins are the most abundant nitrogen-containing compound in the diet and body.
• The classes of complex biomolecules are proteins, DNA, RNA, polysaccharides, and lipids.
• L-α-amino acids polymerize via peptide bond formation.
• Proteins are formed from the sequential linking of amino acids.
• Proteins exhibit primary, secondary, tertiary, and quaternary structure.
• The sequencing of amino acid residues defines the primary structure.
• Secondary structures include alpha helices and beta sheets.
• The tertiary structure is the three-dimensional shape of a protein.
• The quaternary structure is the arrangement of multiple polypeptide chains in a protein.

4.1 Amino Acid Metabolism

• Amino acids are the building blocks of proteins.
• Amino acids are the currency of protein nutrition and metabolism.
• Proteins are essential for cellular processes.
• The genome's base sequence directs amino acid assembly.
• Proteins impact tissue, organ function, and overall health.
• The human genome contains approximately 30,000 genes.
• These genes lead to thousands of proteins that determine characteristics.
• Deficiencies in proteins and amino acids can lead to health issues.

4.1 Amino Acids Structure

• Amino acids are the structural units of proteins.
• Amino acids contain a central carbon atom attached to an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group).
• The side chain (R group) distinguishes one amino acid from another and dictates its chemical properties.
• Amino acids link together to form peptide bonds, creating the polypeptide chains of proteins.

4.3 Structure and Chemistry of Amino Acids

• Amino acids are central to proteins and peptide synthesis.
• They consist of an amino group, a carboxyl group, a hydrogen, and a side-chain (R group) that differentiates them.
• Peptide bonds form when the carboxyl group of one amino acid combines with the amino group of another, releasing a molecule of water.
• Side chains (R groups) influence amino acids' physical and chemical properties.

4.3 Chemical properties and functions of amino acids

• Specific properties enable various functions
• Methionine: Donates a methyl group in one-carbon metabolism.
• Glutamine: The amide group serves as the nitrogen source for pyrimidine synthesis.
• Cysteine: The sulfhydryl group forms disulfide bonds for cross-linking.
• Branched-chain amino acids (BCAAs): Serve as "universal" fuels.
• Derivatives of amino acids act as precursors for important compounds (e.g., creatine, dopamine).

4.3 Role of Amino Acids in Protein Synthesis

• Amino acids are essential precursors in protein synthesis.
• They also serve as signalling molecules that modulate protein synthesis processes.
• In mRNA translation initiation, met-tRNA binds to the 40S ribosomal subunit, forming a 43S preinitiation complex.
• The 43S complex locates the AUG start codon on the mRNA.
• Other initiation factors dissociate from the 40S ribosomal complex to form the 80S initiation complex.

4.3 Post-translational Modifications

• Specific amino acid residues in a polypeptide chain often undergo post-translational modifications after synthesis.
• These modifications are important in affecting protein function, localization and, interactions with other molecules.

4.4 Classification of Amino Acids

• Amino acids are categorized as indispensable (essential), dispensable (non-essential), or conditionally indispensable (essential under certain conditions).
• Indispensable amino acids cannot be synthesized by the organism at a rate sufficient for normal growth.
• Dispensable amino acids can be synthesized by the body.
• Conditionally indispensable amino acids can be synthesized but may not be synthesized at a sufficient rate, so dietary intake may be necessary under conditions of increased demand.

4.4 Importance of "Ordinarily Available"

• Some dispensable amino acids can be synthesized from analogous α-keto acids through transamination.
• These keto acids are not typically available in the diet.
• In conditions like renal failure, these keto acids can help maintain nitrogen metabolism

4.4 Understanding "Normal Growth"

• The definition emphasizes the context of growth for classifying amino acids.
• Arginine is indispensable for growing rats but becomes dispensable for adult rats.
• If the ability to synthesize arginine is impaired, it becomes essential again.

4.4 The Concept of De Novo Synthesis

• The de Novo synthesis of amino acids refers to the creation from non-amino acid precursors.
• Certain amino acids can be synthesized from similar structural analogues via transamination.
• Dispensable amino acids can be synthesized from non-amino acid nitrogen sources (e.g., ammonium ion) and carbon sources (e.g., glucose).

