Lecture Notes Review: Proteins, Metabolism, Energy Balance, Vitamins, and related Concepts

Protein

A biomolecule made of amino acids joined by peptide bonds; performs structural, enzymatic, hormonal, immune, and transport functions.

Amino acid

The building blocks of proteins; each has an amine group, a carboxyl group, and a unique side chain (R).

Amino acid structure

Amine group + carboxyl group + side chain (R); side chain determines the specific amino acid.

Essential amino acids

The nine amino acids that must be obtained from the diet because the body cannot synthesize them in sufficient amounts.

Nine essential amino acids (examples)

Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.

Conditionally essential amino acids

Amino acids that are normally nonessential but must be supplied by the diet under certain conditions (growth, illness, etc.).

Peptide bond

Covalent bond that links the carboxyl group of one amino acid to the amino group of another.

Dipeptide

A molecule consisting of two amino acids linked by a peptide bond.

Tripeptide

A molecule consisting of three amino acids linked together.

Oligopeptide

A short chain of amino acids, typically four to ten residues.

Polypeptide

A long chain of amino acids (eleven or more) that forms a protein.

Protein folding

The process by which a protein assumes its three-dimensional structure: primary (sequence), secondary (hydrogen bonds), tertiary (disulfide bridges), and sometimes quaternary structure.

Primary structure

The linear sequence of amino acids in a polypeptide.

Secondary structure

Folding patterns formed by hydrogen bonds, such as alpha helices and beta sheets.

Tertiary structure

Overall 3D shape of a single polypeptide, determined in part by disulfide bridges.

Quaternary structure

Association of two or more polypeptide chains into a functional protein.

Denaturation

Unfolding of a protein’s structure due to heat, acids, or other agents, altering function.

Protein digestion and absorption

Proteins are denatured in the stomach by acid; enzymes (pepsin, proteases) break them into peptides and amino acids, which are absorbed into capillaries of the villi.

Buffer system (protein)

Proteins help regulate pH; the body also uses phosphate and bicarbonate buffer systems.

Complete protein

A protein that contains all nine essential amino acids in sufficient amounts (typical in animal sources).

Incomplete protein

A protein that lacks one or more essential amino acids (often plant sources).

Biological value

Percentage of nitrogen in a protein that is absorbed and retained; examples: egg 100, milk 93–100, soybean 99, beef 92, wheat 40.

PEM (Protein-Energy Malnutrition)

Malnutrition due to inadequate protein and calories.

Marasmus

Severe energy and protein deficiency (general starvation).

Kwashiorkor

Protein deficiency with edema and poor water balance; usually occurs in children.

Dysentery

Severe diarrhea often associated with malnutrition and intestinal damage.

Aflatoxin

Mold toxin that inhibits protein synthesis and can cause cancer; commonly found on grains and nuts.

Nitrogen balance (N balance)

Comparison of nitrogen intake with nitrogen excretion; positive balance supports growth; negative balance indicates loss or insufficient intake.

RDA for protein

About 0.8 g of protein per kilogram of body weight per day (roughly 10–15% of daily calories).

High-protein diets risks

Potential links to heart disease, kidney stress, mineral losses, and some cancers; often associated with processed proteins.

Tofu

Soybean curd; a plant-based protein source rich in protein.

Nutritional genomics

Study of how nutrients influence gene activity and expression.

Glycolysis

First stage of glucose metabolism; glucose is phosphorylated and split to form pyruvate with modest ATP production.

Krebs cycle (Citric acid cycle)

Second stage of glucose metabolism; acetyl-CoA enters a cycle producing CO2, NADH, FADH2, and some ATP.

Electron Transport System (ETC)

Third stage of glucose metabolism; NADH/FADH2 donate electrons to the chain, driving ATP production and forming water.

Cori cycle

Lactate produced in anaerobic muscle is shuttled to the liver, where it is converted to glucose.

Beta-oxidation

Process of breaking down fatty acids into two-carbon acetyl-CoA units for entry into the Krebs cycle.

Glycerol metabolism

Glycerol can be converted to pyruvate and enter the Krebs cycle.

Deamination

Removal of an amino group from an amino acid, producing ammonia and a carbon skeleton that enters the Krebs cycle.

Transamination

Transfer of an amino group from an amino acid to a keto acid; the amino group donor becomes part of another amino acid.

Ketone bodies

Water-soluble molecules produced from acetyl-CoA when carbohydrate supply is limited; used as an alternative energy source.

Alcohol (ethanol)

A small, water-soluble molecule (CH3CH2OH) that can penetrate cell membranes and impair organelles.

Proof (alcohol)

The percentage of alcohol by volume; diameter of proof equals twice the percentage (e.g., 40% alcohol = 80 proof).

Breathalyzer/urine test

Tests used to estimate blood alcohol concentration based on alcohol in breath or urine.

Alcohol effects on brain

A narcotic depressant; first affects judgment (frontal lobe), then speech/vision, coordination (cerebellum), and brainstem (breathing, heart rate).

Alcohol effects on liver

Alcohol metabolism can lead to fatty liver, fibrosis, and cirrhosis; reduces vitamin B6 and impairs fat metabolism.

Fetal/long-term alcohol effects

Chronic alcohol exposure damages brain and other organs; can contribute to nutritional deficiencies.

Cerebral impact of alcohol

Depresses higher brain centers, impairing coordination, decision making, and reaction time.

Energy balance

The relationship between energy intake (calories) and energy expenditure (basal metabolism, TEF, activity).

Calorie in fat (1 lb)

About 3,500 calories equals one pound of fat.

Hunger vs appetite

Hunger is a physiological drive for food; appetite is the desire to eat driven by various cues.

Ghrelin

Hormone that stimulates hunger and food intake.

Amylin

Hormone that slows gastric emptying and promotes satiety.

Cholecystokinin (CCK)

Hormone that promotes satiety by signaling fullness.

Enterostatin

Hormone that reduces fat intake.

Glucagon-like peptide-1 (GLP-1)

Hormone that slows gastric emptying and reduces appetite.

Oxyntomodulin

Hormone that suppresses appetite.

Peptide YY (PYY)

Hormone that reduces appetite and slows stomach emptying.

Neuropeptide Y

Neuropeptide that stimulates appetite and can decrease energy expenditure.

Thermogenesis

Generation of heat as a metabolic process; part of energy expenditure.

Basal metabolic rate (BMR)

Energy required for basic body functions at rest; about two-thirds of daily energy expenditure; influenced by sex, age, weight, height, and activity.

Thermic effect of food (TEF)

Energy used to digest, absorb, and metabolize food; typically about 10% of daily calories.

BMI

Body Mass Index; a measure of body fat based on height and weight: weight (kg)/height (m)².

Frame size

The size of a person’s bones and muscles; relates to body composition and metabolism.

Minimum body fat percentages

Men: about 5%; Women: about 15%.

Anorexia nervosa

Self-starvation and an abnormal body image; often with medical complications and high relapse risk.

Bulimia

Binge eating followed by purging or other compensatory behaviors; can cause electrolyte imbalance and dental/esophageal damage.

Weight cycling

Repeated cycles of weight loss and regain; can lower resting metabolic rate and complicate future weight loss.

Sibutramine (Meridia)