4.4 Understanding Nonspecific Nitrogen

• Nitrogen is essential for supporting body protein and nitrogen metabolism.
• Traditional methods consider a simple nitrogen-containing mixture sufficient, but evolving perspectives understand the complexities of human nitrogen needs.

4.4 Glutamate Production in Plants and Bacteria

• A key reaction of glutamate production in plants and bacteria is a crucial step in incorporating ammonia from the nitrogen cycle into amino acids.
• In animals, ammonia incorporation primarily occurs through glutamate rather than glutamine..

4.4 Ammonia Incorporation into Glycine

• Glycine synthase reaction incorporates CO2 and NH4+ into glycine, a building block for many essential body components.

4.5 Biology Protein and Amino Acid Requirements

• Dietary α-amino acid nitrogen is essential for organ protein synthesis.
• Protein requirements vary based on body protein mass, age, gender, and physiological state.

4.6 Estimation of Protein and Amino Acid Requirements

• Estimating total protein needs starts from zero nitrogen balance measurements in adults.
• Additional requirements are for growing infants and children, pregnant or lactating women, recovering from trauma, or infection.

4.7 Meeting protein and amino acid needs

• Indispensable amino acid content
• Availability of amino acids

4.7. Protein Digestibility

• Traditional measure = Difference between nitrogen intake and fecal nitrogen output.
• Limitations include bacterial proteins, endogenous nitrogen.
• True digestibility = measures nitrogen truly absorbed
• Uses isotopic labeling.
• True digestibility is higher than apparent.

4.7 Splanchnic Amino Acid Metabolism

• Most dietary proteins get digested in the small intestine.
• A significant proportion of amino acids are metabolized in the splanchnic bed (gut and liver) before reaching other tissues.
• Threonine and glutamate are extensively utilized by the gut.

4.7 Practical Implications for Diet

• Optimize protein intake by ensuring adequate amino acids.
• Use high-quality proteins from animal sources or balanced plant combinations.
• Consider factors like plant protein digestibility in plant-based diets.

4.7 Definition and Differences of Protein Quality

• Protein nutritional quality relates to a protein's ability to meet physiological nitrogen and amino acid needs.
• Indispensable amino acids are essential for protein synthesis.
• Plant proteins may lack some indispensable amino acids (making them incomplete).

4.7 Protein Quality and PDCAAS

• Methods for assessing protein quality include amino acid scoring and the Protein Digestibility-Corrected Amino Acid Score (PDCAAS).
• PDCAAS compares the amino acid composition of a protein to a reference pattern (typically a 2–5-year-old child's requirements).
• PDCAAS helps ensure adequate essential amino acids in the diet.

4.8 Factors Other Than Diet Affecting Protein and Amino Acid Requirements

• Genetic factors (e.g., genetic predispositions can affect metabolism).
• Environmental factors (e.g., environmental conditions influence nutrient needs).
• Physiological factors (e.g., different life stages have distinct protein requirements).
• Pathological factors (e.g., chronic diseases affect protein needs).
• Impact of aging (e.g., aging is associated with increased morbidity, decreased nutrient absorption, and changes in metabolism).

4.8 Metabolic Response to Stress and Illness

• Catabolic phase: Increased nitrogen loss, loss of other nutrients (e.g., potassium, magnesium, vitamin C).
• Anabolic phase: Increased nitrogen retention, enhanced immune cell production, and tissue repair.
• The response duration of the anabolic phase often exceeds the catabolic phase.

4.8 Agent, Host, and Environment Factors in Diet

• Factors affecting protein and amino acid requirements that are not related to diet, such as chemical form of nutrition, age, sex, genetic makeup, existing conditions, drugs, infections, physical trauma, physical conditions (housing, heating), biologic factors (sanitary conditions), socioeconomics factors (poverty, dietary habits, food choices, physical activity).

4.8 References

• Scientific literature contains details on the information found in this unit on protein and amino acid metabolism.